Cr: expected [Ar]3$d^4$ 4$s^2$, actual [Ar]3$d^5$ 4$s^1$. Mo: expected [Kr]4$d^4$ 5$s^2$, actual [Kr]4$d^5$ 5$s^1$. Cu: expected [Ar]3$d^9$ 4$s^2$, actual [Ar]3$d^{10}$ 4$s^1$. Ag: expected [Kr]4$d^9$ 5$s^2$, actual [Kr]4$d^{10}$ 5$s^1$. 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]4$d^{10}$ 5$s^0$ (not 4$d^8$ 5$s^2$ or 4$d^{10}$ 5$s^0$). In ions: 4s electrons are removed first (despite being filled first). $Fe^{2+}$ = [Ar]3$d^6$, not [Ar]3$d^4$ 4$s^2$.