Note: It is also important to know that there are a few exceptions to the electron configuration rules in the transition metals. Half-filled and filled subshells offer greater stability. Elements in Groups 6 and 11 are expected to have nearly half-filled or nearly filled d subshells. Instead they borrow one electron from the highest s subshell so they end up with a half-filled s subshell and a half-filled or filled d subshell. This phenomenon is most likely to appear on the MCAT® with the elements Cr and Cu, which have only one electron in the 4s orbital. The electron configuration of Cr is [Ar] 4s₁ 3d₅ and the electron configuration of Cu is [Ar] 4s₁ 3d₁₀. Know these two notable exceptions in case they appear on the MCAT®.