Atom. An atom is defined in modern chemistry as the smallest portion of matter which can take part in a chemical change. It is not divisible by any forces at present at our disposal. Atoms are not conceived to be capable of existing singly; but always in combination with at least one other atom to form a molecule. If the atoms in the molecule are alike, as with oxygen, mercury, gold, etc., the substance formed by an aggregation of such molecules is called an element; otherwise it is a compound. There can, of course, be no atom of a compound body, and the term is therefore restricted to the ultimate particles of the elements.