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why do the covalent character increase, when smaller the cation and larger the anion?
When the cation is very small, it will have a higher charge density than a larger one, as if both are M+, the smaller one will have a greater positive charge per unit area as it were.

A large anion is vulnerable to being polarised, that is to say distorted in shape, by such a cation with high charge density, as the outer electrons are further away from the +ve nucleus, and so there is less attraction to the nucleus, as charge attraction follows an inverse square law.

This polarisation is caused by the cation 'pulling' some of the electron density towards itself from the anion. This constitutes some sharing of elections between two nuclei, i.e. a partial covalent bond. Although the ions are still charged, the interactions between them are not purely electrostatic, and so their is partial ionic and partial covalent character present.


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