IE1 generally increases across the 3d series but with irregularities. Values are between s-block (low) and p-block (high). Cr and Cu have slightly higher IE1 than expected (removing electron from half-filled/fully filled d is difficult). However, more important is IE2 and IE3 for +2 and +3 state stability. IE2 is high for Cr (removing from stable 3$d^5$) and Cu (removing from 3$d^{10}$). The sum IE1 + IE2 determines ease of forming $M^{2+}$. For Mn, IE1 + IE2 is unusually high ($Mn^{2+}$ is $d^5$, very stable once formed, but hard to form). Zn has high IE2 (3$d^{10}$ → 3$d^9$ is unfavourable) — no higher oxidation state.