Zeff is the change in energy that occurs when an electron is added to an atom. So atoms in group II (alkaline metals) have very low Zeff, while halogens have very high Zeff (becuase they want to accept an electron to fill the octet).
Hope that helps!
I thought the change in energy when an electron is added is electron affinity and Z eff is the the attractive force b/w valence electrons and the nucleus. I could be wrong on this tho...can anyone confirm?