Element, chemical

A chemical element can be defined in one of two ways: experimentally or theoretically. Experimentally, an element is any substance that cannot be broken down into any simpler substance. Imagine that you are given a piece of pure iron and asked to break it down using any device or method ever invented by chemists. Nothing you can do will ever change the iron into anything simpler. Iron, therefore, is an element.

The experimental definition of an element can be explained by using a second definition: an element is a substance in which all atoms are of the same kind. If there were a way to look at each of the individual atoms in the bar of pure iron mentioned above, they would all be the same—all atoms of iron. In contrast, a chemical compound, such as iron oxide, always contains at least two different kinds of atoms, in this case, atoms of iron and atoms of oxygen.

Words to Know

Atomic mass: The mass of the protons, neutrons, and electrons that make up an atom.

Atomic number: The number of protons in the nucleus of an element's atom.

Chemical symbol: A letter or pair of letters that represents some given amount of an element.

Compound, chemical: A substance that consists of two or more chemical elements joined to each other in a specific proportion.

Metal: An element that loses electrons in chemical reactions with other elements.

Metalloid: An element that acts sometimes like a metal and sometimes like a nonmetal.

Nonmetal: An element that tends to gain electrons in chemical reactions with other elements.

Periodic table: A system of classifying the chemical elements according to their atomic number.

Synthetic element: An element that is made artificially in a laboratory but is generally not found in nature.

Natural and synthetic elements

Ninety-two chemical elements occur naturally on Earth. The others have been made synthetically or artificially in a laboratory. Synthetic elements are usually produced in particle accelerators (devices used to increase the velocity of subatomic particles such as electrons and protons) or nuclear reactors (devices used to control the energy released by nuclear reactions). The first synthetic element to be produced was technetium, discovered in 1937 by Italian American physicist Emilio Segrè (1905–1989) and his colleague C. Perrier. Except for technetium and promethium, all synthetic elements have larger nuclei than uranium.

Two Dozen Common and Important Chemical Elements

		Percent of all atoms*
Element	Symbol	In the universe	In Earth's crust	In sea water	In the human body	Characteristics under ordinary room conditions
*If no number is entered, the element constitutes less than 0.1 percent.
Aluminum	Al	—	6.3	—	—	A lightweight, silvery metal
Calcium	Ca	—	2.1	—	.02	Common in minerals, seashells, and bones
Carbon	C	—	—	—	10.7	Basic in all living things
Chlorine	Cl	—	—	0.3	—	A toxic gas
Copper	Cu	—	—	—	—	The only red metal
Gold	Au	—	—	—	—	The only yellow metal
Helium	He	7.1	—	—	—	A very light gas
Hydrogen	H	92.8	2.9	66.2	60.6	The lightest of all elements; a gas
Iodine	I	—	—	—	—	A nonmetal; used as antiseptic
Iron	Fe	—	2.1	—	—	A magnetic metal; used in steel
Lead	Pb	—	—	—	—	A soft, heavy metal
Magnesium	Mg	—	2.0	—	—	A very light metal
Mercury	Hg	—	—	—	—	A liquid metal; one of the two liquid elements
Nickel	Ni	—	—	—	—	A noncorroding metal; used in coins
Nitrogen	N	—	—	—	2.4	A gas; the major component of air
Oxygen	O	—	60.1	33.1	25.7	A gas; the second major component of air
Phosphorus	P	—	—	—	0.1	A nonmetal; essential to plants
Potassium	K	—	1.1	—	—	A metal; essential to plants; commonly called "potash"
Silicon	Si	—	20.8	—	—	A semiconductor; used in electronics
Silver	Ag	—	—	—	—	A very shiny, valuable metal
Sodium	Na	—	2.2	0.3	—	A soft metal; reacts readily with water, air
Sulfur	S	—	—	—	0.1	A yellow nonmetal; flammable
Titanium	Ti	—	0.3	—	—	A light, strong, noncorroding metal used in space vehicles
Uranium	U	—	—	—	—	A very heavy metal; fuel for nuclear power

At the beginning of the twenty-first century, there were 114 known elements, ranging from hydrogen (H), whose atoms have only one electron, to the as-yet unnamed element whose atoms contain 114 electrons. New elements are difficult to produce. Only a few atoms can be made at a time, and it usually takes years before scientists agree on who discovered what and when.

Classifying elements

More than 100 years ago, chemists began searching for ways to organize the chemical elements. At first, they tried listing them by the size (mass) of their nucleus, their atomic mass. Later, they found that using the number of protons in their atomic nuclei was a more effective technique. They invented a property known as atomic number for this organization. The atomic number of an element is defined as the number of protons in the nucleus of an atom of that element. Hydrogen has an atomic number of 1, for example, because the nuclei of hydrogen atoms each contain one—and only one—proton. Similarly, oxygen has an atomic number of 8 because the nuclei of all oxygen atoms contain 8 protons. The accompanying table (periodic table of the elements) contains a list of the known chemical elements arranged in order according to their atomic number.

Notice that the chemical symbol for each element is also included in the table. The chemical symbol of an element is a letter or pair of letters that stands for some given amount of the element, for example, for one atom of the element. Thus, the symbol Ca stands for one atom of calcium, and the symbol W stands for one atom of tungsten. Chemical symbols, therefore, are not really abbreviations.

Chemical elements can be fully identified, therefore, by any one of three characteristics: their name, their chemical symbol, or their atomic number. If you know any one of these identifiers, you immediately know the other two. Saying "Na" to a chemist immediately tells that person that you are referring to sodium, element #11. Similarly, if you say "element 19," the chemist knows that you're referring to potassium, known by the symbol K.

The system of classifying elements used by chemists today is called the periodic table. The law on which the periodic table is based was first discovered almost simultaneously by German chemist Julius Lothar Meyer (1830–1895) and Russian chemist Dmitry Mendeleev (1834–1907) in about 1870. The periodic table is one of the most powerful tools in chemistry because it organizes the chemical elements in groups that have similar physical and chemical properties.

Properties of the elements

One useful way of describing the chemical elements is according to their metallic or nonmetallic character. Most metals are hard with bright, shiny surfaces, often white or grey in color. Since important exceptions to this rule exist, metals are more properly defined according to the way they behave in chemical reactions. Metals, by this definition, are elements that lose electrons to other elements. By comparison, nonmetals are elements that gain electrons from other elements in chemical reactions. (They may be gases, liquids, or solids but seldom look like a metal.) The vast majority (93) of the elements are metals; the rest are nonmetals.

A Who's Who of the Elements

Element	Distinction	Comment
Astatine (At)	The rarest	Rarest of the naturally occurring elements
Boron (B)	The strongest	Highest stretch resistance
Californium (Cf)	The most expensive	Sold at one time for about $1 billion a gram
Carbon (C)	The hardest	As diamond, one of its three solid forms
Germanium (Ge)	The purest	Has been purified to 99.99999999 percent purity
Helium (He)	The lowest melting point	−271.72°C at a pressure of 26 times atmospheric pressure
Hydrogen (H)	The lowest density	Density 0.0000899 g/cc at atmospheric pressure and 0°C
Lithium (Li)	The lowest–density metal	Density 0.534g/cc
Osmium (Os)	The highest density	Density 22.57 g/cc
Radon (Rn)	The highest–density gas	Density 0.00973 g/cc at atmospheric pressure and 0°C
Tungsten (W)	The highest melting point	3,420°C

Historical background

The concept of a chemical element goes back more than 2,000 years. Ancient Greek philosophers conceived of the idea that some materials are more fundamental, or basic, than others. They listed obviously important materials such as earth, air, fire, and water as possibly being such "elemental" materials. These speculations belonged in the category of philosophy, however, rather than science. The Greeks had no way of testing their ideas to confirm them.

In fact, a few elements were already known long before the speculations of the Greek philosophers. No one at that time called these materials elements or thought of them as being different from the materials we call compounds today. Among the early elements used by humans were iron, copper, silver, tin, and lead. We know that early civilizations knew about and used these elements because of tools, weapons, and pieces of art that remain from the early periods of human history.

Another group of elements was discovered by the alchemists, the semimystical scholars who contributed to the early development of chemistry. These elements include antimony, arsenic, bismuth, phosphorus, and zinc.

Formation of the Elements

How were the chemical elements formed? Scientists believe the answer to that question lies in the stars and in the processes by which stars are formed. The universe is thought to have been created at some moment in time 12 to 15 billion years ago. Prior to that moment, nothing other than energy is thought to have existed. But something occurred to transform that energy into an enormous explosion: the big bang. In the seconds following the big bang, matter began to form.

According to the big bang theory, the simplest forms of matter to appear were protons and electrons. Some of these protons and electrons combined to form atoms of hydrogen. A hydrogen atom consists of one proton and one electron; it is the simplest atom that can exist. Slowly, over long periods of time, hydrogen atoms began to come together in regions of space forming dense clouds. The hydrogen in these clouds was pulled closer and closer together by gravitational forces. Eventually these clouds of hydrogen were dense enough to form stars.

A star is simply a mass of matter that generates energy by nuclear reactions. The most common of these reactions involves the combination of four hydrogen atoms to make one helium atom. As soon as stars began to form, then, helium became the second element found in the universe.

As stars grow older, they switch from hydrogen-to-helium nuclear reactions to other nuclear reactions. In another such reaction, helium atoms combine to form carbon atoms. Later carbon atoms combine to form oxygen, neon, sodium, and magnesium. Still later, neon and oxygen combine with each other to form magnesium. As these reactions continue, more and more of the chemical elements are formed.

