1. The problem statement, all variables and given/known data What is the electron configuration for Gd? 2. Relevant equations None 3. The attempt at a solution Since Gd has 64 electrons, I would assume that the electron config. is: [Xe]6s24f8 Apparently this is wrong. The hint the question gives is that the last two orbitals are only half full... so I guess it would be something like: [Xe]6s24f75d1 But why? According to my diagram, the 4f level fills up before the 5d level does, so why wouldn't it take all 14 spaces for the electrons? And if the 4f level doesn't really fill up before the 5d level, then why do lower levels fill up in weird orders like (1s2,2s2,2p6,3s2). Eg. Why does it jump from s to p then back to s?