How to remember exceptional case of electronic configuration of transition element

Exceptional cases of electronic configuration.

The electronic configuration depends upon stability. Exceptions are related with half-filled and fully filled subshells that are more stable.

Trick:

Look carefully last 2 subshell of transition metal (n-1d) ns, if d orbital becomes  half-filled and completely filled by moving 1 electron from s orbital then transition element will follow exceptional case.

E.g.:  Cr having atomic number 24

Expected: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁴ 4s²

Actual:   1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁵   4s¹                 

Other examples are given below

Copper [Ar] 3d¹⁰ 4s¹

 Molybdenum [Kr] 4d⁵ 5s¹

 Silver [Kr] 4d ¹⁰ 5s¹

Gold [Xe] 4f¹⁴ 5d¹⁰ 6s¹