[seanifred]

here's the question:

predict which has the higher first ionization energy. provide a short explanation...

Cu or Au

now i've looked at everything i can think of that would help to deduce the answer to this question.  i have considered effective nuclear charges and electron configurations and slater's rules.  i feel like i am missing something.  i know that gold has the higher value, but i have no idea why.

thanks for the help

[Yggdrasil]

It probably has something to do with the Lanthanide contraction. 

Basically, as the 4f orbitals fill, the electrons go into very weakly shielding orbitals.  Hence, as one goes across the lanthanides in the periodic table, the effective nuclear charge (Z_eff) increases more than usual.  Therefore, the elements after the lanthaides (e.g. Au) will have higher Z_eff values than expected.  This can lead to phenomena such as smaller atomic radii than expected and higher first ionization energies than expected.