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 then 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 2pβ1s repulsion than it is for the 2sβ1s repulsion.Comment: 4 page