[ortho/meta/parasite]

why is the ionisation energy for Cd (configuration [Kr]4d¹⁰5s² ) greater than that of Sr [Kr]5s². shouldn't the 4d e^- in Cd shield the valence e^- from the nucleus making them easier to remove?

[fledarmus]

Only partially. If you compare a 4d orbital to a 5s orbital you will note that a single electron in a 4d orbital doesn't shield the nucleus much. A lot of the electron density in the 5s orbital is in areas that are completely unblocked from the d electron. Since the nuclear charge has risen by 1 as well, the electrons in the 5s orbital are feeling an increased nuclear charge with very little shielding, and the radius shrinks. This process continues until you have 1 electron in each d orbital to get a full sphere, and even a little beyond, before there are enough electrons in the d orbitals to really have an effect. If you look at the radius of all the atoms between Sr and Cd, you will note that they shrink up to Ru before starting to get a little larger again. By the time you get to Cd, you have increased the nuclear charge by +10, and haven't blocked all of it, so the 5s electrons are feeling a stronger charge.

Selected atomic radii below:

[ortho/meta/parasite]

thanks man massive help. I wasn't really thinking about the physical shape of the orbitals but the diagrams help the explanation make perfect sense