Why is the 4s subshell filled before the 3d subshell in the electronic configuration of potassium?

**(i)** Full electronic configuration of potassium (K): 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹ **(ii)** The 4s subshell is filled before 3d because it has lower energy due to shielding and penetration effects. Electrons in the 4s orbital experience less effective nuclear charge compared to 3d, making it energetically favorable to fill first.