[Darren]

Why does Cr have a lower Ionisation Energy compared to V? Chromiun has a 3d⁵4s¹ while vanadium has a 3d³4s².

This effect cant be due to the shielding of the 3d subshell right? Since they offer little screening effect on the valence electrons. Then i was thinking that V should have a lower ionisation energy instead of chromium as V has a pair of electrons in the 4s subshell which causes inter-electron repulsion, making it easier to remove the valence electron from V than Cr.


Why does Cu have a larger atomic radius than Ni but a higher ionisation energy than Ni?
Copper has a 3d¹⁰4s¹ while nickel has a 3d⁸4s².

And here, i was thinking in terms of inter-electron repulsion again, which would determine Cu to have a smaller atomic radius than Ni. But the data shows that Ni has a smaller atomic radius instead. Why the contradiction in these two examples?

[ramboacid]

Full subshells will be more stable than half-filled subshells, which will in turn be more stable than other partially filled subshells. In this case, the s orbital is more stable with 2 electrons than with just one, and so it takes more energy to remove an electron from the more stable subshell.

The phenomenon is more clear with full, half-filled, and partially filled p-orbitals. Take a look at this graph of ionization energies of different elements as one moves across a period:
http://4.bp.blogspot.com/_OdZZiyNKEoU/SmETmcVPFjI/AAAAAAAAAd4/H8aeFVmpa7E/s320/Periodicity+-+IE.jpg
Notice how the ionization energy drops after adding a fourth electron to a p orbital; that's mainly due to electron repulsion.

As for the second question, the ionization energy has to do with electrons in the s orbitals, because that's where the first electron will be removed. I don't know the exact reason why Cu would be bigger than Ni, but I can guess the atomic radius has to do with the electrons in the d orbitals, which are probably farther away from the nucleus than the electrons in the s orbital because they are worse at shielding than s electrons (and consequently more penetrating).