polarisation of ionic bonds

Polarisation of ionic bonds

Covalent or ionic?

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 character. 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:

Just as a covalent bond can have some ionic nature to it when it becomes a polar covalent bond an ionic bond can have a degree of covalent character. In a purely ionic bond the ions are spherical. However when the non-metal ion present in an ionic bond is large it can become distorted and is no longer spherical, it can become oval or egg shaped. To help and explain how this happens consider the sizes of some common metal cations and non-metal anions. Metals lose electrons to form cations and as a consequence of this they shrink down in size. Whereas non-metal atoms tend to gain electrons to form anions and as a consequence of this their size increases a lot. This is shown in the diagram below:

relative sizes of metal cations and non-metal anions when compared to the metal ans non-metal atoms.

The polarising power of metal cations

The polarising power of metal cations-Metal cation with a large charge to size ratio are able to polarise large non-metal anions such as the iodide ion. In an ionic compound when the size difference between the metal cation and the non-metal anion is not that different the ions are spherical and this results in a typical ionic bond. 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 some way). 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:

The Al3+ cation has such a large charge to size ratio that it is able to distort smaller anions than the iodide ion. The Al3+ cation is able to distort or polarise the much smaller chloride anion to such an extent that aluminium chloride is a covalent substance and not ionic as you might have expected.

Key Points