SSC CHSL 2023 PYQ — Question 29

The correct answer is option 4 i.e. [Ar}3d¹⁰4s¹

 

The correct electronic configuration of Copper (Cu) is:

1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s¹

OR in short form (using the noble gas core configuration):

[Ar] 3d¹⁰ 4s¹

• Atomic number of Copper (Cu) = 29, meaning it has 29 electrons.
• According to Aufbau's principle, electrons fill orbitals in the order of increasing energy (1s → 2s → 2p → 3s → 3p → 4s → 3d).

However, instead of the expected [Ar] 3d⁹ 4s², copper has [Ar] 3d¹⁰ 4s¹ because:

1. Half-filled (d⁵) and fully-filled (d¹⁰) configurations are more stable due to exchange energy and symmetry.
2. One electron from the 4s orbital shifts to the 3d orbital to achieve greater stability.

This is an exception to the regular filling order and is observed in transition metals like Cu (29) and Cr (24).