Covalent bonds are tied with ionic bonds as the strongest chemical bonds known and they occur when atoms share at least one pair of valence electrons (half are provided by each atom). Valence electrons are electrons found in an atom's outermost electron layer. The number of valence electrons an atom has to share is referred to as its valency. Valency dictates the number of covalent bonds it can form; the periodic table gives values on valency. Elements in the group IV (i.e., the column numbered IV) such as carbon have a valency of 4, hence can form four covalent bonds at most, group V elements have a valency of 3 so can form 3 covalent bonds at most, group VI elements have a valency of 2 so can form 2 covalent bonds, group VII elements have a valency of 1 so can form just one covalent bond. Most covalent bonds in the human body involve nonmetals.
Covalent bonds can be further categorized according to the number (no.) of electron pairs involved: single bonds involve one pair, double bonds involve two pairs and triple bonds involve three pairs.