# In which cases are there electron-electron repulsion?

Does electron-electron repulsion takes place within an electron shell, between shells or both? So if it takes place between shells, can we say that electron-electron repulsion between electron shells increases the atomic radius?

Thanks!

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The atomic or ionic radius actually is a product of two factors, one of which is electron-electron repulsion. The other is attraction between the nucleus and the outermost electron.

We often talk about Zeff, the effective nuclear charge, which is the charge the outermost electron would "see" the nucleus have when all of the repulsion between electrons is taken into account.

A simple set of rules, called Slater's rules, gives s decent approximation of Zeff. http://en.wikipedia.org/wiki/Slater's_rules

The bottom line, any time we are talking about attraction or repulsion, is Coulomb's law.

F = Qq1q2 / d^2

The force of attraction or repulsion is given by a constant times the magnitudes of the charges divided by the distance between them squared.

So, finally, for your question. Electron-electron repulsion occurs between any two electrons in the atom. The amount of repulsion depends on the distances between them. The amount of repulsion experienced by the outermost electron, compared to the attraction the electron feels for the nucleus, dictates the distance between them and hence the atomic or ionic radius.

"...there's an example : Zn = [Ar]~3d10 - 4s2 ... the repulsion of full 3d shell is very big so the atomic radious of zinc id much bigger than the atomic radious of copper which is immediately before zinc ..."

I must protest. It would appear that Sadra has no idea about what he/she is talking. Both copper and zinc have a filled 3d-sublevel.

Cu -- [Ar] 4s1 3d10 ...........145 pm

Zn -- [Ar] 4s2 3d10 ...........142 pm

Copper is in fact slightly larger than zinc. The fact that a zinc atom is slightly smaller can be attributed to the effective nuclear charge of Zn. Zinc has one more proton and essentially identical electron repulsion as does copper. Therefore, the greater nuclear charge of zinc pulls on the outermost electron(s) with a bit more force and makes the zinc atom slightly smaller.