Physics. The smallest component of an element having the chemical properties of the element. The atom consists of a dense, positively charged nucleus with neutrons and protons surrounded by a system of electrons. It has an approximate diameter of 0.1 nm and characteristically remains undivided in chemical reactions. The number of protons determines the identity of the chemical element.
An atom that has either lost or gained one or more electrons is called ion. Positive ions (cations) are formed by the loss of electrons; negative ions (anions) are formed by the gain of electrons.