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 Parentheses indicate most stable isotope; brackets enclose lower and upper bounds of weight variation.
Element Symbol Atomic Number Atomic Weight Melting Point(Degrees Celsius) Boiling Point(Degrees Celsius)
actinium Ac 89 (227) 1050. 3200. ±300
aluminum Al 13 26.98154 660.37 2467.
americium Am 95 (243) 1172. 2600.
antimony Sb 51 121.760 630.74 1750.
argon Ar 18 39.948 -189.2 -185.7
arsenic As 33 74.92160 817. (at 28 atmospheres) 613. (sublimates)
astatine At 85 (210) 302. (est.) 337. (est.)
barium Ba 56 137.327 725. 1640.
berkelium Bk 97 (247) 1050. 2590.
beryllium Be 4 9.01218 1278. ±5 2970.
bismuth Bi 83 208.98040 271.3 1560. ±5
bohrium Bh 107 (270)
boron B 5 [10.806; 10.821] 2300. 2550. (sublimates)
bromine Br 35 79.904 -7.2 58.78
cadmium Cd 48 112.411 320.9 765.
calcium Ca 20 40.078 839. ±2 1484.
californium Cf 98 (251) 900. 1470.
carbon C 6 [12.0096; 12.0116] ∼3550. 4827.
cerium Ce 58 140.116 799. 3426.
cesium Cs 55 132.90545 28.40 669.3
chlorine Cl 17 [35.446; 35.457] -100.98 -34.6
chromium Cr 24 51.9961 1857. ±20 2672.
cobalt Co 27 58.9332 1495. 2870.
copernicium Cn 112 (285)
copper Cu 29 63.546 1083.4 ±0.2 2567.
curium Cm 96 (247) 1340. ±40 3110.
darmstadtium Ds 110 (281)
dubnium Db 105 (268)
dysprosium Dy 66 162.500 1412. 2562.
einsteinium Es 99 (252) 857.
erbium Er 68 167.259 1529. 2863.
europium Eu 63 151.964 822. 1597.
fermium Fm 100 (257) 1527.
flerovium Fl 114 (289)
fluorine F 9 18.9984 -219.62 -188.14
francium Fr 87 (223) (27) (est.) (677) (est.)
gadolinium Gd 64 157.25 1313. ±1 3266.
gallium Ga 31 69.723 29.78 2403.
germanium Ge 32 72.63 937.4 2830.
gold Au 79 196.96657 1064.43 2808.
hafnium Hf 72 178.49 2227. ±20 4602.
hassium Hs 108 (277)
helium He 2 4.0026 <-272.2 -268.934
holmium Ho 67 164.93032 1474. 2425.
hydrogen H 1 [1.00784; 1.00811] -259.14 -252.87
indium In 49 114.818 156.61 2080.
iodine I 53 126.90447 113.5 184.35
iridium Ir 77 192.217 2410. 4130.
iron Fe 26 55.845 1535. 2750.
krypton Kr 36 83.798 -156.6 -152.30 ±0.10
lanthanum La 57 138.90547 921. 3457.
lawrencium Lr 103 (262) 1627.
lead Pb 82 207.2 327.502 1740.
lithium Li 3 [6.938; 6.997] 180.54 1342.
livermorium Lv 116 (292)
lutetium Lu 71 174.9668 1663. 3395.
magnesium Mg 12 24.3050 648.8 ±0.5 1090.
manganese Mn 25 54.93805 1244. ±3 1962.
meitnerium Mt 109 (276)
mendelevium Md 101 (258) 827.
mercury Hg 80 200.59 -38.842 356.58
molybdenum Mo 42 95.96 2617. 4612.
neodymium Nd 60 144.242 1021. 3068.
neon Ne 10 20.1797 -248.67 -246.048
neptunium Np 93 (237) 640. ±1 3902. (est.)
nickel Ni 28 58.6934 1453. 2732.
niobium Nb 41 92.90638 2468. ±10 4742.
nitrogen N 7 [14.00643; 14.00728] -209.86 -195.8
nobelium No 102 (259) 827.
osmium Os 76 190.23 3045. ±30 5027. ±100
oxygen O 8 [15.99903; 15.99977] -218.4 -182.962
palladium Pd 46 106.42 1554. 2970.
phosphorus P 15 30.97376 44.1 (white) 280. (white)
platinum Pt 78 195.084 1772. 3827. ±100
plutonium Pu 94 (244) 641. 3232.
polonium Po 84 (209) 254. 962.
potassium K 19 39.0983 63.25 760.
praseodymium Pr 59 140.90765 931. 3512.
promethium Pm 61 (145) 1042 3000. (est.)
protactinium Pa 91 231.03588 <1600. 4026.
radium Ra 88 (226) 700. 1140.
radon Rn 86 (222) -71. -61.8
rhenium Re 75 186.207 3180. 5627. (est.)
rhodium Rh 45 102.90550 1966. ±3 3727. ±100
roentgenium Rg 111 (280)
rubidium Rb 37 85.4678 38.89 686.
ruthenium Ru 44 101.07 2310. 3900.
rutherfordium Rf 104 (265)
samarium Sm 62 150.36 1072. ±5 1791.
scandium Sc 21 44.95591 1541. 2831.
seaborgium Sg 106 (271)
selenium Se 34 78.96 217. 684.9 ±1.0
silicon Si 14 [28.084; 28.086] 1410. 2355.
silver Ag 47 107.8682 961.93 2212.
sodium Na 11 22.98977 97.81 ±0.03 882.9
strontium Sr 38 87.62 269. 1384.
sulfur S 16 [32.059; 32.076] 112.8 444.674
tantalum Ta 73 180.94788 2996. 5425. ±100
technetium Tc 43 (98) 2200. 4877.
tellurium Te 52 127.60 449.5 ±0.3 989.8 ±3.8
terbium Tb 65 158.92535 1356. 3123.
thallium Tl 81 [204.382; 204.385] 303.5 1457. ±10
thorium Th 90 232.03806 1750. ∼4790.
thulium Tm 69 168.93421 1545. ±15 1947.
tin Sn 50 118.710 231.9681 2270.
titanium Ti 22 47.867 1660. ±10 3287.
tungsten W 74 183.84 3410. ±20 5660.
ununoctium Uuo 118 (294)
ununpentium Uup 115 (288)
ununseptium Uus 117 (294)
ununtrium Uut 113 (284)
uranium U 92 238.02891 1132.3 ±0.8 3818.
vanadium V 23 50.9415 1890. ±10 3380.
xenon Xe 54 131.293 -111.9 -107.1 ±3
ytterbium Yb 70 173.054 819. 1194.
yttrium Y 39 88.90585 1522. ±8 3338.
zinc Zn 30 65.38 419.58 907.
zirconium Zr 40 91.224 1852. ±2 4377.