At some point, all stars die. The nuclear reactions on which they depend for their energy come to an end. In some cases, a star's death is dramatic. It may actually blow itself apart, like an atomic bomb. The elements of which the star was made are then spread throughout the universe. They remain in space until they are drawn into the core of other stars or other astronomical bodies, such as our own Earth. If this theory is correct, then the atoms of iron, silver, and oxygen you see around you every day actually started out life in the middle of a star billions of miles away.

The modern definition of an element was first provided by English chemist Robert Boyle (1627–1691). Boyle defined elements as "certain primitive and simple, or perfectly unmingled bodies; which not being made of any other bodies, or of one another, are the ingredients of which all those call'd perfectly mixed bodies are immediately compounded, and into which they are ultimately resolved." For all practical purposes, Boyle's definition of an element has remained the standard working definition for a chemical element ever since.

By the year 1800, no more than about 25 true elements had been discovered. During the next hundred years, however, that situation changed rapidly. By the end of the century, 80 elements were known. The rapid pace of discovery during the 1800s can be attributed to the development of chemistry as a science, to the improved tools of analysis available to chemists, and to the new predictive power provided by the periodic law of 1870.

During the twentieth century, the last remaining handful of naturally occurring elements were discovered and the synthetic elements were first manufactured.

Chemical elements

By the end of the nineteenth century, the elements and matter comprising all things could no long be viewed as immutable. The dramatic rise of scientific methodology and experimentation during the later half of the eighteenth century set the stage for the fundamental advances in chemistry and physics made during the nineteenth century. In less than a century, European society moved from an understanding of the chemical elements grounded in mysticism to an understanding of the relationships between elements found in a modern periodic table . During the eighteenth century, there was a steady march of discovery with regard to the chemical elements. Isolations of hydrogen and oxygen allowed for the formation of water from its elemental components. Nineteenth century scientists built experiments on new-found familiarity with elements such as nitrogen, beryllium, chromium and titanium.

By the mid-nineteenth century, chemistry was in need of organization. New elements were being discovered at an increasing pace. Accordingly, the challenge for chemists and physicists was to find a key to understand the increasing volume of experimental evidence regarding the properties of the elements. In 1869, the independent development of the periodic law and tables by the Russian chemist Dmitry Mendeleev (1834–1907) and German chemist Julius Meyer (1830–95) brought long sought order and understanding to the elements.

Mendeleev and Meyer did not work in a vacuum. English chemist J.A.R. Newlands (1837–1898) had already published several works that ventured relationships among families of elements, including his "law of octaves" hypothesis. Mendeleev's periodic chart of elements, however, spurred important discoveries and isolation of chemical elements. Most importantly, Mendeleev's table provided for the successful prediction of the existence of new elements and these predictions proved true with the discovery of gallium (1875), scandium (1879) and germanium (1885).

By the end of the nineteenth century, the organization of the elements was so complete that British physicists Lord Rayleigh (born John William Strutt, 1842–1919) and William Ramsay (1852–1916) were able to expand the periodic table and to predict the existence and properties of the noble gases argon and neon.

Nineteenth century advances were, however, not limited to mere identification and isolation of the elements. By 1845, German chemist Adolph Kolbe (1818–84) synthesized an organic compound and, in 1861, another German chemist Friedrich Kekule (1829–1896) related the properties of molecules to their geometric shape. These advances led to the development of wholly new materials (e.g., plastics, celluloids) that had a dramatic impact on a society in midst of industrial revolution.

The most revolutionary development with regard to the elucidation of the elements during the nineteenth century came in the waning years of the century. In 1895, Wilhelm Röntgen (1845–1923) published a paper titled: "On a New Kind of Rays." Röntgen's work offered the first description of x rays and offered compelling photographs of photographs of a human hand. The scientific world quickly grasped the importance of Röntgen's discovery. At a meeting of the French Academy of Science, Henri Becquerel (1852–1908) observed the pictures taken by Röntgen of bones in the hand. Within months Becquerel presented two important reports concerning "uranium rays" back to the Academy. Becquerel, who was initially working with phosphorescence, described the phenomena that later came to be understood as radioactivity . Less than two years later, two other French scientists, Pierre (1859–1906) and Marie Curie (born in Poland, 1867–1934) announced the discovery of the radioactive elements polonium and radium. Marie Curie then set out on a systematic search for radioactive elements and was able, eventually, to document the discovery of radioactivity in uranium and thorium minerals .

As the nineteenth century drew to a close, Ernest Rutherford (1871–1937), using an electrometer, identified two types of radioactivity, which he labeled alpha radiation and beta radiation. Rutherford actually thought he had discovered a new type of x ray. Subsequently alpha and beta radiation were understood to be particles. Alpha radiation is composed of alpha particles (the nucleus of helium). Because alpha radiation is easily stopped, alpha radiation-emitting elements are usually not dangerous to biological organisms (e.g., humans) unless the emitting element actually enters the organism. Beta radiation is composed of a stream of electrons (electrons were discovered by J. J. Thomson in 1897) or positively charged particles called positrons.

The impact of the discovery of radioactive elements produced immediate and dramatic impacts upon society. Within a few years, high-energy electromagnetic radiation in the form of x rays, made possible by the discovery of radioactive elements, was used by physicians to diagnose injury. More importantly, the rapid incorporation of x rays into technology established a precedent increasingly followed throughout the twentieth century. Although the composition and nature of radioactive elements was not fully understood, the practical benefits to be derived by society outweighed scientific prudence.

Italian scientist Alessandro Volta's (1745–1827) discovery, in 1800, of a battery using discs of silver and zinc gave rise to the voltaic pile or the first true batteries. Building on Volta's concepts, English chemist Humphry Davy (1778–1829) first produced sodium from the electrolysis of molten sodium hydroxide in 1807. Subsequently, Davy isolated potassium, another alkali metal, from potassium hydroxide in the same year. Lithium was discovered in 1817.

Studies of the spectra of elements and compounds spawned further discoveries. German chemist Robert Bunsen's (1811–1999) invention of the famous laboratory burner that bears his name allowed for the development of new methods for the analysis of the elemental structure of compounds. Working with Russian-born scientist Gustav Kirchhoff (1824–1887) Bunsen's advances made possible flame analysis (a technique now commonly known as atomic emission spectroscopy [AES]) and established the fundamental principles and techniques of spectroscopy. Bunsen examined the spectra (i.e., component colors), emitted when a substance was subjected to intense flame. Bunsen's keen observation that flamed elements that emit light only at specific wavelengths—and that each element produces a characteristic spectra—along with Kirchhoff's work on black body radiation set the stage for subsequent development of quantum theory . Using his own spectroscopic techniques, Bunsen discovered the elements cesium and rubidium.

Using the spectroscopic techniques pioneered by Bunsen, other nineteenth century scientists began to deduce the chemical composition of stars. These discoveries were of profound philosophical importance to society because they proved that Earth did not lie in a privileged or unique portion of the universe. Indeed, the elements found on Earth, particularly those associated with life, were found to be commonplace in the cosmos. In 1868, French astronomer P.J.C. Janssen (1824–1907) and English astronomer, Norman Lockyer (1836–1920), used spectroscopic analysis to identify helium on the Sun . For the first time an element was first discovered outside the confines of Earth.

See also Atomic mass and weight; Atomic number; Big Bang theory; Stellar life cycle

