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
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.