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The elements are at the heart of chemistry, and indeed they are at the heart of life as well. Every physical substance encountered in daily life is either an element, or more likely a compound containing more than one element. There are millions of compounds, but only about 100 elements, of which only 88 occur naturally on Earth. How can such vast complexity be created from such a small amount of elements? Consider the English alphabet, with just 26 letters, with which an almost infinite number of things can be said or written. Even more is possible with the elements, which greatly outnumber the letters of the alphabet. Despite the relatively great quantity of elements, however, just two make up almost the entire mass of the universeand these two are far from abundant on Earth. A very small number of elements, in fact, are essential to life on this planet, and to the existence of human beings.


The Focal Point of Chemistry

Like physics, chemistry is concerned with basic, underlying processes that explain how the universe works. Indeed, these two sciences, along with astronomy and a few specialized fields, are the only ones that address phenomena both on the Earth and in the universe as a whole. By contrast, unless or until life on another planet is discovered, biology has little concern with existence beyond Earth's atmosphere, except inasmuch as the processes and properties in outer space affect astronauts.

While physics and chemistry address many of the same fundamental issues, they do so in very different ways. To make a gross generalizationsubject to numerous exceptions, but nonetheless useful in clarifying the basic difference between the two sciencesphysicists are concerned with external phenomena, and chemists with internal ones.

For instance, when physicists and chemists study the interactions between atoms, unless physicists focus on some specialized area of atomic research, they tend to treat all atoms as more or less the same. Chemists, on the other hand, can never treat atoms as though they are just undifferentiated particles colliding in space. The difference in structure between one kind of atom and another, in fact, is the starting-point of chemical study.

Structure of the Atom

An atom is the fundamental particle in a chemical element, or a substance that cannot be broken down into another substance by chemical means. Clustered at the center, or nucleus, of the atom are protons, which have a positive electric charge, and neutrons, which possess no charge.

Spinning around the nucleus are electrons, which are negatively charged. The vast majority of the atom's mass is made up by the protons and neutrons, which have approximately the same mass, whereas that of the electron is much smaller. The mass of an electron is about 1/1836 that of proton, and 1/1839 that of a neutron.

It should be noted that the nucleus, though it constitutes most of the atom's mass, is only a tiny portion of the atom's volume. If the nucleus were the size of a grape, in fact, the electrons would, on average, be located about a mile away.


Atoms of the same element always have the samenumber of protons, and since this figure isunique for a given element, each element is assigned an atomic number equal to the number of protons in its nucleus. Two atoms may have the same number of protons, and thus be of the same element, yet differ in their number of neutrons. Such atoms are called isotopes.

Isotopes are generally represented as follows: where S is the symbol of the element, a is the atomic number, and m is the mass numberthe sum of protons and neutrons in the atom's nucleus. For the stable silver isotope designated as for instance, Ag is the element symbol (discussed below); 47 its atomic number; and 93 the mass number. From this, it is easy to discern that this particular stable isotope has 46 neutrons in its nucleus.

Because the atomic number of any element is established, sometimes isotopes are represented simply with the mass number thus: 93Ag. They may also be designated with a subscript notation indicating the number of neutrons, so that one can obtain this information at a glance without having to do the arithmetic. For the silver isotope shown here, this is written as Isotopes are sometimes indicated by simple nomenclature as well: for instance, carbon-12 or carbon-13.


The number of electrons in an atom is usually the same as the number of protons, and thus most atoms have a neutral charge. In certain situations, however, the atom may lose or gain one or more electrons and acquire a net charge, becoming an ion. The electrons are not "lost" when an atom becomes an ion: they simply go elsewhere.

