Using the concepts of shielding and penetration, explain why a ground state configuratin 1s2 2s2 for an?

Question

Litium atom is energetically preferred over 1s2 2p1

Answer 1

Li is 1s2 2s1, not 1s2 2s2.

The 2s orbital is, on average, closer to the nucleus than 2p, and so is lower in energy. 2p is slightly shielded by 2s, and very much shielded by 1s, and so is higher in energy. So the outer electron goes into the sub-level of lowest energy, which is 2s.