As the difference between the electronegativities values for the two atoms in a covalent bond increase the more polar the bond becomes, as the bond polarity increases the covalent bond becomes more ionic like in nature. Eventually as the difference in electronegativity values rises to around 1.7-2 the bond is more ionic than covalent and we consider it to an ionic bond with the usual properties we expect of an ionic compound, that is:
In an ionic compound when the size difference between the
and the non-metal anion
is not that different the ions are spherical and this results in a typical
However as the metal cation get smaller and their charge increases they are able to attract the
electron density around the non-metal anions back towards the metal ion. This results in the
spherical non-metal anions becoming oval or egg shaped, we say they are
polarised (polarised simply means that the electron density around an atom
has been squashed or stretched or distorted in someway). The larger
the non-metal anion and the smaller and more highly charged the
metal cation the more the anion is polarised.
As the metal cation attracts the electron density back towards it from the non-metal anion then the electron density is in effect being shared between the two ions, this is what we expect in a covalent bond. What we end up with is an ionic bond with some degree of covalent character. The amount of covalent character present in an ionic bond will depend upon the charge/size ratio of the metal cation and the size of the non-metal anion. The diagram opposite uses the large iodide anion as an example.
Large metal cations with a 1+ charge are not able to distort or polarise the iodide anion and the spherical ions means the bonding is ionic, however small cations with large charges such as Mg2+ or Al3+ will polarise the large iodide anion and result in an ionic compound with covalent characteristics. Covalent characteristics are: