Atoms are somewhat unstable if valence electrons do not completely fill outer shells. They are very stable when the outer shells are fully occupied. So, atoms commonly give up, or borrow electrons, producing ions, to obtain this stability. As a result, they may become cations, which have a net positive charge because there are more protons than electrons, or anions, with a net negative charge because there are more electrons than protons.
2.11 Forming an oxygen anion
The schematic drawings in Figure 2.11 show how a neutral oxygen atom can gain two electrons to become an anion. The neutral atom has 8 protons in its nucleus and 8 electrons. It contains two 1s electrons and six electrons in its outer shell (2s and 2p orbitals). Thus, 2 additional electrons are needed to fill the outer shell. Commonly, two electrons move in to stabilize the atom – and the result is an oxygen anion. The ionic charge (-2) is the number of protons (still 8) less the number of electrons (now 10).
Different cations and anions may have different ionic charge. We call all ions with a charge of +1 or -1 monovalent. Those with a charge of +2 or -2 are divalent. Trivalent and tetravalent refer to charges of +3 or -3 and +4 or -4. The oxygen ion shown above is a diatomic anion.
Some elements ionize more easily than others. Those that ionize most easily to become cations are called metallic elements. The degree to which elements are metallic generally decreases from left to right in the Periodic Table. The elements on the right side of the table, which ionize to become anions, are nonmetallic.