Why is the ground-state electron configuration for lithium 1s²2s?

¹ Department of Physics, University of Alberta - Edmonton, Alberta, Canada, T6G 2E1
² Physics Division, School of Science and Technology, University of Camerino - I-62032 Camerino (MC), Italy, EU

Received: 6 September 2012
Accepted: 13 November 2012

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.