Aluminum (Al), for instance, has an atomic number of 13, which tells us that an aluminum atom will have 13 protons. Given the fact that every proton has a positive charge, and that most atoms tend to be neutral in charge, this means that there are usually 13 electrons, with a negative charge, present in an atom of aluminum. Aluminum may, however, form an ion by losing three electrons.


After its three electrons have departed, the remaining aluminum ion has a net positive charge of 3, represented as +3. How do we know this? Initially the atom had a charge of +13 + (13) = 0. With the exit of the 3 electrons, leaving behind only 10, the picture changes: now the charge is +13 + (10) = +3.

When a neutral atom loses one or more electrons, the result is a positively charged ion, or cation (pronounced KAT-ie-un). Cations are represented by a superscript number and plus sign after the element symbol: Al3+, for instance, represents the aluminum cation described above. (Some chemists represent this with the plus sign before the numberfor example, Al+3.) A cation is named after the element of which it is an ion: thus the ion we have described is called either the aluminum ion, or the aluminum cation.


When a neutrally charged atom gains electrons, and as a result acquires a negative charge, this type of ion is known as an anion (AN-ie-un). Anions can be represented symbolically in much the same way of cations: Cl, for instance, is an anion of chlorine that forms when it acquires an electron, thus assuming a net charge of 1. Note that the 1 is not represented in the superscript notation, much as people do not write 101. In both cases, the 1 is assumed, whereas any number higher than 1 is shown.

The anion described here is never called a chlorine anion; rather, anions have a special nomenclature. If the anion represents, as is the case here, a single element, it is named by adding the suffix -ide to the name of the original element name: hence it would be called chloride. Other anions involve more than one element, and in these cases other rules apply for designating names. A few two-element anions use the-ide ending; such is the case, for instance, with a deadly mixture of carbon and nitrogen (CN), better known as cyanide.

For anions involving oxygen, there may be different prefixes and suffixes, depending on the relative number of oxygen atoms in the anion.


The Periodic Table

Note that an ion is never formed by a change in the number of protons: that number, as noted earlier, is a defining characteristic of an element. If all we know about a particular atom is that it has one proton, we can be certain that it is an atom of hydrogen. Likewise, if an atom has 79 protons, it is gold.

Knowing these quantities is not a matter of memorization: rather, one can learn this and much more by consulting the periodic table of elements. The periodic table is a chart, present in virtually every chemistry classroom in the world, showing the elements arranged in order of atomic number. Elements are represented by boxes containing the atomic number, element symbol, and average atomic mass, in atomic mass units, for that particular element. Vertical columns within the periodic table indicate groups or "families" of elements with similar chemical characteristics.

These groups include alkali metals, alkaline earth metals, halogens, and noble gases. In the middle of the periodic table is a wide range of vertical columns representing the transition metals, and at the bottom of the table, separated from it, are two other rows for the lanthanides and actinides.

An Overview of the Elements

As of 2001, there 112 elements, of which 88 occur naturally on Earth. (Some sources show 92 naturally occurring elements; however, a few of the elements with atomic numbers below 92 have not actually been found in nature.) The others were created synthetically, usually in a laboratory, and because these are highly radioactive, they exist only for fractions of a second. The number of elements thus continues to grow, but these "new" elements have little to do with the daily lives of ordinary people. Indeed, this is true even for some of the naturally occurring elements: few people who are not chemically trained, for instance, are able to identify thulium, which has an atomic number of 69.

Though an element can theoretically exist as a gas, liquid, or a solid, in fact the vast majority of elements are solids. Only 11 elementsthe six noble gases, along with hydrogen, nitrogen, oxygen, fluorine, and chlorineexist in the gaseous state at a normal temperature of about 77°F (25°C). Just two are liquids at normal temperature: mercury, a metal, and the non-metal bromine. (The metal gallium becomes liquid at just 85.6°F, or 29.76°C.) The rest are all solids.

The noble gases are monatomic, meaning that they exist purely as single atoms. So too are the "noble metals," such as gold, silver, and platinum. "Noble" in this context means "set apart": noble gases and noble metals are known for their tendency not to react to, and hence not to bond with, other elements. On the other hand, a number of other elements are described as diatomic, meaning that two atoms join to form a molecule.

Names of the Elements


As noted above, the periodic table includes the element symbol or chemical symbola one-or two-letter abbreviation for the name of the element. Many of these are simple one-letter designations: O for oxygen, or C for carbon. Others are two-letter abbreviations, such as Ne for neon or Si for silicon. Note that the first letter is always capitalized, and the second is always lowercase.

In many cases, the two-letter symbols indicate the first and second letters of the element's name, but this is far from universal. Cadmium, for example, is abbreviated Cd, while platinum is Pt. In other cases, the symbol seems to have nothing to do with the element name as it is normally used: for instance, Au for gold or Fe for iron.

Many of the one-letter symbols indicate elements discovered early in history. For instance, carbon is represented by C, and later "C" elements took two-letter designations: Ce for cerium, Cr for chromium, and so on. But many of those elements with apparently strange symbols were among the first discovered, and this is precisely why the symbols make little sense to a person who does not recognize the historical origins of the name.


For many years, Latin was the language of communication between scientists from different nations; hence the use of Latin names such as aurum ("shining dawn") for gold, or ferrum, the Latin word for iron. Likewise, lead (Pb) and sodium (Na) are designated by their Latin names, plumbum and natrium, respectively.

Some chemical elements are named for Greek or German words describing properties of the elementfor example, bromine (Br), which comes from a Greek word meaning "stench." The name of cobalt comes from a German term meaning "underground gnome," because miners considered the metal a troublemaker. The names of several elements with high atomic numbers reflect the places where they were originally discovered or created: francium, germanium, americium, californium.

Americium and californium, with atomic numbers of 95 and 98 respectively, are among those elements that do not occur naturally, but were created artificially. The same is true of several elements named after scientistsamong them einsteinium, after Albert Einstein (1879-1955), and nobelium after Alfred Nobel (1833-1896), the Swedish inventor of dynamite who established the Nobel Prize.

Abundance of Elements


The first two elements on the periodic table, hydrogen and helium, represent 99.9% of the matter in the entire universe. This may seem astounding, but Earth is a tiny dot within the vastness of space, and hydrogen and helium are the principal elements in stars.

All elements, except for those created artificially, exist both on Earth and throughout the universe. Yet the distribution of elements on Earth is very, very different from that in other placesas well it should be, given the fact that Earth is the only planet known to support life. Hydrogen, for instance, constitutes only about 0.87%, by mass, of the elements found in the planet's crust, waters, and atmosphere. As for helium, it is not even among the 18 most abundant elements on Earth.


That great element essential to animal life, oxygen, is by far the most plentiful on Earth, representing nearly half49.2%of the total mass of atoms found on this planet. (Here the term "mass" refers to the known elemental mass of the planet's atmosphere, waters, and crust; below the crust, scientists can only speculate, though it is likely that much of Earth's interior consists of iron.) Together with silicon (25.7%), oxygen accounts for almost exactly three-quarters of the elemental mass of Earth. Add in aluminum (7.5%), iron (4.71%), calcium (3.39%), sodium (2.63%), potassium (2.4%), and magnesium (1.93%), and these eight elements make up about 97.46% of Earth's material.

In addition to hydrogen, whose distribution is given above, nine other elements account for a total of 2% of Earth's composition: titanium (0.58%), chlorine (0.19%), phosphorus (0.11%), manganese (0.09%), carbon (0.08%), sulfur (0.06%), barium (0.04%), nitrogen (0.03%), and fluorine (0.03%). The remaining 0.49% is made up of various elements.

Elements In the Human Body

Fans of science-fiction are familiar with the phrase "carbon-based life form," which is used, for instance, by aliens in sci-fi movies to describe humans. In fact, the term is a virtual redundancy on Earth, since all living things contain carbon.

Essential though it is to life, carbon as a component of the human body takes second place to oxygen, which is an even larger proportion of the body's mass65.0%than it is of the Earth. Carbon accounts for 18%, and hydrogen for 10%, meaning that these three elements make up 93% of the body's mass. Most of the remainder is taken up by 10 other elements: nitrogen (3%), calcium (1.4%), phosphorus (1.0%), magnesium (0.50%), potassium (0.34%), sulfur (0.26%), sodium (0.14%), chlorine (0.14%), iron (0.004%), and zinc (0.003%).


As small as the amount of zinc is in the human body, there are still other elements found in even smaller quantities. These are known as trace elements, because only traces of them are present in the body. Yet they are essential to human well-being: without enough iodine, for instance, a person can develop a goiter, a large swelling in the neck area. Chromium helps the body metabolize sugars, which is why people concerned with losing weight and/or toning their bodies through exercise may take a chromium supplement.

Even arsenic, lethal in large quantities, is a trace element in the human body, and medicines for treating illnesses such as the infection known as "sleeping sickness" contain tiny amounts of arsenic. Other trace elements include cobalt, copper, fluorine, manganese, molybdenum, nickel, selenium, silicon, and vanadium.


Though these elements are present in trace quantities within the human body, that does not mean that exposure to large amounts of them is healthy. Arsenic, of course, is a good example; so too is aluminum. Aluminum is present in unexpected places: in baked goods, for instance, where a compound containing aluminum (baking powder) is sometimes used in the leavening process; or even in cheeses, as an aid to melting when heated. The relatively high concentrations of aluminum in these products, as well as fluorine in drinking water, has raised concerns among some scientists.

Generally speaking, an element is healthy for the human body in proportion to its presence in the body. With trace elements and others that are found in smaller quantities, however, it is sometimes possible and even advisable to increase the presence of those elements by taking dietary supplements. Hence a typical multivitamin contains calcium, iron, iodine, magnesium, zinc, selenium, copper, chromium, manganese, molybdenum, boron, and vanadium.

For most of these, recommended daily allowances (RDA) have been established by the federal government. Usually, people do not take sodium as a supplement, thoughAmericans already get more than their RDA of sodium through salt, which is overly abundant in the American diet.


Challoner, Jack. The Visual Dictionary of Chemistry. New York: DK Publishing, 1996.

"Classification of Elements and Compounds" (Web site). <http://dl.clackamas.cc.or.us/ch104-04/classifi.htm> (May 14, 2001).

"Elements and Compounds" (Web site). <http://wine1.sb.fsu.edu/chm1045/notes/Intro/Elements/Intro02.htm> (May 14, 2001).

