Cao is the chemical formula for calcium oxide, which is an ionic compound. Ionic compounds are formed when atoms lose or gain electrons to achieve a stable electron configuration. In the case of calcium oxide, calcium atoms lose two electrons to become Ca2+ ions, while oxygen atoms gain two electrons to become O2- ions. The Ca2+ and O2- ions are then attracted to each other by their opposite charges, forming an ionic bond.

Covalent bonds, on the other hand, are formed when atoms share electrons in order to achieve a stable electron configuration. This is in contrast to ionic bonds, where electrons are transferred from one atom to another.