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Chemical Element

A Survey Of The Elements, History Of The Elements, Organization Of The Elements, "orphan" Elements

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

    Percent of all atomsa  
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

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.

Additional topics

Science EncyclopediaScience & Philosophy: Electrophoresis (cataphoresis) to Ephemeral