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

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