chemical elements

Name	Symbol	Atomic number	Relative atomic mass1	Valency point °C	Melting point °C	Boiling Point °C	Date of discovery
1 Relative atomic mass: values given in parentheses are for radioactive elements whose relative atomic mass cannot be given precisely without knowledge of origin, and is the atomic mass number of the isotope of longest known half-life
2 Also called hahnium, nielsbohrium, rutherfordium, or element 105
3 Also called unnilquadium (Unq) or element 104
4 Also called wolfram
Actinium	Ac	89	(227)	――	1230	3200	1899
Aluminium	Al	13	26.98154	3	660.2	2350	1827
Americium	Am	95	(243)	3, 4, 5, 6	995	2600	1944
Antimony	Sb	51	121.75	3.5	630.5	1750	c.1000 bc
Argon	Ar	18	39.948	0	―189.4	―185.9	1894
Arsenic	As	33	74.9216	3.5	613	――	1250
Astatine	At	85	(210)	1, 3, 5, 7	302	377	1940
Barium	Ba	56	137.34	2	725	1640	1808
Berkelium	Bk	97	(247)	3, 4	986	――	1949
Beryllium	Be	4	9.01218	2	1285	2470	1798
Bismuth	Bi	83	208.9804	3, 5	271.3	1560	1753
Boron	B	5	10.81	3	2079	3700	1808
Bromine	Br	35	79.904	1, 3, 5, 7	―7.2	58.8	1826
Cadmium	Cd	48	112.40	2	320.9	765	1817
Caesium	Cs	55	132.9054	1	284	678	1860
Calcium	Ca	20	40.08	2	839	1484	1808
Californium	Cf	98	(251)	――	――	――	1950
Carbon	C	6	12.011	2.4	3550	4200	――
Cerium	Ce	58	140.12	3, 4	798	3257	1803
Chlorine	Cl	17	35.453	1, 3, 5, 7	―101	―34.6	1774
Chromium	Cr	24	51.996	2, 3, 6	1890	2672	1797
Cobalt	Co	27	58.9332	2, 3	1495	2870	1735
Copper	Cu	29	63.546	1, 2	1083	2567	c.8000 bc
Curium	Cm	96	(247)	3	1340	――	1944
Dubnium3	Db	104	(261)	――	――	――	1969
Dysprosium	Dy	66	162.50	3	1409	2335	1896
Einsteinium	Es	99	(254)	――	――	――	1952
Erbium	Er	68	167.26	3	1522	2863	1843
Europium	Eu	63	151.96	2, 3	822	1597	1896
Fermium	Fm	100	(257)	――	――	――	1952
Fluorine	F	9	18.9984	1	―219.6	―188.1	1886
Francium	Fr	87	(223)	1	30	650	1939
Gadolinium	Gd	64	157.25	3	1311	3233	1880
Gallium	Ga	31	69.72	2, 3	29.78	2403	1875
Germanium	Ge	32	72.59	4	937.4	2830	1886
Gold	Au	79	196.9665	1, 3	1063	2800	――
Hafnium	Hf	72	178.49	4	2227	4602	1923
Helium	He	2	4.0026	0	―272	268.9	1895
Holmium	Ho	67	164.9304	3	1470	2300	1878
Hydrogen	H	1	1.0079	1	―259.1	―252.9	1766
Indium	In	49	114.82	3	156.6	2080	1863
Iodine	I	53	126.9045	1, 3, 5, 7	113.5	184.4	1811
Iridium	Ir	77	192.22	3, 4	2410	4130	1804
Iron	Fe	26	55.847	2, 3	1540	2760	c.4000 bc
Krypton	Kr	36	83.80	0	―156.6	―152.3	1898
Lanthanum	La	57	138.9055	3	920	3454	1839
Lawrencium	Lr	103	(256)	――	――	――	1961
Lead	Pb	82	207.2	2, 4	327.5	1740	――
Lithium	Li	3	6.941	1	180.5	1347	1817
Lutetium	Lu	71	174.97	3	1656	3315	1907
Magnesium	Mg	12	24.305	2	648.8	1090	1808
Manganese	Mn	25	54.9380	2, 3, 4, 6, 7	1244	1962	1774
Mendelevium	Md	101	(258)	――	――	――	1955
Mercury	Hg	80	200.59	1, 2	―38.9	356.6	c.1500 bc
Molybdenum	Mo	42	95.94	3, 4, 6	2610	5560	1778
Neodymium	Nd	60	144.24	3	1010	3068	1885
Neon	Ne	10	20.179	0	―248.7	―246.1	1898
Neptunium	Np	93	237.0482	4, 5, 6	640	3902	1940
Nickel	N i	28	58.70	2, 3	1453	2732	1751
Niobium	Nb	41	92.9064	3, 5	2468	4742	1801
Nitrogen	N	7	14.0067	3, 5	―210	―195.8	1772
Nobelium	No	102	(255)	――	――	――	1958
Osmium	Os	76	190.2	2, 3, 4, 8	3045	5027	1903
Oxygen	O	8	15.9994	2	―218.4	―183	1774
Palladium	Pd	46	106.4	2, 4, 6	1552	3140	1803
Phosphorus	P	15	30.97376	3, 5	44.1	280	1669
Platinum	Pt	78	195.09	2, 4	1772	3800	1735
Plutonium	Pu	94	(244)	3, 4, 5, 6	641	3232	1940
Polonium	Po	84	(209)	――	254	962	1898
Potassium	K	19	39.098	1	63.2	777	1807
Praseodymium	Pr	59	140.9077	3	931	3512	1885
Promethium	Pm	61	(145)	3	1080	2460	1941
Protactinium	Pa	91	231.0359	――	1200	4000	1913
Radium	Ra	88	226.0254	2	700	1140	1898
Radon	Rn	86	(222)	0	―71	―61.8	1899
Rhenium	Re	75	186.207	――	3180	5627	1925
Rhodium	Rh	45	102.9055	3	1966	3727	1803
Rubidium	Rb	37	85.4678	1	38.8	688	1861
Ruthenium	Ru	44	101.07	3, 4, 6, 8	2310	3900	1827
Samarium	Sm	62	150.35	2, 3	1072	1791	1879
Scandium	Sc	21	44.9559	3	1539	2832	1879
Selenium	Se	34	78.96	2, 4, 6	217	684.9	1817
Silicon	Si	14	28.086	4	1410	2355	1823
Silver	Ag	47	107.868	1	961.9	2212	c.4000 bc
Sodium	Na	11	22.98977	1	97.8	882	1807
Strontium	Sr	38	87.62	2	769	1384	1808
Sulphur	S	16	32.06	2, 4, 6	112.8	444.7	――
Tantalum	Ta	73	180.9479	5	2996	5425	1802
Technetium	Tc	43	(97)	6, 7	2172	4877	1937
Tellurium	Te	52	127.60	2, 4, 6	449.5	989.8	1782
Terbium	Tb	65	158.9254	3	1360	3041	1843
Thallium	Tl	81	204.37	1, 3	303.5	1457	1861
Thorium	Th	90	232.0381	4	1750	4790	1828
Thulium	Tm	69	168.9342	3	1545	1947	1879
Tin	Sn	50	118.69	2, 4	232	2270	c.3500 bc
Titanium	Ti	22	47.90	3, 4	1660	3287	1791
Tungsten§	W	74	183.85	6	3410	5660	1783
Unnilpentium2	Unp	105	(262)	――	――	――	1970
Uranium	U	92	238.029	4, 6	1132	3818	1789
Vanadium	V	23	50.9414	3, 5	1890	3380	1801
Xenon	Xe	54	131.30	0	―111.9	―107.1	1898
Ytterbium	Yb	70	173.04	2, 3	824	1193	1907
Yttrium	Y	39	88.9059	3	1510	3300	1828
Zinc	Zn	30	65.38	2	419.6	907	1800
Zirconium	Zr	40	91.22	4	1852	4377	1789

Element, Chemical

A survey of the elements

History of the elements

Organization of the elements

Orphan elements

Resources

A chemical element is a fundamental substance of the material world, one that cannot be divided into a more elementary substance by chemical processes. Each element has an identity; for example, gold consists of only gold atoms, and a gold atom is unlike any other atom. Indeed, a gold atom can be split, but the subatomic particles (electrons, protons, and neutrons) that constitute a gold atom are not gold. It could be said that subatomic particles are generic, interchangeable. Atoms, on the other hand, have an identity, and constitute the identity of an element.

A chemical element is a substance made up of only one kind of atom (atoms having the same atomic number). A compound, on the other hand, is made up of two or more kinds of atom combined together in specific proportions.

The atomic number of an element is the number of protons found in the nucleus of each atom of that element; the number of protons in the nucleus equals the number of electrons that can bind to the atom. (Since electrons and protons have equal but opposite electrical charges, atoms can bind as many electrons to themselves as they have protons in their nuclei.) Because the chemical properties of an atom—the ways in which it binds to other atoms—are determined by the number of electrons that can bind to its nucleus, every element has a unique set of chemical properties.

Some elements, such as the rare gases, exist as collections of single atoms; such a substance is monatomic.

Two dozen of the most common and/or important chemical elements
		Percent of all atoms (a)
Element	Symbol	In the universe	In the Earth’s crust	In sea water	In the human body	Characteristics under ordinary room conditions
(a) If no number is entered, the element constitutes less than 0.1 percent.
Aluminum	Al	—	6.3	—	—	A lightweight, silvery metal
Calcium	Ca	—	2.1	—	0.2	Common in minerals, seashells, and bones
Carbon	C	—	—	—	10.7	Basic in all living things
Chlorine	Cl	—	—	0.3	—	A toxic gas
Copper	Cu	—	—	—	—	The only red metal
Gold	Au	—	—	—	—	The only yellow metal
Helium	He	7.1	—	—	—	A very light gas
Hydrogen	H	92.8	2.9	66.2	60.6	The lightest of all elements; a gas
Iodine	I	—	—	—	—	A nonmetal; used as antiseptic
Iron	Fe	—	2.1	—	—	A magnetic metal; used in steel
Lead	Pb	—	—	—	—	A soft, heavy metal
Magnesium	Mg	—	2.0	—	—	A very light metal
Mercury	Hg	—	—	—	—	A liquid metal; one of the two liquid elements
Nickel	Ni	—	—	—	—	A noncorroding metal; used in coins
Nitrogen	N	—	—	—	2.4	A gas; the major component of air
Oxygen	O	—	60.1	33.1	25.7	A gas; the second major component of air
hosphorus	P	—	—	—	0.1	A nonmetal; essential to plants
otassium	K	—	1.1	—	—	A metal; essential to plants; commonly called “potash”
Silicon	Si	—	20.8	—	—	A semiconductor; used in electronics
Silver	Ag	—	—	—	—	A very shiny, valuable metal
Sodium	Na	—	2.2	0.3	—	A soft metal; reacts readily with water, air
Sulfur	S	—	—	—	0.1	A yellow nonmetal; flammable
Titanium	Ti	—	0.3	—	—	A light, strong, noncorroding metal used in space vehicles
Uranium	U	—	—	—	—	A very heavy metal; fuel for nuclear power

Others may exist as molecules that consist of two or more atoms of that element bonded together. For example, oxygen (O) can remain stable as either a diatomic (two-atom) molecule (O₂ ) or a triatomic (three-atom) molecule (O₃ ). (O₂ is the form of oxygen that humans breathe; O₃ [ozone] is toxic to animals and plants, yet ozone in the upper atmosphere screens Earth from harmful solar radiation.) Phosphorus (P) is stable as a four-atom molecule (P₄ ), while sulfur (S) is stable as an eight-atom molecule (S₈ ).

