Increase in attraction as size of cation decreases

[Lakshya]

We explain that as the size of metal cation decreases, the covalent character of the formed molecule increases. The reason given is that as the size decreases, the electron cloud is attracted more towards it as charge density is increased.

Now, if I assume metal cation to be uniformly charged sphere, then I can show from Gauss Law that no matter what is the radius of cation, the field outside at a particular distance from centre remains same. Moreover, if I don't consider it to be uniformly distributed, how can I be sure that the field will increase as size is decreased?

 

[jbsteele]

The answer to this is that the size of a metal cation is not necessarily a uniform charge distribution, but instead depends on the electron configuration of the metal. As the size of the metal cation decreases, the electron configuration changes and thus the charge density increases at the nucleus. This increase in charge density causes an increase in the attraction of the electron cloud to the nucleus, resulting in an increase in the covalent character of the formed molecule.