Explain why lithium has lower ionisation energy than helium even though lithium has 1 proton more.
He has an electronic configuration of 1s2, Li has an electronic configuration of 1s22s1.
Shielding in He is lower than that of Li since Li will start filling a new shell.
Since the electrons are further away from the nucleus the pull on the outermost electrons will be less.
The Ionisation Energy for Li will be lower than that of He.