A cation is formed when an atom loses electrons. An atom that loses electrons loses negative charge; therefore, it becomes positively charged. These atoms have a low affinity or attraction for electrons and are called electropositive atoms.
According to the octet rule, atoms seek to gain or lose electrons to achieve eight electrons in their outermost energy levels, which is the maximum allowable number of electrons in these energy shells and the most stable form of an atom. Elements on the periodic table that have the maximum number of electrons in their outermost energy levels are the noble gases. These elements are the most stable of all elements, and they are reluctant to form compounds with others. All other elements wish to mimic the noble gases in terms of electron configuration.
Atoms that have few electrons in their outer energy levels tend to lose electrons to other atoms. The elements most likely to become cations are those of the alkali metal and alkaline earth metal groups of the periodic table, because they are the most electropositive elements.
Electrons lost by cations are picked up by strongly electronegative elements, which lie on the right side of the periodic table. The atoms that pick up electrons to become more stable are called anions. Since opposite charges attract, cations and anions typically form bonds, thus becoming neutral compounds. The formation of bonds between cations and anions is called an ionic bond, and these compounds are called ionic compounds.