An atom is a particle of matter that uniquely defines achemical element. An atom consists of a central nucleus that is usually surrounded by one or more electrons. Each electron is negatively charged. The nucleus is positively charged, and contains one or more relatively heavy particles known as protons and neutrons.

A proton is positively charged. The number of protons in the nucleus ofan atom is the atomic number for the chemical element. A proton has a rest mass, denoted mp, of approximately 1.673 x 10-27 kilogram (kg). A neutron is electrically neutral and has a rest mass, denoted mn, of approximately 1.675 x 10-27 kg. The mass of a proton or neutron increases when the particle attains extreme speed, for example in a cyclotron or linear accelerator.

An early model of the atom was developed by the physicist ErnestRutherford in 1912. He was the first to suggest that atoms are like miniature solar systems, except that the attractive force is not caused by gravity, but by opposing electrical charges. In the so-called Rutherford atom, electrons orbit the nucleus in circular paths. Niels Bohr revised Rutherford's theory in 1913. In the Bohr atom,the negatively charged electrons orbit the nucleus at specific mediandistances. These distances are represented by spheres, called shells, surrounding the nucleus. Electrons can move from shell to shell. When an electron absorbs enough energy, it moves to a larger, or higher, shell. When it loses a certain amount of energy, it falls to a smaller, or lower, shell.

The total mass of an atom, including the protons, neutrons andelectrons, is the atomic mass or atomic weight. Electrons contribute only a tiny part of this mass. For most practical purposes, the atomic weight can be thought of as the number of protons plus the number of neutrons. Because the number ofneutrons in an atom can vary, there can be several different atomic weights for mostelements.

Atoms having the same number of protons, but different numbers ofneutrons, represent the same element, but are known as different isotopes of that element. Theisotope for an element is specified by the sum of the number of protons andneutrons. Examples of different isotopes of an element are carbon 12(the most common, non-radioactive isotope of carbon) and carbon 14 (a less common, radioactive isotope of carbon).

Protons and electrons have equal and opposite charge, and normally anatom has equal numbers of both. Thus, atoms are usually neutral. Anion is an atomwith extra electrons or with a deficiency of electrons, resulting in itsbeing electrically charged. An ion with extra electrons is negatively charged and is called an anion; an ion deficient in electrons is positively charged and is called a cation.

This was last updated in October 2009

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