"Elements, Compounds, and Mixtures." Purdue University Department of Chemistry. (Web site). <http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch2/mix.html> (May 14, 2001).

"Elements, Compounds, and Mixtures" (Web site). <http://www.juniorcert.net/serve/cont.php3?pg=SC2CEC0339> (May 14, 2001).

Knapp, Brian J. Elements. Illustrated by David Woodroffe and David Hardy. Danbury, CT: Grolier Educational, 1996.

"MatterElements, Compounds, and Mixtures" (Web site). <http://chem.sci.gu.edu.au/help_desk/Matter.htm> (May 14, 2001).

Oxlade, Chris. Elements and Compounds. Chicago, IL: Heinemann Library, 2001.

Stwertka, Albert. A Guide to the Elements. New York: Oxford University Press, 1998.

Zumdahl, Steven S. Introductory Chemistry: A Foundation, 4th ed. Boston: Houghton Mifflin, 2000.



The negative ion that results when an atom gains one or more electrons. An anion (pronounced "AN-ie-un") of an element is never called, for instance, the chlorine anion. Rather, for an anion involving a single element, it is named by adding the suffix -ide to the name of the original elementhence, "chloride." Other rules apply for more complex anions.


The smallest particle of an element. An atom can exist either alone or in combination with other atoms in a molecule. Atoms are made up of protons, neutrons, and electrons. An atom that loses orgains one or more electrons, and thus has a net charge, is an ion. Atoms that have the same number of protonsthat is, are of the same elementbut differ in number of neutrons are known as isotopes.


An SI unit (abbreviated amu), equal to 1.66 · 1024 g, for measuring the mass of atoms.


The number of protons in the nucleus of an atom. Since this number is different for each element, elements are listed on the periodic table in order of atomic number.


A figure used by chemists to specify the massin atomic mass unitsof the average atom in a large sample. If a substance is a compound, the average atomic mass of all atoms in a molecule of that substance must be added together to yield the average molecular mass of that substance.


The positive ion that results when an atom loses one or more electrons. A cation (pronounced KAT-ie-un) is named after the element of which it is anion and thus is called, for instance, the aluminum ion or the aluminum cation.


Another term for element symbol.


A substance made up of atoms of more than one element. These atoms are usually joined in molecules.


A term describing an element that exists as molecules composed of two atoms. This is in contrast to monatomic elements.


Negatively charged particles in an atom. Electrons, which spin around the protons and neutrons that make up the atom's nucleus, constitute a very small portion of the atom's mass. The number of electrons and protons is the same, thus canceling out one another. But when an atom loses or gains one or more electrons, howeverthus becoming anionit acquires a net electric charge.


A substance made up of only one kind of atom. Unlike compounds, elements cannot be chemically broken into other substances.


A one-or two-letter abbreviation for the name of an element. These may be a single capitalized letter (O for oxygen), or a capitalized letter followed by a lowercase one (Ne for neon). Sometimes the second letter is not the second letter of the element name (for example, Pt for platinum). In addition, the symbol may refer to an original Greek or Latinname, rather than the name used now, is the case with Au (aurum) for gold.


An atom that has lost or gained one or more electrons, and thus has a net electric charge.


Atoms that have an equal number of protons, and hence are of the same element, but differ in their number of neutrons.


The sum of protons and neutrons in an atom's nucleus. Where an isotope is represented, the mass number is placed above the atomic number to the left of the element symbol.


A group of atoms, usu-allybut not always representing more than one element, joined in a structure. Compounds are typically made of up molecules.


A term describing an element that exists as single atoms. This in contrast to diatomic elements.


A subatomic particle that has no electric charge. Neutrons are found at the nucleus of an atom, alongside protons.


The center of an atom, a region where protons and neutrons are located, and around which electrons spin.


A chart that shows the elements arranged in order of atomic number, along with element symbol and the average atomic mass (in atomic mass units) for that particular element. Vertical columns within the periodic table indicate groups or "families" of elements with similar chemical characteristics.


A positively charged particle in an atom. Protons and neutrons, which together form the nucleus around which electrons spin, have approximately the same massa mass that is many times greater than that of an electron. The number of protons in the nucleus of an atom is the atomic number of an element.

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element, in chemistry, a substance that cannot be decomposed into simpler substances by chemical means. A substance such as a compound can be decomposed into its constituent elements by means of a chemical reaction, but no further simplification can be achieved. An element can, however, be decomposed into simpler substances, such as protons and neutrons or various combinations of them, by the methods of particle physics, e.g., by bombardment of the nucleus.

The Atom

The smallest unit of a chemical element that has the properties of that element is called an atom. Many elements (e.g., helium) occur as single atoms. Other elements occur as molecules made up of more than one atom. Elements that ordinarily occur as diatomic molecules include hydrogen, nitrogen, oxygen, and the halogens, but oxygen also occurs as a triatomic form called ozone. Phosphorus usually occurs as a tetratomic molecule, and crystalline sulfur occurs as molecules containing eight atoms.

