Calcium oxide is an ionic, chemical compound created by equal transfer of atoms. As described, calcium loses two electrons, giving it a +2 charge and an atom of oxygen picks up two electrons, giving it a –2 charge. This event is called ionic bonding where there’s a complete transfer of valence electrons between atoms. Usually, metals are the ones donating their valence electron (e.g. calcium) and it’s the non-metals who accepts the donated electrons (e.g. oxygen).