What is the general order of filling orbitals up to 4p, and why is the 4s subshell filled before 3d?

**(i)** General order of filling orbitals up to 4p: 1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p **(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.