1211.3240 (W. S. Stacey et al.)

Why is the ground state electron configuration for Lithium $1s^22s$ ?    [PDF]

W. S. Stacey, F. Marsiglio

The electronic ground state for Lithium is $1s^22s$, and not $1s^22p$. 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 $2p-1s$ repulsion than it is for the $2s-1s$ repulsion.

View original: http://arxiv.org/abs/1211.3240