Atom

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Just as a MOLECULE is the smallest quantity of a substance that retains the properties of that substance, so an atom is the smallest quantity of an ELEMENT possessing the properties of that element—that is, it is the smallest amount of an element able to take part in chemical reactions. Atoms are composed of a NUCLEUS (which has a proton and a neutron) and electrons, circling the nucleus. Protons are positively charged; neutrons are neutral; and electrons have a negative electrical charge.

The ATOMIC NUMBER of an element is the same as the number of protons in the nucleus. (Discussion of the phenomena of subatomic particles is not appropriate here; see Campbell, Reece & Mitchell for further detail.) The mass (or weight) of an atom is measured in Daltons, after John Dalton (1766–1844), the eminent British scientists who pioneered atomic theory.

See also: atomic weight; periodic table of the elements

Campbell N.A., Reece J.B. & Mitchell L.G. (1999) Biology, 5th edn, Addison-Wesley: Menlo Park CA.

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