Cr: expected [Ar]3d^4 4s^2, actual [Ar]3d^5 4s^1. Mo: expected [Kr]4d^4 5s^2, actual [Kr]4d^5 5s^1. Cu: expected [Ar]3d^9 4s^2, actual [Ar]3d^10 4s^1. Ag: expected [Kr]4d^9 5s^2, actual [Kr]4d^10 5s^1. Reason: half-filled d^5 (5 exchange pairs = 10) and fully-filled d^10 (10 exchange pairs = 45) have higher exchange energy than d^4 (6 pairs) or d^9 (36 pairs). The gain in exchange energy exceeds the small energy cost of promoting one 4s electron to 3d. Pd: exceptional [Kr]4d^10 5s^0 (not 4d^8 5s^2 or 4d^10 5s^0). In ions: 4s electrons are removed first (despite being filled first). Fe^2+ = [Ar]3d^6, not [Ar]3d^4 4s^2.