Typically observed between reactive metals and non-metals.
Atoms have large differences in their tendencies to lose or gain electrons.
The metal atoms lose their electrons and become cation(positively charged).
The non-metal atoms gain electrons and become anion (negatively charged).
Ionic compound is formed by the electrostatic attraction between these cations and anions into a 3D array. Chemical formula represents the cation-to-anion ratio.