| Definition: | basic constituent particle of any substance, electrically neutral, consisting of a single central part having a positive electric charge and nearly all the mass, surrounded by electrons NOTE 1 The motions and combinations of atoms explain macroscopic properties of the substance. NOTE 2 If an atom loses or accepts one or more electrons, it becomes an ion. NOTE 3 The electric charge of the central part, called atomic nucleus, determines the number of electrons; and by this the chemical behaviour of the atom, i.e. its ability to produce combinations with other atoms. A chemical element consists of atoms, the atomic nuclei of which have the same number of protons.
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