Question

Why chromium has electronic configuration 3d⁵ 4s¹ and not 3d⁴ 4s² ?

Answer 1

Chromium has atomic number 24. 

Its expected and actual electronic configurations are:

₂₄Cr (Expected): 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁴ 4s²

(Actual): 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁵ 4s¹

Explanation: 

i. The energy difference between the 3d and 4s orbitals is very low. 

ii. The d-orbital being degenerate, acquires more stability when it is half-filled (3d⁵) or completely filled (3d¹⁰). 

iii. Electron – electron repulsion results in transfer of one electron from 4s orbital to 3d orbital of Cr. 

This results in half filled 3d⁵ orbital which in turn results in extra stability.