Hi, For single-electron atomic systems, the electron can be specified by four quantum numbers n, l, m_l, m_s (principal, orbital, z-orbital, z-spin). The orbital quantum numbers are well defined since the problem is spherically symmetric. However, for many-electron systems, the spherical symmetry is broken due to the inter-electronic interaction. It seems to me that the orbital quantum numbers would then lose their meaning. Still, the configurations are labelled as 1s1s2p, as if the electrons didn't interact at all. This might be a good approximation for highly stripped positive ions such as Lithium-like Uranium, but surely this can't be the case for neutral Lithium?