Transition metals show variable oxidation states because ns and (n-1)d electrons have similar energies. Maximum oxidation state = total (ns + nd) electrons, up to Mn: Sc(+3), Ti(+4), V(+5), Cr(+6), Mn(+7). Beyond Mn, the maximum decreases: Fe(+6), Co(+5), Ni(+4), Cu(+3), Zn(+2). The +2 state is universal (ns^2 removal). Stability of +2 increases left to right. Mn^2+ (d^5) and Zn^2+ (d^10) are exceptionally stable. Higher oxidation states are stabilised by electronegative ligands (F^-, O^2-): CrO4^2- (Cr^+6), MnO4^- (Mn^+7). Lower states stabilised by pi-acceptor ligands (CO, CN^-).