Even though all atoms of a given element have the same number of protons in their nuclei, they may not have the same number of neutrons in their nuclei. Atoms of the same element having different numbers of neutrons in their nuclei are termed isotopes of that element. An isotope is named according to the sum of the number of protons and the number of neutrons in its nucleus. For example, 99% of all carbon (C), atomic number 6, has 6 neutrons in the nucleus of each atom; this isotope of carbon is called carbon 12 (¹²C). An isotope is termed stable if its nuclei are permanent, and unstable (or radioactive) if its nuclei occasionally explode. Some elements have only one stable (non-radioactive) isotope, while others have two or more. Two stable isotopes of carbon are¹²C (6 protons, 6 neutrons) and¹³C (6 protons, 7 neutrons); a radioactive isotope of carbon is¹⁴ (6 protons, 8 neutrons). Tin (Sn) has ten stable isotopes. Some elements have no stable isotopes; all their isotopes are radioactive. All isotopes of a given element have the same outer electron structure and therefore the same chemical properties.

Ninety-two different chemical elements occur naturally on Earth; 81 of these have at least one stable isotope. Other elements have been made synthetically

Table 2. A Who’s Who of the Elements. (Thomson Gale.)
A who’s who of the elements
Element	Distinction	Comment
Astatine (At)	The rarest	Rarest of the naturally occurring elements
Boron (B)	The strongest	Highest stretch resistance
Californium (Cf)	The most expensive	Sold at one time for about $1 billion a gram
Carbon (C)	The hardest	As diamond, one of its three solid forms
Germanium (Ge)	The purest	Has been purified to 99.99999999 percent purity
Helium (He)	The lowest melting point	–457.09°F (–271.72°C) at a pressure of 26 times atmospheric pressure
Hydrogen (H)	The lowest density	Density 0.0000899 g/cc at atmospheric pressure and 32°F (0°C)
Lithium (Li)	The lowest-density metal	Density 0.534g/cc
Osmium (Os)	The highest density	Density 22.57 g/cc
Radon (Rn)	The highest-density gas	Density 0.00973 g/cc at atmospheric pressure and 32°F (0°C)
Tungsten (W)	The highest melting point	6, 188°F (3, 420°C)

(artificially), usually by causing the nuclei of two atoms to collide and merge. Since 1937, when technetium (Tc, atomic number 43), the first synthetic element, was made, the number of known elements has grown as nuclear chemists made new elements. Most of these synthetic elements have atomic numbers higher than 92 (i.e., more than 92 protons in their nuclei); since 92 is the atomic number of uranium (U), these artificial heavy elements are called transuranium (past-uranium) elements. The heaviest element discovered and verified so far is Element 111, Roentgenium (Rg), which was discovered in 1994. Since then, Elements 112 (Ununbium [Uub], discovered in 1996), 113 (Ununtrium [Uut], 2003), 114 (Ununquadium [Uuq], 1998), 115 (Ununpentium [Uup], 2003), and 116 (Ununhexium [Uuh], 2000) have been discovered but not verified by independent scientific studies.

A survey of the elements

Of the 116 currently known elements, 11 are gases, two are liquids, and 103 are solids. (The transuranium elements are presumed to be solids, but since only a few atoms at a time can be synthesized it is impossible to be sure.) Many elements, such as iron (Fe), copper (Cu), and aluminum (Al), are familiar everyday substances, but many are unfamiliar, either because they are not abundant on Earth or because they are not used much by human beings. Less-common naturally occurring elements include dysprosium (Dy), thulium (Tm) and protactinium (Pa).

Every element (except a few of the transuranium elements) has been assigned a name and a one- or two-letter symbol for convenience in writing formulas and chemical equations; these symbols are shown above in parentheses. For example, to distinguish the four elements that begin with the letter c, calcium is symbolized as Ca, cadmium as Cd, californium as Cf, and carbon as C.

Many of the symbols for chemical elements do not seem to make sense in terms of their English names— Fe for iron, for example. Those are mostly elements that have been known for thousands of years and that already had Latin names before chemists began handing out the symbols. Iron is Fe for its Latin name, ferrum. Gold is Au for aurum, sodium is Na for natrium, copper is Cu for cuprum, and mercury is Hg for hydrargyrum, meaning liquid silver, which is exactly what it looks like, but is not.

Notice that only two elements taken together— hydrogen and helium—make up 99.9% of the atoms in the entire universe. That is because virtually all the mass in the universe is in the form of stars, and stars are made mostly of H and He. Only H and He were produced in the big bang that (theoretically) began the universe; all other elements have been built up by nuclear reactions since that time, either naturally (in the cores of stars) or artificially (in laboratories). On Earth, only three elements—oxygen, silicon and aluminum—make up more than 87% of the Earth’s crust (the rigid, rocky outer layer of the planet, about 10.5 mi [17 km] under most dry land [less under the oceans]). Only six more elements—hydrogen, sodium, calcium, iron, magnesium, and potassium—account for more than 99% of Earth’s crust.

The abundance of an element can be quite different from its importance to humans. Nutritionists believe that some 24 elements are essential to life, even though many are fairly rare and are needed in only tiny amounts.

History of the elements

Many substances now known as elements have been known since ancient times. Gold (Au) was found and made into ornaments during the late Stone Age, some 10, 000 years ago. More than 5, 000 years ago, in Egypt, the metals iron (Fe), copper (Cu), silver (Ag), tin (Sn), and lead (Pb) were also used for various purposes. Arsenic (As) was discovered around 1250 AD, and phosphorus (P) was discovered around 1674. By 1700, about 12 elements were known, but they were not yet recognized as they are today.

The concept of elements—i.e., the theory that there are a limited number of fundamental pure substances out of which all other substances are made—goes back to the ancient Greeks. Empedocles (c. 495–435 BC) proposed that there are four basic roots of all materials: earth, air, fire and water. Plato (c. 427–347 BC) referred to these four roots as stoicheia elements. Aristotle (384–322 BC), a student of Plato’s, proposed that an element is “one of those simple bodies into which other bodies can be decomposed and which itself is not capable of being divided into others.” Except for nuclear fission and other nuclear reactions discovered more than 2, 000 years later, by which the atoms of an element can be decomposed into smaller parts, this definition remains accurate.

Several other theories were generated throughout the years, most of which have been dispelled. For example, Swiss physician and alchemist Theophrastus Bombastus von Hohenheim (c. 1493–1541), also known as Paracelsus, proposed that everything was made of three principles: salt, mercury, and sulfur. An alchemist named van Helmont (c. 1577–c.1644) tried to explain everything in terms of just two elements: air and water.

Eventually, English chemist Robert Boyle (1627– 1691) revived Aristotle’s definition and refined it. In 1789, French chemist Antoine Lavoisier (1743–1794) was able to publish a list of chemical elements that met Boyle’s definition. Even though some of Lavoisier’s elements later turned out to be compounds (combinations of actual elements), his list set the stage for the adoption of standard names and symbols for the various elements.

Swedish chemist J. J. Berzelius (1779–1848) was the first person to employ the modern method of classification: a one- or two-letter symbol for each element. These symbols could be put easily together to show how the elements combine into compounds.

For example, writing two Hs and one O together as H₂O would mean that the particles (molecules) of water consist of two hydrogen atoms and one oxygen atom, bonded together. Berzelius published a table of 24 elements, including their atomic weights, most of which are close to the values used today.

By the year 1800 only about 25 true elements were known, but progress was relatively rapid throughout the nineteenth century. By the time Russian scientist Dmitri Ivanovich Mendeleev (1834–1907) organized his periodic table in 1869, he had about 60 elements to reckon with. By 1900, there were more than 80. The list quickly expanded to 92, ending at uranium (atomic number 92). There it stayed until 1940, when synthesis of the transuranium elements began.

Organization of the elements

The task of organizing more than a hundred very different elements into some simple, sensible arrangement would seem difficult. Mendeleev’s periodic table, however, is the answer. It even accommodates the synthetic transuranium elements without strain. In this encyclopedia, each individual chemical element is discussed under at least one of the following types of entry: (1) Fourteen particularly important elements are discussed in their own entries. They are aluminum, calcium, carbon, chlorine, copper, hydrogen, iron, lead, nitrogen, oxygen, silicon, sodium, sulfur, and uranium. (2) Elements that belong to any of seven families of elements—groups of elements that have similar chemical properties—are discussed under their family-name headings. These seven families are the actinides, alkali metals, alkaline earth metals, halogens, lanthanides, rare gases, and transuranium elements. (3) Elements that are not discussed either under their own name or as part of a family (orphan elements) are discussed briefly below. Any element that is not discussed below can be found in the headings described above.

Orphan elements

Actinium. The metallic chemical element of atomic number 89, with symbol Ac, specific gravity 10.07, melting point 1, 924°F (1, 051°C), and boiling point 5, 788°F (3, 198°C). All isotopes of this element are radioactive; the half-life of its most stable isotope, actinium-227, is 21.8 years. Its name is from the Greek aktinos, meaning ray.

