As the difference between the electronegativities values for the two atoms in a covalent bond increase the bond will become more polar. As the bond polarity increases the covalent bond becomes more ionic like in character. Eventually as the difference in electronegativity values rises from the two atoms forming the bond rise 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:

• To have a giant ionic lattice type structure
• High melting and boiling points.
• Brittle.
• Spherical anions and cations.
• Dissolves in water to form a solution that will conduct electricity

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 the formation of a model 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 or distorted.

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.

As you can see from the image opposite large metal cations with a +1 charge are not able to distort or polarise the iodide anion and the spherical anion and cation ions would show that the bonding is ionic, however small metal 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 would include:

• Have a molecular structure.
• Insoluble in water.
• Low melting and boiling points

Key Points

• One of the requirements of an ideal ionic bond is that the ions are spherical. However when the non-metal anion in an ionic compound is large and has a low charge it can be distorted or polarised by small highly charged metal cations.
• Small highly charged metal cations can polarise or distort large non-metal anions to form ionic compounds with covalent character. This reinforces the idea that your chemistry teacher has probably mentioned- ionic and covalent bonding are extreme examples, most of the bonding present in compounds has properties of both.

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