Atomic Number and Mass Number

Regardless of how many atoms the element is composed of, each atom has the same number of protons in its nucleus, and this is different from the number in the nucleus of any other element. Thus this number, called the atomic number (at. no.), defines the element. For example, the element carbon consists of atoms all with at. no. 6, i.e., all having 6 protons in the nucleus; any atom with at. no. 6 is a carbon atom. By 2006, 117 elements were known, ranging from hydrogen with an at. no. of 1 to an as yet unnamed element (temporarily known as ununoctium) with an at. no. of 118. (See the table entitled Elements for an alphabetical list of all the elements, including their symbols, atomic numbers, atomic weights, and melting and boiling points.) The nuclei of most atoms also contain neutrons. The total number of protons and neutrons in the nucleus of an atom is called the mass number. For example, the mass number of a carbon atom with 6 protons and 6 neutrons in its nucleus is 12.


Although all atoms of an element have the same number of protons in their nuclei, they may not all have the same number of neutrons. Atoms of an element with the same mass number make up an isotope of the element. All known elements have isotopes; some have more than others. Hydrogen, for example, has only 3 isotopes, while xenon has 16. Approximately 300 naturally occurring isotopes are known, and more than 2,500 radioactive isotopes have been artificially produced (see synthetic elements). There are 13 isotopes of carbon, having from 2 to 14 neutrons in the nucleus and therefore mass numbers from 8 to 20.

Not all of the elements have stable isotopes. Some have only radioactive isotopes, which decay to form other isotopes, usually of other elements (see radioactivity). In some cases all the isotopes of an element are very unstable, and the element is therefore not found in nature. Only 94 of the elements are known to occur naturally on earth. Of these, 6 occur in minute amounts produced by the decay of other elements. These 6 extremely scarce elements and those that do not occur at all naturally were discovered when they were produced in the laboratory; they are often called the man-made, artificially produced, or synthetic elements.

Atomic Mass and Atomic Weight

Atoms are not very massive; a carbon atom weighs about 2 × 10-23 grams. Because atoms have so little mass, a unit much smaller than the gram is used. In the current system (adopted in 1960–61) the unit of atomic mass, called atomic mass unit (amu), is defined as exactly 1/12 the mass of an atom of carbon-12. The atomic weight of an element is the mean (weighted average) of the atomic masses of all the naturally occurring isotopes. Carbon has two principal naturally occurring isotopes, carbon-12 and carbon-13. Carbon-12, whose mass is defined as exactly 12 amu, constitutes 98.89% of naturally occurring carbon; carbon-13, whose mass is 13.00335 amu, constitutes 1.11%. (There are also small traces of the radioactive isotope carbon-14.) The atomic weight of the element is determined by multiplying the percent abundance of each isotope by the atomic mass of the isotope, adding these products, and dividing by 100. However, isotope abundance is often determined by the medium of the source, solid, liquid, or gas, and the average atomic weight may fluctuate. Thus, for carbon, [(98.89 × 12.000) + (1.11 × 13.00335)]/100 = 12.01115, which is the atomic weight of the element carbon in amu, but because the proportions of the isotopes vary depending on where the carbon is found, carbon's atomic weight is now expressed as an interval defined by the lower and upper bounds within which the atomic weight ranges: [12.0096; 12.0116]. Certain synthetic elements exist only momentarily in the form of a few short-lived isotopes; in such cases the concept of atomic weight cannot be applied.

Properties of the Elements

Properties of an element are sometimes classed as either chemical or physical. Chemical properties are usually observed in the course of a chemical reaction, while physical properties are observed by examining a sample of the pure element. The chemical properties of an element are due to the distribution of electrons around the atom's nucleus, particularly the outer, or valence, electrons; it is these electrons that are involved in chemical reactions. A chemical reaction does not affect the atomic nucleus; the atomic number therefore remains unchanged in a chemical reaction.

Some properties of an element can be observed only in a collection of atoms or molecules of the element. These properties include color, density, melting point, boiling point, and thermal and electrical conductivity. While some of these properties are due chiefly to the electronic structure of the element, others are more closely related to properties of the nucleus, e.g., mass number.

The elements are sometimes grouped according to their properties. One major classification of the elements is as metals, nonmetals, and metalloids. Elements with very similar chemical properties are often referred to as families; some families of elements include the halogens, the inert gases, and the alkali metals. In the periodic table the elements are arranged in order of increasing atomic weight in such a way that the elements in any column have similar properties.

Official Symbols and Names for the Elements

Each element is assigned an official symbol by the International Union of Pure and Applied Chemistry (IUPAC). For example, the symbol for carbon is C, and the symbol for silver is Ag [Lat. argentum = silver]. There are several ways of designating an isotope. One designation consists of the name or symbol of the element followed by a hyphen and the mass number of the isotope; thus the isotope of carbon with mass number 12 can be designated carbon-12 or C-12. The mass number is often written as a superscript, e.g., C12; sometimes the atomic number is written as a subscript preceding the symbol, e.g., 6C12. The IUPAC rules for nomenclature of inorganic chemistry state that the subscript atomic number and superscript mass number should both precede the symbol, e.g., 126C.

Many isotopes were given special names and symbols when they were first discovered in natural radioactive decay series (e.g., uranium-235 was called actinouranium and represented by the symbol AcU). This practice is discouraged in the modern nomenclature except in the case of hydrogen. The isotopes hydrogen-2 and hydrogen-3 are usually called deuterium and tritium, respectively. Hydrogen-1, the most abundant isotope, has the name protium but is usually simply called hydrogen. Newly discovered elements that have been synthesized by one laboratory and not yet confirmed by a second are given a provisional name based on Greek and Latin roots; when the discovery is confirmed, the laboratory that first made it may suggest a name for the element.