Antimony. The metallic chemical element of atomic number 51, with symbol Sb, atomic weight 121.8, specific gravity 6.69, melting point 1, 167°F(630.63°C), and boiling point 2, 889°F (1, 587°C). One of its main uses is to alloy with lead in automobile batteries; actinium makes the lead harder.

Arsenic. The metallic chemical element of atomic number 33, with symbol As, atomic weight 74.92, specific gravity 5.73 in gray metallic form, and melting point 1, 503°F (817°C). It sublimes (solid turns to gas) at 1, 137°F (614°C). Arsenic compounds are poisonous.

Bismuth. The metallic chemical element of atomic number 83, with symbol Bi, atomic weight 208.98, specific gravity 9.75, melting point 520.5°F (271.4°C), and boiling point 2, 847.2°F (1, 564°C). Bismuth oxychloride is used in pearlized cosmetics. Bismuth subsalicylate, an insoluble compound, is the major ingredient in Pepto-Bismol^®. The soluble compounds of bismuth, however, are poisonous.

Boron. The non-metallic chemical element of atomic number 5, with symbol B, atomic weight 10.81, specific gravity (amorphous form) 2.37, melting point 3, 767°F(2, 075°C), and boiling point 7, 232°F(4, 000°C). Common compounds are borax, Na₂B₄O₇ •10H₂O, used as a cleansing agent and water softener, and boric acid, H₃BO₃, a mild antiseptic and an effective cockroach poison.

Cadmium. The metallic chemical element of atomic number 48, with symbol Cd, atomic weight 112.4, specific gravity 8.65, melting point 609.92°F (321.07°C), and boiling point 1, 413°F (767°C). A soft, highly toxic metal used in silver solder, in many other alloys, and in nickel-cadmium rechargeable batteries. Because it is an effect absorber of moving neutrons, it is used in control rods for nuclear reactors to slow the chain reaction.

Chromium. The metallic chemical element of atomic number 24, with symbol Cr, atomic weight 51.99, specific gravity 7.19, melting point 3, 465°F (1, 907°C), boiling point 4, 840°F (2, 671°C). It is a hard, shiny metal that takes a high polish. It is used to electroplate steel for protection against corrosion and as the major ingredient (next to iron) in stainless steel. Alloyed with nickel, it makes Nichrome^®, a high-electrical-resistance metal that gets red hot when electric current passes through it; toaster and heater coils are made of Nichrome^® wire. Chromium is named from the Greek chroma, meaning color, because most of its compounds are highly colored. Chromium is responsible for the green color of emeralds.

Cobalt. The metallic chemical element of atomic number 27, with symbol Co and atomic weight 58.93. Cobalt is a grayish, hard, brittle metal closely resembling iron and nickel. These three metals are the only naturally occurring magnetic elements on Earth.

Gallium. The metallic chemical element of atomic number 31, with symbol Ga, atomic weight 69.72, melting point 85.6°F (29.78°C), and boiling point 3, 999°F (2, 204°C). Gallium is frequently used in the electronics industry and in thermometers that measure a wide range of temperatures.

Germanium. The metallic chemical element of atomic number 32, with symbol Ge and atomic weight 72.59. In pure form, germanium is a brittle crystal. It was used to make the world’s first transistor and is still used as a semiconductor in electronics devices.

Gold. The metallic chemical element of atomic number 79, with symbol Au and atomic weight 196.966. This most malleable of metals was probably one of the first elements known to humans. It is usually alloyed with harder metals for use in jewelry, coins, or decorative pieces.

Hafnium. The metallic chemical element of atomic number 72, with symbol Hf, atomic weight 178.49, melting point 4, 040.6 ±68°F (2, 227 ±20°C), and boiling point 8, 315.6°F (4, 602°C). Hafnium is strong and resistant to corrosion. It also absorbs neutrons well, making it useful in control rods of nuclear reactors.

Indium. The metallic chemical element of atomic number 49, with symbol In, atomic weight 114.82, melting point 313.89°F (156.61°C), and boiling point 3, 776°F (2, 080°C). Indium is a lustrous, silvery metal that bends easily. It is often alloyed with other metals in solid-state-electronics devices.

Iridium. The metallic chemical element of atomic number 77, with symbol Ir and atomic weight 192.22. Iridium is an extremely dense metal that resists corrosion better than most others. In its pure state, it is often used in aircraft spark plugs.

Manganese. The metallic chemical element of atomic number 25, with symbol Mn and atomic weight 54.93. The biggest use of manganese is in steelmaking, where it is alloyed with iron. This element is required by all plants and animals, so it is sometimes added as manganese oxide to animal feed.

Mercury. The metallic chemical element of atomic number 80, with symbol Hg, atomic weight 200.59, melting point -37.96°F (-38.87°C), and boiling point 673.84°F (356.58°C). Mercury is highly poisonous and causes irreversible damage to the nervous and excretory systems. This element was long used in thermometers because it expands and contracts at a nearly constant rate; however, mercury thermometers are being phased out in favor of alcohol-based and electronic thermometers because of mercury’s high toxicity.

Molybdenum. The metallic chemical element of atomic number 42, with symbol Mo, atomic weight 95.94, melting point 4, 753°F (2, 623°C), and boiling point 8, 382°F (4, 639°C). Molybdenum is used to make superalloyed metals designed for high-temperature processes. It is also found as a trace element in plant and animal tissues.

Nickel. The metallic chemical element of atomic number 28, with symbol Ni and atomic weight 58.71. Nickel is often mixed with other metals, such as copper and iron, to increase the alloy’s resistance to heat and moisture.

Niobium. The metallic chemical element of atomic number 41, with symbol Nb, atomic weight 92.90, melting point 4, 474.4 ±50°F (2, 468 ±10°C), and boiling point 8, 571.2°F (4, 744°C). Niobium is used to strengthen alloys used to make lightweight aircraft frames.

Osmium. The metallic chemical element of atomic number 76, with symbol Os and atomic weight 190.2. Osmium is hard and dense, weighing twice as much as lead. The metal is used to make fountain pen tips and electrical devices.

Palladium. The metallic chemical element of atomic number 46, with symbol Pd and atomic weight 106.42. Palladium is soft. It also readily absorbs hydrogen and is, therefore, used to purify hydrogen gas.

Phosphorus. The nonmetallic chemical element of atomic number 15, with symbol P and atomic weight 30.97. Phosphorus is required by all plant and animal cells. Most of the phosphorus in human beings is in the bones and teeth. Phosphorus is heavily used in agricultural fertilizers.

Platinum. The metallic chemical element of atomic number 78, with symbol Pt, atomic weight 195.08, melting point 3, 215.1°F (1, 768.4°C), and boiling point 6, 920.6 ±212°F (3, 827 ±100°C). Platinum withstands high temperatures well and is used in rocket and jet-engine parts. It is also used as a catalyst in chemical reactions.

Polonium. The metallic chemical element of atomic number 84, with symbol Po and atomic weight 209. Polonium is a product of uranium decay and is 100 times as radioactive as uranium.

Rhenium. The metallic chemical element of atomic number 75, with symbol Re, atomic weight 186.207, specific gravity 21.0, melting point 5, 766.8°F(3, 186°C), and boiling point 10, 104.8°F (5, 596°C). Rhenium is used in chemical and medical instruments, as a catalyst for the chemical and petroleum industries, and in photoflash lamps.

Rhodium. The metallic chemical element of atomic number 45, with symbol Rh and atomic weight 102.91. This element is similar to palladium. Electroplated rhodium, which is hard and highly reflective, is used as a reflective material for optical instruments.

Ruthenium. The metallic chemical element of atomic number 44, with symbol Ru, atomic weight 101.07, specific gravity 12.5, melting point 4, 233.2°F (2, 334°C), and boiling point 7, 502°F (4, 150°C). This element is alloyed with platinum and palladium to form hard, resistant contacts for electrical equipment that must withstand a great deal of wear.

Scandium. The metallic chemical element of atomic number 21, with symbol Sc, atomic weight 44.96, melting point 2, 805.8°F (1, 541°C), and boiling point 5, 127.8°F (2, 831°C). Scandium is a silvery-white metal that develops a yellowish or pinkish cast when exposed to air. It has relatively few commercial applications.

Selenium. The nonmetallic chemical element of atomic number 34, with symbol Se and atomic weight 78.96. Selenium is able to convert light directly into electricity, and its resistance to electrical current decreases when it is exposed to light. Both properties make this element useful in photocells, exposure meters, and solar cells.

Silver. The metallic chemical element of atomic number 47, with symbol Ag and atomic weight 107.87. Silver has long been used in the manufacture of coins. It is also an excellent conductor of heat and electricity. Some compounds of silver are light-sensitive, making silver important in the manufacture of photographic films and papers.

Tantalum. The metallic chemical element of atomic number 73, with symbol Ta, atomic weight 180.95, melting point 5, 462.6°F (3, 017°C), and boiling point of 9, 797 ±212°F (5, 425 ±100°C). Tantalum is a heavy, gray, hard metal that is used in alloys to pen points and analytical weights.

Technetium. The metallic chemical element of atomic number 43, with symbol Tc and atomic weight 98. Technetium was the first element to be produced synthetically; scientists have never detected the natural presence of this element on Earth.

