Re: Why is the first ionization energy of Zinc higher than copper?

The radius of the atom has less of an effect than the way the orbitals are 
filled. For copper, the electron state of the unexcited atom is [Ar]4s1 
3d10; for zinc, it's [Ar]4s2 3d10. So the zinc atom wants to keep the s 
shell filled more than the copper atom wants to hold on to the single 
electron it has. As a general rule, the farther to the right you go on the 
periodic table, the more tightly electrons are held. For an excellent 
refernce about the electron states and other data on the elements, I 
recommend the Los Alamos site,