Why is the ground-state electron configuration for lithium 1s

Abstract

The electronic ground state for lithium is 1s22s, and not 1s22p. The traditional argument for why this is so is based on a screening argument that claims that the 2p electron is better shielded by the 1s electrons, and therefore higher in energy than the configuration that includes the 2s electron. We show that this argument is flawed, and in fact the actual reason for the ordering is because the electron-electron interaction energy is higher for the 2p-1s repulsion than it is for the 2s-1s repulsion