Tellurium. The nonmetallic chemical element of atomic number 52, with symbol Te, atomic weight 127.60, melting point 841.1 ± 32.54°F(449.5 ±0.3°C), and boiling point 1, 813.64 ±38.84°F (989.8 ±3.8°C).

Tellurium is a grayish-white, lustrous, brittle metal. It is a semiconductor and is used in the electronics industry.

Thallium. The metallic chemical element of atomic number 81, with symbol Tl and atomic weight 204.38. Thallium is a bluish-gray metal that is soft enough to be cut with a knife. Thallium sulfate is used as a rodenticide and ant poison.

Tin. The metallic chemical element of atomic number 50, with symbol Sn and atomic weight 118.69. Tin is alloyed with copper and antimony to make pewter. It is also used as a soft solder and as coating to prevent other metals from corrosion.

Titanium. The metallic chemical element of atomic number 22, with symbol Ti, atomic weight 47.90, melting point 3, 020 ±50°F (1, 660 ±10°C), and boiling point 5, 948.6°F (3, 287°C). This element occurs as a bright, lustrous brittle metal or dark gray powder. Titanium alloys are strong for their weight and can withstand large changes in temperature.

Tungsten. The metallic chemical element of atomic number 74, with symbol W, atomic weight 183.85, and melting point 6,170 ±68°F (3,410 ±20°C). The melting point of tungsten is higher than that of any other metal. Its chief use is as a filament in electric light bulbs.

Vanadium. The metallic chemical element of atomic number 23, with symbol V and atomic weight 50.94. Pure vanadium is bright white. This metal finds its biggest use in strengthening steel.

Yttrium. The metallic chemical element of atomic number 39, with symbol Y, atomic weight 88.91, melting point 2, 771.6 ±46.4°F (1, 522 ±8°C), and boiling point 6, 040.4°F (3, 338°C). Yttrium is a relatively active metal that decomposes in cold water slowly and in boiling water rapidly. Certain compounds containing yttrium have been shown to become super-conducting at relatively high temperatures.

Zinc. The metallic chemical element of atomic number 30, with symbol Zn and atomic weight 65.39. Zinc—a brittle metal at room temperature—forms highly versatile alloys in industry. One zinc alloy is nearly as strong as steel, but has the malleability of plastic.

Zirconium. The metallic chemical element of atomic number 40, with symbol Zr, atomic weight 91.22, melting point 3, 365.6 ± 35.6°F (1, 852 ±2°C), and boiling point 7, 910.6°F (4, 377°C). Neutrons can pass through this metal without being absorbed; this makes it highly desirable as a construction material for the metal rods containing the fuel pellets in nuclear power plants.

See also Ammonia; Compound, chemical; Deuterium; Element, transuranium; Tritium; Valence.

Resources

BOOKS

Lide, David R. CRC Handbook of Chemistry and Physics. 86th ed. Boca Raton, FL: CRC Press, 2005.

Ede, Andrew. The Chemical Element: A Historical Perspective. Westport, CT: Greenwood Press, 2006.

Emsley, John. Nature’s Building Blocks: An A-Z Guide to the Elements. Oxford, UK: Oxford University Press, 2001.

Merck. The Merck Index. Whitehouse Station, NJ: Merck; London: Harcourt, 2001.

Scerri, Eric, R. The Periodic Table: Its Story and its Significance. New York: Oxford University Press, 2006.

Siekierski, Slawomir. Concise Chemistry of the Elements. Chichester, UK: Horwood Publishing, 2002.

Robert L. Wolke

Element, chemical

A chemical element is a substance made up of only one kind of atom (atoms having the same atomic number ). A compound, on the other hand, is made up of two or more kinds of atom combined together in specific proportions.

The atomic number of an element is the number of protons found in the nucleus of each atom of that element; the number of protons in the nucleus equals the number of electrons that can bind to the atom. (Since electrons and protons have equal but opposite electrical charges, atoms can bind as many electrons to themselves as they have protons in their nuclei.) Because the chemical properties of an atom—the ways in which it binds to other atoms—are determined by the number of electrons that can bind to its nucleus, every element has a unique set of chemical properties.

Some elements, such as the rare gases , exist as collections of single atoms; such a substance is monatomic. Others may exist as molecules that consist of two or more atoms of that element bonded together. For example, oxygen (O) can remain stable as either a diatomic (two-atom) molecule (O2) or a triatomic (three-atom) molecule (O3). (O2 is the form of oxygen that we breathe; O3 [ozone] is toxic to animals and plants, yet ozone in the upper atmosphere screens Earth from harmful solar radiation.) Phosphorus (P) is stable as a four-atom molecule (P4), while sulfur (S) is stable as an eight-atom molecule (S8).

Even though all atoms of a given element have the same number of protons in their nuclei, they may not have the same number of neutrons in their nuclei. Atoms of the same element having different numbers of neutrons in their nuclei are termed isotopes of that element. An isotope is named according to the sum of the number of protons and the number of neutrons in its nucleus. For example, 99% of all carbon (C), atomic number 6, has 6 neutrons in the nucleus of each atom; this isotope

TABLE 1. TWO DOZEN OF THE MOST COMMON AND/OR IMPORTANT CHEMICAL ELEMENTS.

		Percent of all atoms a
Element	Symbol	In the universe	In the Earth's crust	In sea water	In the human body	Characteristics under ordinary room conditions
aIf no number is entered, the element constitutes less than 0.1 percent.
Aluminum	Al	—	6.3	—	—	A lightweight, silvery metal
Calcium	Ca	—	2.1	—	0.2	Common in minerals, seashells, and bones
Carbon	C	—	—	—	10.7	Basic in all living things
Chlorine	Cl	—	—	0.3	—	A toxic gas
Copper	Cu	—	—	—	—	The only red metal
Gold	Au	—	—	—	—	The only yellow metal
Helium	He	7.1	—	—	—	A very light gas
Hydrogen	H	92.8	2.9	66.2	60.6	The lightest of all elements; a gas
Iodine	I	—	—	—	—	A nonmetal; used as antiseptic
Iron	Fe	—	2.1	—	—	A magnetic metal; used in steel
Lead	Pb	—	—	—	—	A soft, heavy metal
Magnesium	Mg	—	2.0	—	—	A very light metal
Mercury	Hg	—	—	—	—	A liquid metal; one of the two liquid elements
Nickel	Ni	—	—	—	—	A noncorroding metal; used in coins
Nitrogen	N	—	—	—	2.4	A gas; the major component of air
Oxygen	O	—	60.1	33.1	25.7	A gas; the second major component of air
Phosphorus	P	—	—	—	0.1	A nonmetal; essential to plants
Potassium	K	—	1.1	—	—	A metal; essential to plants; commonly called "potash"
Silicon	Si	—	20.8	—	—	A semiconductor; used in electronics
Silver	Ag	—	—	—	—	A very shiny, valuable metal
Sodium	Na	—	2.2	0.3	—	A soft metal; reacts readily with water, air
Sulfur	S	—	—	—	0.1	A yellow nonmetal; flammable
Titanium	Ti	—	0.3	—	—	A light, strong, noncorroding metal used in space vehicles
Uranium	U	—	—	—	—	A very heavy metal; fuel for nuclear power

TABLE 2. A WHO'S WHO OF THE ELEMENTS.

Element	Distinction	Comment
Astatine (At)	The rarest	Rarest of the naturally occurring elements
Boron (B)	The strongest	Highest stretch resistance
Californium (Cf)	The most expensive	Sold at one time for about $1 billion a gram
Carbon (C)	The hardest	As diamond, one of its three solid forms
Germanium (Ge)	The purest	Has been purified to 99.99999999 percent purity
Helium (He)	The lowest melting point	-457.09°F (-271.72˚C) at a pressure of 26 times atmospheric pressure
Hydrogen (H)	The lowest density	Density 0.0000899 g/cc at atmospheric pressure and 32°F (0˚C)
Lithium (Li)	The lowest-density metal	Density 0.534g/cc
Osmium (Os)	The highest density	Density 22.57 g/cc
Radon (Rn)	The highest-density gas	Density 0.00973 g/cc at atmospheric pressure and 32°F (0˚C)
Tungsten (W)	The highest melting point	6,188°F (3,420˚C)

of carbon is called carbon 12 (12C). An isotope is termed stable if its nuclei are permanent, and unstable (or radioactive) if its nuclei occasionally explode. Some elements have only one stable (nonradioactive) isotope, while others have two or more. Two stable isotopes of carbon are 12C (6 protons, 6 neutrons) and 13C (6 protons, 7 neutrons); a radioactive isotope of carbon is 14C (6 protons, 8 neutrons). Tin (Sn) has ten stable isotopes. Some elements have no stable isotopes; all their isotopes are radioactive. All isotopes of a given element have the same outer electron structure and therefore the same chemical properties.

Ninety-two different chemical elements occur naturally on Earth; 81 of these have at least one stable isotope. Other elements have been made synthetically (artificially), usually by causing the nuclei of two atoms to collide and merge. Since 1937, when technetium (Tc, atomic number 43), the first synthetic element, was made, the number of known elements has grown as nuclear chemists made new elements. Most of these synthetic elements have atomic numbers higher than 92 (i.e., more than 92 protons in their nuclei); since 92 is the atomic number of uranium (U), these artificial heavy elements are called "transuranium" (past-uranium) elements. The heaviest element so far is Element 114, whose synthesis was announced in January 1999. In June, 1999, scientists at Lawrence Berkeley National Laboratory in California announced the synthesis of elements 116 and 118; however, it was later revealed that these announcements had been based on fabricated (made-up) data, and the claim to have synthesized these elements was publicly retracted. The same researcher who falsified the data had participated in the work leading up to the announcements of elements 110 and 112 in 1994 and 1996, but later analysis confirmed that enough authentic evidence existed to support the announcement that 110 and 112 had been synthesized.

