Cấu hình electron của Cu (Z = 29) là:
A. 1s$^{2}$2s$^{2}$2p$^{6}$3s$^{2}$3p$^{6}$d$^{9}$4s$^{2}$.
B. 1s$^{2}$2s$^{2}$2p$^{6}$3s$^{2}$3p$^{6}$3d$^{10}$4s$^{1}$.
C. 1s$^{2}$2s$^{2}$2p$^{6}$3s$^{2}$3p$^{6}$4s$^{2}$3d$^{9}$.
D. 1s$^{2}$2s$^{2}$2p$^{6}$3s$^{2}$3p$^{6}$4s$^{1}$3d$^{10}$.