The Elements through the Ages

Some elements have been known since antiquity. Gold ornaments from the Neolithic period have been discovered. Gold, iron, copper, lead, silver, and tin were used in Egypt and Mesopotamia before 3000 BC However, recognition of these metals as chemical elements did not occur until modern times.

Greek Concept of the Elements

The Greek philosophers proposed that there are basic substances from which all things are made. Empedocles proposed four basic "roots," earth, air, fire, and water, and two forces, harmony and discord, joining and separating them. Plato called the roots stoicheia (elements). He thought that they assume geometric forms and are made up of some more basic but undefined matter. A different theory, that of Leucippus and his followers, held that all matter is made up of tiny indivisible particles (atomos).

This theory was rejected by Aristotle, who expanded on Plato's theory. Aristotle believed that different forms (eidos) were assumed by a basic material, which he called hulé. The hulé had four basic properties, hotness, coldness, dryness, and moistness. The four elements differ in their embodiment of these properties; fire is hot and dry, earth cold and dry, water cold and moist, and air hot and moist. Although Aristotle 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," he thought the metals to be made of water, and called mercury "silver water" (chutos arguros). His idea that matter was a single basic substance that assumed different forms led to attempts by the alchemists to transmute other metals into gold.

Evolution of Modern Concepts

Although much early work was done in chemistry, especially with metals, and many recipes were recorded, there were few developments in the conception of the elements. In the 16th cent. Paracelsus proposed salt, mercury, and sulfur as three "principles" of which bodies were made, although he apparently also believed in the four "elements." Van Helmont (c.1600) rejected the four elements and three principles, substituting two elements, air and water.

Robert Boyle rejected these early theories and proposed a definition of chemical elements that led to the currently accepted definition. His definition is strikingly similar to Aristotle's earlier definition. In The Sceptical Chymist (1661) Boyle wrote, "I now mean by elements … 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 called perfectly mixed bodies [chemical compounds] are immediately compounded, and into which they are ultimately resolved."

Whereas Aristotle and other early philosophers tried to determine the identity of the elements solely by reason, Boyle and later scientists used the results of numerous experiments to identify the elements. In 1789 Antoine Lavoisier published a list of chemical elements based on Boyle's definition; this encouraged adoption of standard names for the elements. Although some of his elements are now known to be compounds, such as metallic oxides and salts, they were at the time accepted as elements since they could not be decomposed by any method then known.

In 1803 John Dalton proposed (as part of his atomic theory) that all atoms of an element have identical properties (including mass), that these atoms are unchanged by chemical action, and that atoms of different elements react with one another in simple proportions. Although symbols for some of the elements already existed, they were by no means universally accepted, and each compound also had a unique symbol that was unrelated to its chemical composition. Dalton devised a new set of circular symbols for the elements and used a combination of elemental symbols to represent a compound. For example, his symbol for oxygen was ○, and for hydrogen [symbol]. Since he thought water contained one atom of hydrogen for every atom of oxygen, he formed the symbol for water by writing the symbols for hydrogen and oxygen touching one another, [symbol]○. J. J. Berzelius was the first to use the modern method, letting one or two letters of the element's name serve as its symbol. He also published an early table of atomic weights of 24 elements with most values very close to those now in use.

Discovery of the Elements

As noted above, some of the elements were discovered in prehistoric times but were not recognized as elements. Arsenic was discovered around 1250 by Albertus Magnus, and phosphorus was discovered about 1674 by Hennig Brand, an alchemist, who prepared it by distilling human urine. Only 12 elements were known before 1700, and only about twice that many by 1800, but by 1900 over 80 elements had been identified. In 1919 Ernest Rutherford found that hydrogen was given off when nitrogen was bombarded with alpha particles. This first transmutation encouraged further study of nuclear reactions, and eventually led to the discovery in 1937 of technetium, the first synthetic element. Neptunium (atomic number 93) was the first transuranium element to be synthesized (1940). Its discovery prompted the search that has led to the ongoing synthesis of additional transuranium elements.


See J. Emsley, The Elements (1991); A. Swertka, A Guide to the Elements (1996); P. W. Atkins, The Periodic Kingdom (1997); N. N. Greenwood and A. Earnshaw, Chemistry of the Elements (2d ed. 1997).

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An element may be identified in any subject matter that is capable of analysis and synthesis and in any science or art in which such processes are studied. Compounds are known to be such when they can be resolved into simpler parts. The least parts into which anything can be divided, i.e., the ultimate units or parts out of which other things are formed by combination, are called elements. However, elements are only relatively indivisible, that is, in a certain way or context, or from a certain point of view. Thus, the letters of the alphabet and their sounds are elements of speech and writing, although in other respects they are divisible and are not elements.

Element in Philosophy. An element is a kind of material cause from which something is composed in a primary way, as bread is made of flour and water, and a meal of bread and wine (see matter; matter and form). Furthermore, an element is somehow in the compound in a positive way and remains in the compound. It is not like illness, which does not remain in the body after health has been recovered, but rather like nourishing food that does remain after eating. Moreover, an element has a specific character of its own, and one which is simple, that is, not further divisible into other species in the same line of division. Again, just as principles are related to consequences and causes to effects, so elements are related to compounds.