A survey of the elements

Of the 114 currently known elements, 11 are gases, two are liquids, and 101 are solids. (The transuranium elements are presumed to be solids, but since only a few atoms at a time can be synthesized it is impossible to be sure.) Many elements, such as iron (Fe), copper (Cu), and aluminum (Al), are familiar everyday substances, but many are unfamiliar, either because they are not abundant on Earth or because they are not used much by human beings. Less-common naturally occurring elements include dysprosium (Dy), thulium (Tm) and protactinium (Pa).

Every element (except a few of the transuranium elements) has been assigned a name and a one- or two-letter symbol for convenience in writing formulas and chemical equations; these symbols are shown above in parentheses. For example, to distinguish the four elements that begin with the letter c, calcium is symbolized as Ca, cadmium as Cd, californium as Cf, and carbon as C.

Many of the symbols for chemical elements do not seem to make sense in terms of their English names—Fe for iron, for example. Those are mostly elements that have been known for thousands of years and that already had Latin names before chemists began handing out the symbols. Iron is Fe for its Latin name, ferrum. Gold is Au for aurum, sodium is Na for natrium, copper is Cu for cuprum, and mercury is Hg for hydrargyrum, meaning liquid silver, which is exactly what it looks like, but is not.

Table 1 lists some of the most common and important chemical elements. Note that many of these are referred to in the last column as metals. In fact, 93 out of the 114 elements are metals; the others are nonmetals.

Notice that only two elements taken together—hydrogen and helium—make up 99.9% of the atoms in the entire universe. That is because virtually all the mass in the universe is in the form of stars, and stars are made mostly of H and He. Only H and He were produced in the big bang that began the universe; all other elements have been built up by nuclear reactions since that time, either naturally (in the cores of stars) or artificially (in laboratories). On Earth, only three elements—oxygen, silicon and aluminum—make up more than 87% of the earths's crust (the rigid, rocky outer layer of the planet , about 10.5 mi [17 km] under most dry land [less under the oceans]). Only six more elements—hydrogen, sodium, calcium, iron, magnesium , and potassium—account for more than 99% of Earth's crust.

The abundance of an element can be quite different from its importance to humans. Nutritionists believe that some 24 elements are essential to life, even though many are fairly rare and are needed in only tiny amounts.

History of the elements

Many substances now known as elements have been known since ancient times. Gold (Au) was found and made into ornaments during the late stone age, some 10,000 years ago. More than 5,000 years ago, in Egypt, the metals iron (Fe), copper (Cu), silver (Ag), tin (Sn), and lead (Pb) were also used for various purposes. Arsenic (As) was discovered around a.d. 1250, and phosphorus (P) was discovered around 1674. By 1700, about 12 elements were known, but they were not yet recognized as they are today.

The concept of elements—i.e., the theory that there are a limited number of fundamental pure substances out of which all other substances are made—goes back to the ancient Greeks. Empedocles (c. 495–435 b.c.) proposed that there are four basic "roots" of all materials: earth, air, fire, and water . Plato (c. 427–347 b.c.) referred to these four "roots" as stoicheia elements. Aristotle (384–322 b.c.), a student of Plato's, proposed that an element is "one of those simple bodies into which other bodies can be decomposed and which itself is not capable of being divided into others." Except for nuclear fission and other nuclear reactions discovered more than 2,000 years later, by which the atoms of an element can be decomposed into smaller parts, this definition remains accurate.

Several other theories were generated throughout the years, most of which have been dispelled. For example, the Swiss physician and alchemist Theophrastus Bombastus von Hohenheim (c. 1493–1541), also known as Paracelsus, proposed that everything was made of three "principles:" salt , mercury, and sulfur. An alchemist named van Helmont (c. 1577–c.1644) tried to explain everything in terms of just two elements: air and water.

Eventually, English chemist Robert Boyle (1627–1691) revived Aristotle's definition and refined it. In 1789, French chemist Antoine Lavoisier (1743–94) was able to publish a list of chemical elements that met Boyle's definition. Even though some of Lavoisier's "ele ments" later turned out to be compounds (combinations of actual elements), his list set the stage for the adoption of standard names and symbols for the various elements.

The Swedish chemist J. J. Berzelius (1779–1848) was the first person to employ the modern method of classification: a one- or two-letter symbol for each element. These symbols could be put easily together to show how the elements combine into compounds. For example, writing two Hs and one O together as H2O would mean that the particles (molecules) of water consist of two hydrogen atoms and one oxygen atom, bonded together. Berzelius published a table of 24 elements, including their atomic weights, most of which are close to the values used today.

By the year 1800 only about 25 true elements were known, but progress was relatively rapid throughout the nineteenth century. By the time Russian scientist Dmitri Ivanovich Mendeleev (1834–1907) organized his periodic table in 1869, he had about 60 elements to reckon with. By 1900 there were more than 80. The list quickly expanded to 92, ending at uranium (atomic number 92). There it stayed until 1940, when synthesis of the transuranium elements began.

Organization of the elements

The task of organizing more than a hundred very different elements into some simple, sensible arrangement would seem difficult. Mendeleev's periodic table, however, is the answer. It even accommodates the synthetic transuranium elements without strain. In this encyclopedia, each individual chemical element is discussed under at least one of the following types of entry: (1) Fourteen particularly important elements are discussed in their own entries. They are aluminum, calcium, carbon, chlorine , copper, hydrogen, iron, lead, nitrogen , oxygen, silicon, sodium, sulfur, and uranium. (2) Elements that belong to any of seven families of elements—groups of elements that have similar chemical properties—are discussed under their family-name headings. These seven families are the actinides , alkali metals , alkaline earth metals , halogens , lanthanides , rare gases, and transuranium elements. (3) Elements that are not discussed either under their own name or as part of a family ("orphan elements") are discussed briefly below. Any element that is not discussed below can be found in the headings described above.

"Orphan" elements

Actinium. The metallic chemical element of atomic number 89. Symbol Ac, specific gravity 10.07, melting point 1,924°F (1,051°C), boiling point 5,788°F (3,198°C). All isotopes of this element are radioactive; the half-life of its most stable isotope, actinium-227, is 21.8 years. Its name is from the Greek aktinos, meaning ray.

Antimony. The metallic chemical element of atomic number 51. Symbol Sb, atomic weight 121.8, specific gravity 6.69, melting point 1,167°F (630.63°C), boiling point 2,889°F (1,587°C). One of its main uses is to alloy with lead in automobile batteries; actinium makes the lead harder.

Arsenic. The metallic chemical element of atomic number 33. Symbol As, atomic weight 74.92, specific gravity 5.73 in gray metallic form, melting point 1,503°F (817°C), sublimes (solid turns to gas) at 1,137°F (614°C). Arsenic compounds are poisonous.

Bismuth. The metallic chemical element of atomic number 83. Symbol Bi, atomic weight 208.98, specific gravity 9.75, melting point 520.5°F (271.4°C), boiling point 2,847.2°F (1,564°C). Bismuth oxychloride is used in "pearlized" cosmetics. Bismuth subsalicylate, an insoluble compound, is the major ingredient in Pepto-Bismol. The soluble compounds of bismuth, however, are poisonous.

Boron. The non-metallic chemical element of atomic number 5. Symbol B, atomic weight 10.81, specific gravity (amorphous form) 2.37, melting point 3,767°F (2,075°C), boiling point 7,232°F (4,000°C). Common compounds are borax, Na2B4O7•10H2O, used as a cleansing agent and water softener, and boric acid , H3BO3, a mild antiseptic and an effective cockroach poison.

Cadmium. The metallic chemical element of atomic number 48. Symbol Cd, atomic weight 112.4, specific gravity 8.65, melting point 609.92°F (321.07°C), boiling point 1,413°F (767°C). A soft, highly toxic metal used in silver solder, in many other alloys, and in nickel-cadmium rechargeable batteries. Because it is an effect absorber of moving neutrons, it is used in control rods for nuclear reactors to slow the chain reaction.

Chromium. The metallic chemical element of atomic number 24. Symbol Cr, atomic weight 51.99, specific gravity 7.19, melting point 3,465°F (1,907°C), boiling point 4,840°F (2,671°C). A hard, shiny metal that takes a high polish. Used to electroplate steel for protection against corrosion and as the major ingredient (next to iron) in stainless steel. Alloyed with nickel, it makes Nichrome, a high-electrical-resistance metal that gets red hot when electric current passes through it; toaster and heater coils are made of Nichrome wire. Chromium is named from the Greek chroma, meaning color , because most of its compounds are highly colored. Chromium is responsible for the green color of emeralds.

Cobalt. The metallic chemical element of atomic number 27. Symbol Co, atomic weight 58.93. Cobalt is a grayish, hard, brittle metal closely resembling iron and nickel. These three metals are the only naturally occurring magnetic elements on Earth.

Gallium. The metallic chemical element of atomic number 31. Symbol Ga, atomic weight 69.72, melting point 85.6°F (29.78°C), boiling point 3,999°F (2,204°C). Gallium is frequently used in the electronics industry and in thermometers that measure a wide range of temperatures.

