Ionic bonding occurs between charged particles. These may be atoms or groups of atoms, but this discuss will be conducted in terms of single atoms. Ionic bonding occurs between metal atoms and nonmetal atoms. Metals usually have 1, 2, or 3 electrons in their outermost shell. Nonmetals have 5, 6, or 7 electrons in their outer shell. Atoms with outer shells that are only partially filled are unstable. To become stable, the metal atom wants to get rid of one or more electrons in its outer shell. Losing electrons will either result in an empty outer shell or get it closer to having an empty outer shell. It would like to have an empty outer shell because the next lower energy shell is a stable shell with eight electrons.
Since electrons have a negative charge, the atom that gains electrons becomes a negatively charged ions (aka anion) because it now has more electrons than protons. Alternately, an atom that loses electrons becomes a positively charged ion (aka cations). The particles in an ionic compound are held together because there are oppositely charged particles that are attracted to one another.
The images above schematically show the process that takes place during the formation of an ionic bond between sodium and chlorine atoms. Note that sodium has one valence electron that it would like to give up so that it would become stable with a full outer shell of eight. Also note that chlorine has seven valence electrons and it would like to gain an electron in order to have a full shell of eight. The transfer of the electron causes the previously neutral sodium atom to become a positively charged ion (cation), and the previously neutral chlorine atom to become a negatively charged ion (anion). The attraction for the cation and the anion is called the ionic bond.
Generally, solid materials with ionic bonds:
- are hard because particles cannot easily slide past one another.
- are good insulators because there are no free electrons or ions (unless dissolved or melted).
- are transparent because their electrons are not moving from atom to atom and less likely to interact with light photons.
- are brittle and tend to cleave rather than deform because bonds are strong.
- have high melting point because ionic bonds are relatively strong.