Question

is the energy of 2s orbital of lithium greater than that of sodium

Answer 1

yes because as atomic no. increases the energy of same orbital decreases as here 2s of lithium has more energy than 2s of Sodium

now be u think what is the role of atomic number so ,
here is ur answer

as atomic number increases no.of protons increases thus nuclear charge increase and electrons get more attracted towards nucleus and u know as the radius of electron from nucleus decrease electron get more stable due to low energy .therefore energy also depends on atomic no. of atoms for Same orbital.

Hope it helps!