(i) 1s2 2s2 2p6 3s2 3p6 3d8 4s2
(ii) 1s2 2s2 2p6 3s2 3p6 3d9 4s2
(iii) 1s2 2s2 2p6 3s2 3p6 3d10 4s1
(iv) 1s2 2s2 2p6 3s2 3p6 3d5 4s1
Correct Answer: (ii) 1s2 2s2 2p6 3s2 3p6 3d9 4s2
Explanation: In this case 1s2 2s2 2p6 3s2 3p6 3d9 4s2, the orbitals are not filled properly which affects the stability.