Atoms tend to lose and or gain electrons thus forming ions, primarily because of the fact that the atoms want to achieve stability in the sense that they want to have a stable octet and or electron shell. By doing this metals, will most likely obtain a stable, and or full shell, and likewise the nonmetals, and as a result the atoms are in a state, of stability, they are not as reactive prior to having the electron in their shell, they will thus form an ionic bond, due to the attraction that occurs between the ions.