1. The problem statement, all variables and given/known data I'm a complete chemistry novice, just on chapter 1 of my book here. I am tasked with writing down the electron configuration of Copper, atom number 29. 2. Relevant equations I've got my orbital chart here. 29 electrons to fill in. 3. The attempt at a solution I go with: 1s2 2s2 2p6 3s2 3p6 4s2 3d9 On my chart, and I am sorry if I am making a mess of the terms of chemistry here, it shows 4s filling up before electrons fill up 3d. And this is correct according to my school book. So I try to draw it using the normal onion layer model. 2-8-17-2 Then I got wikipedia, and it shows this picture: 2-8-18-1 Is my book wrong? Am I wrong? Is wikipedia wrong here?