The ionisation energies increase with increase in atomic number. However, the trend is irregular among some d-block elements. On the basis of electronic configuration, the Zn:1s22s2p63s2p6d104s2 Fe:1s22s2p63s2p6d64s2 Cu:1s22s2p63s2p6d104s1 Cr:1s22s2p63s2p6d54s1 IE1 follows the order Zn>Fe>Cu>Cr