Compounds can be categorized according to their mode of formation into covalent and ionic compounds. Covalent compounds are also referred to as molecular compounds while ionic compounds are also called electrovalent compounds.
In ionic compounds ions are involved in the formation of the compounds. Sodium chloride is an ionic compound formed by the reaction between the positive ions of sodium and the negative ions of chlorine. The Na+ ion and Cl- are formed when an atom of sodium gives away one electron to the chlorine atom. Such compounds have strong inter-ionic forces and thus have high melting and boiling points as well as being good conductors of heat and electricity.
Covalent compounds are formed by the sharing of pairs of electrons. Atoms can share one, two or even three pairs of electrons leading to the formation of a single, a double or a triple covalent bond. Sulphur dioxide and carbon dioxide are examples of such covalent compounds. Such compounds have low melting and boiling points and are poor conductors of heat and electricity.