Various Usages. In arithmetic, the unit is the element of which numbers are composed. In geometry, lines determined by points are elements, and also surfaces and solids determined by lines. According to ancient and medieval science, the whole world was thought to be composed of four kinds of elementary bodiesviz, earth, water, air, and fireall transformable one into another, and further entering into the formation of mixtures and compounds. Moreover it was thought that the animal body in temperament and health depends upon the mixture and proportion of four vital elements or humors, viz, blood, phlegm, yellow bile, and black bile.

In modern biology, cells and tissues are the elements of complex organisms, while simple organisms are resolved into nucleus or nuclear materials and cytoplasm with its organic and inorganic parts. In modern physical science the elements are the chemically simple bodies, about 100 in number of kinds, that are classified with natural sequence in the periodic table (see below).

According to another meaning of the word, the elements of a science or art are its primary conceptions and demonstrations. These are the demonstrations that are made, not by long chains of reasoning, but by simple arguments consisting of three terms and employing only one medium of demonstration. In this sense one speaks of the elements of ethics or metaphysics, or of any science or art.

Presence in Compounds. A philosophical problem of importance is concerned with the manner in which elements exist in a compound. Some compounds seem to be mere mixtures of elements that retain their own characteristics and their own identity, as sea water is a mixture of water and various salts. On the other hand, a compound such as water or salt has its own characteristics, and seems to be a natural unit of a kind specifically different from its elements, which do not retain their own characteristics unmodified. In compounds such as these, the elements do not seem to retain their own existence, but have become parts of a new unit and exist in virtue of the new unit (virtualiter ), somewhat as food becomes part of one organism through digestion and assimilation. If the elements retained their own identity, the compound would not be one in kind, but many.

See Also: atomism; principle; causality.

Bibliography: m. j. adler, The Great Ideas: A Syntopicon of Great Books of the Western World (Chicago 1952) 1:400412. a. van melsen, From Atomos to Atom, tr. h. j. koren (New York 1960). v. miano, Enciclopedia filosofica (Venice-Rome 1957) 1:184750.

[w. h. kane]

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el·e·ment / ˈeləmənt/ • n. 1. a part or aspect of something abstract, esp. one that is essential or characteristic: the death had all the elements of a great tabloid story. ∎  a small but significant presence of a feeling or abstract quality: it was the element of danger he loved in flying. ∎  (elements) the rudiments of a branch of knowledge: legal training may include the elements of economics and political science. ∎  (often elements) a group of people of a particular kind within a larger group or organization: extreme right-wing elements in the army. ∎  Math. & Logic an entity that is a single member of a set. 2. (also chemical element) each of more than one hundred substances that cannot be chemically interconverted or broken down into simpler substances and are primary constituents of matter. Each element is distinguished by its atomic number, i.e., the number of protons in the nuclei of its atoms. ∎  any of the four substances (earth, water, air, and fire) regarded as the fundamental constituents of the world in ancient and medieval philosophy. ∎  one of these substances considered as a person's or animal's natural environment: for the islanders, the sea is their kingdom, water their element fig. she was in her element with doctors and hospitals. ∎  (the elements) the weather, esp. strong winds, heavy rain, and other kinds of bad weather: there was no barrier against the elements. ∎  (elements) (in church use) the bread and wine of the Eucharist. 3. a part in an electric teapot, heater, or stove that contains a wire through which an electric current is passed to provide heat. ∎ on some electric typewriters, a ball with raised letters that print when the keys are pressed.

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element Substance that cannot be split into simpler substances by chemical means. All atoms of a given element have the same atomic number (at.no.) and thus the same number of protons and electrons. The atoms can have different atomic mass numbers and a natural sample of an element is generally a mixture of isotopes. The known elements range from hydrogen (at.no. 1) to unnilenium (at.no. 109); elements of the first 95 atomic numbers exist in nature, the higher numbers have been synthesized. See also periodic table

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1. In Christianity, the materials of bread and wine used in the eucharist.

2. In Hinduism, the components and forces which constitute the universe: see BHŪTA.

3. In Buddhism, constituents of appearance, dharma (2).

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element any of the four substances (earth, water, air, and fire) regarded as the fundamental constituents of the world in ancient and medieval philosophy. The word is recorded from Middle English (denoting fundamental constituents of the world or celestial objects) and comes via Old French from Latin elementum ‘principle, rudiment’, translating Greek stoikheion ‘step, component part’.

In late Middle English, elements denoted the letters of the alphabet; from this developed the sense of the rudiments of learning, the first principles of a subject.

From the mid 16th century, element (usually in plural) has also denoted the bread or wine used in the Christian Eucharist.

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element one of the four constituents of the universe (earth, water, air, fire) XIII (whence ult. the use in mod. chem. XIX); constituent portion; pl. rudiments XIV. — (O)F. élément — L. elementum esp. pl. principles, rudiments, letters of the alphabet (used to tr. Gr. stoikheîon step, base, element, etc.), of unkn. orig.
Hence elemental XV. So elementary XVII (earlier elementare XIV, -air XVI). — L. elementārius.