1a. A part or particle considered to be an irreducible constituent of a specified system. b. The irreducible, indestructible material unit postulated by ancient atomism. 2. An extremely small part, quantity, or amount. 3.Physics & Chemistrya. A unit of matter, the smallest unit of an element, having all the characteristics of that element and consisting of a dense, central, positively charged nucleus surrounded by a system of electrons. The entire structure has an approximate diameter of 10-8 centimeter and characteristically remains undivided in chemical reactions except for limited removal, transfer, or exchange of certain electrons. b. This unit regarded as a source of nuclear energy. See table at subatomic particle.
ETYMOLOGY:
Middle English attome, from Latin atomus, from Greek atomos, indivisible, atom : a-, not; see a–1 + tomos, cutting (from temnein, to cut; see tem- in Appendix I).
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