definition of atoms
atom. n. 1. (Atomic Physics). a. the smallest quantity of an element that can take part in a chemical reaction. b. this entity as a source of nuclear energy: the power of the atom.
atÂ·om. The smallest unit of an element, consisting of protons and neutrons in a dense central nucleus orbited by a number of electrons. In electrically neutral atoms, the number of protons equals the number of electrons.