Atoms tend to unite forming molecules to achieve electronic configurations of greater stability. There are two basic types of links:

  • Ionic bonding results from the transfer of one or more electrons from the less electronegative atom to the more electronegative atom.
  • The covalent bond results from the sharing of one or more electrons between the atoms that form the bond.

The ionic and covalent bonds represent theoretical extremes that do not usually occur in nature, the real bonds have both ionic and covalent character.
When the ionic character clearly predominates over the covalent, the compound is called ionic. In the case where the covalent character of the bond predominates, we speak of a covalent compound. Molecules formed by a single type of atom do not have an ionic character, they are pure covalent bonds (N 2 , O 2 , Cl 2 )