An ionic bond can form between two elements when one has a small number of electrons in the valence shell (metal) and one has an almost full outer shell (non-metal). Atoms are more likely to accept electrons if they have an almost full outer shell (elements on the right of the periodic table).

Sodium and chlorine form an ionic bond, with sodium giving up an electron from its valence shell and donating it to the chlorine atom to complete its valence shell. 

The non metal atoms attract valence electrons from metal atoms to become negatively charged. Cooperatively, the metal atom becomes positively charged from losing one or more electrons.

It is the attraction between two oppositely charged ions that forms the bond. This bond is non-directional, leading to freedom in the way they pack… BUT ions of opposite sign must surround each other to retain the attraction between ions

Ionic bonds are strong and stiff. As a result they generally give a material with:

  • High strength
  • High elastic modulus
  • High melting point
  • Poor electrical conductivity

Some examples of ionic bonding are:

  • Magnesia (MgO)
  • Alumina (Al2O3)
  • Cement