Calcium, a group 2 element, naturally loses two electrons in order to become electronically stable. PO43- naturally gains 3 electrons to become more electronically stable. In order to bond with electroneutrality, it is necessary for the ions to bond to create a molecule with 3 Ca2+ ions and 2 PO43- ions, since 3 Calcium ions have a charge of 6+ and 2 phosphate ions have a net charge of 6-. This creates a molecular net charge of 0.
