[quote name='naiku']From what I remember...its will be the 4s, you lose and gain electrons from the outer most shell. This would require the least amount of energy. Anion or cations are also formed at the furthest shell, which has the lowest subshell energy.[/QUOTE]
But, if the 4s shell is full it takes more energy to take those, I think the 3d shell is the valence shell. Chromium and copper don't have the 4s shell full, that would be the valence shell.