Germanium. The metallic chemical element of atomic number 32. Symbol Ge, atomic weight 72.59. In pure form, germanium is a brittle crystal . It was used to make the world's first transistor and is still used as a semiconductor in electronics devices.

Gold. The metallic chemical element of atomic number 79. Symbol Au, atomic weight 196.966. This most malleable of metals was probably one of the first elements known to humans. It is usually alloyed with harder metals for use in jewelry, coins, or decorative pieces.

Hafnium. The metallic chemical element of atomic number 72. Symbol Hf, atomic weight 178.49, melting point 4,040.6 ±S68°F (2,227 ±20°C), boiling point 8,315.6°F (4,602°C). Hafnium is strong and resistant to corrosion. It also absorbs neutrons well, making it useful in control rods of nuclear reactors.

Indium. The metallic chemical element of atomic number 49. Symbol In, atomic weight 114.82, melting point 313.89°F (156.61°C), boiling point 3,776°F (2,080°C). Indium is a lustrous, silvery metal that bends easily. It is often alloyed with other metals in solid-stateelectronics devices.

Iridium. The metallic chemical element of atomic number 77. Symbol Ir, atomic weight 192.22. Iridium is an extremely dense metal that resists corrosion better than most others. In its pure state, it is often used in aircraft spark plugs.

Manganese. The metallic chemical element of atomic number 25. Symbol Mn, atomic weight 54.93. The biggest use of manganese is in steelmaking, where it is alloyed with iron. This element is required by all plants and animals, so it is sometimes added as manganese oxide to animal feed.

Mercury. The metallic chemical element of atomic number 80. Symbol Hg, atomic weight 200.59, melting point -37.96°F (-38.87°C), boiling point 673.84°F (356.58°C). Mercury is highly poisonous and causes irreversible damage to the nervous and excretory systems. This element was long used in thermometers because it expands and contracts at a nearly constant rate ; however, mercury thermometers are being phased out in favor of alcohol-based and electronic thermometers because of mercury's high toxicity.

Molybdenum. The metallic chemical element of atomic number 42. Symbol Mo, atomic weight 95.94, melting point 4,753°F (2,623°C), boiling point 8,382°F (4,639°C). Molybdenum is used to make superalloyed metals designed for high-temperature processes. It is also found as a trace element in plant and animal tissues.

Nickel. The metallic chemical element of atomic number 28. Symbol Ni, atomic weight 58.71. Nickel is often mixed with other metals, such as copper and iron, to increase the alloy's resistance to heat and moisture.

Niobium. The metallic chemical element of atomic number 41. Symbol Nb, atomic weight 92.90, melting point 4,474.4 ±50°F (2,468 ±10°C), boiling point 8,571.2°F (4,744°C). Niobium is used to strengthen alloys used to make lightweight aircraft frames.

Osmium. The metallic chemical element of atomic number 76. Symbol Os, atomic weight 190.2. Osmium is hard and dense, weighing twice as much as lead. The metal is used to make fountain pen tips and electrical devices.

Palladium. The metallic chemical element of atomic number 46. Symbol Pd, atomic weight 106.42. Palladium is soft. It also readily absorbs hydrogen, and is therefore used to purify hydrogen gas.

Phosphorus. The nonmetallic chemical element of atomic number 15. Symbol P, atomic weight 30.97. Phosphorus is required by all plant and animal cells. Most of the phosphorus in human beings is in the bones and teeth. Phosphorus is heavily used in agricultural fertilizers .

Platinum. The metallic chemical element of atomic number 78. Symbol Pt, atomic weight 195.08, melting point 3,215.1°F (1,768.4°C), boiling point 6,920.6 ±212°F (3,827 ±100°C). Platinum withstands high temperatures well and is used in rocket and jet-engine parts. It is also used as a catalyst in chemical reactions .

Polonium. The metallic chemical element of atomic number 84. Symbol Po, atomic weight 209. Polonium is a product of uranium decay and is 100 times as radioactive as uranium.

Rhenium. The metallic chemical element of atomic number 75. Symbol Re, atomic weight 186.207, specific gravity 21.0, melting point 5,766.8°F (3,186°C), boiling point 10,104.8°F (5,596°C). Rhenium is used in chemical and medical instruments, as a catalyst for the chemical and petroleum industries, and in photoflash lamps.

Rhodium. The metallic chemical element of atomic number 45. Symbol Rh, atomic weight 102.91. This element is similar to palladium. Electroplated rhodium, which is hard and highly reflective, is used as a reflective material for optical instruments.

Ruthenium. The metallic chemical element of atomic number 44. Symbol Ru, atomic weight 101.07, specific gravity 12.5, melting point 4,233.2°F (2,334°C), boiling point 7,502°F (4,150°C). This element is alloyed with platinum and palladium to form hard, resistant contacts for electrical equipment that must withstand a great deal of wear.

Scandium. The metallic chemical element of atomic number 21. Symbol Sc, atomic weight 44.96, melting point 2,805.8°F (1,541°C), boiling point 5,127.8°F (2,831°C). Scandium is a silvery-white metal that develops a yellowish or pinkish cast when exposed to air. It has relatively few commercial applications.

Selenium. The nonmetallic chemical element of atomic number 34. Symbol Se, atomic weight 78.96. Selenium is able to convert light directly into electricity , and its resistance to electrical current decreases when it is exposed to light. Both properties make this element useful in photocells, exposure meters, and solar cells.

Silver. The metallic chemical element of atomic number 47. Symbol Ag, atomic weight 107.87. Silver has long been used in the manufacture of coins. It is also an excellent conductor of heat and electricity. Some compounds of silver are light-sensitive, making silver important in the manufacture of photographic films and papers.

Tantalum. The metallic chemical element of atomic number 73. Symbol Ta, atomic weight 180.95, melting point 5,462.6°F (3,017°C), boiling point of 9,797 ±212°F (5,425 ±100°C). Tantalum is a heavy, gray, hard metal that is used in alloys to pen points and analytical weights.

Technetium. The metallic chemical element of atomic number 43. Symbol Tc, atomic weight 98. Technetium was the first element to be produced synthetically; scientists have never detected the natural presence of this element on Earth.

Tellurium. The nonmetallic chemical element of atomic number 52. Symbol Te, atomic weight 127.60, melting point 841.1 ±32.54°F (449.5 ±0.3°C), boiling point 1,813.64 ±38.84°F (989.8 ±3.8°C). Tellurium is a grayish-white, lustrous, brittle metal. It is a semiconductor and is used in the electronics industry.

Thallium. The metallic chemical element of atomic number 81. Symbol Tl, atomic weight 204.38. Thallium is a bluish-gray metal that is soft enough to be cut with a knife. Thallium sulfate is used as a rodenticide and ant poison.

Tin. The metallic chemical element of atomic number 50. Symbol Sn, atomic weight 118.69. Tin is alloyed with copper and antimony to make pewter. It is also used as a soft solder and as coating to prevent other metals from corrosion.

Titanium. The metallic chemical element of atomic number 22. Symbol Ti, atomic weight 47.90, melting point 3,020 ±50°F (1,660 ±10°C), boiling point 5,948.6°F (3,287°C). This element occurs as a bright, lustrous brittle metal or dark gray powder. Titanium alloys are strong for their weight and can withstand large changes in temperature .

Tungsten. The metallic chemical element of atomic number 74. Symbol W, atomic weight 183.85, melting point 6,170 ±68°F (3,410 ±20°C). The melting point of tungsten is higher than that of any other metal. Its chief use is as a filament in electric light bulbs.

Vanadium. The metallic chemical element of atomic number 23. Symbol V, atomic weight 50.94. Pure vanadium is bright white. This metal finds its biggest use in strengthening steel.

Yttrium. The metallic chemical element of atomic number 39. Symbol Y, atomic weight 88.91, melting point 2,771.6 ±46.4°F (1,522 ±8°C), boiling point 6,040.4°F (3,338°C). Yttrium is a relatively active metal that decomposes in cold water slowly and in boiling water rapidly. Certain compounds containing yttrium have been shown to become superconducting at relatively high temperatures.

Zinc. The metallic chemical element of atomic number 30. Symbol Zn, atomic weight 65.39. Zinc, a brittle metal at room temperature, forms highly versatile alloys in industry. One zinc alloy is nearly as strong as steel, but has the malleability of plastic.

Zirconium. The metallic chemical element of atomic number 40. Symbol Zr, atomic weight 91.22, melting point 3,365.6 ±35.6°F (1,852 ±2°C), boiling point 7,910.6°F (4,377°C). Neutrons can pass through this metal without being absorbed; this makes it highly desirable as a construction material for the metal rods containing the fuel pellets in nuclear power plants.

See also Ammonia; Compound, chemical; Deuterium; Element, transuranium; Tritium; Valence.

Resources

books

Lide, David R. CRC Handbook of Chemistry and Physics. 7th ed. Boca Raton, FL: CRC Press LLC, 1997.

Emsley, J. The Elements. 3rd ed. New York: Oxford Univ. Press, Inc., 1998.

Greenwood, N. N., and A. Earnshaw. Chemistry of the Elements. 2nd ed. Woburn, MA: Butterworth-Heinemann, 1997.

periodicals

Seife, Charles. "Heavy-Element Fizzle Laid to Falsified Data." Science (July 19, 2002): 313–315.

Robert L. Wolke