2s2p

2s2p is a shorthand label used in atomic spectroscopy and quantum chemistry to denote a two-electron configuration in the second electron shell (n=2), with one electron occupying the 2s subshell (l=0) and the other occupying the 2p subshell (l=1). This configuration can arise as an excited-state arrangement in many atoms and ions that have two electrons in the n=2 shell, alongside other electrons in inner shells.

In LS coupling, the two electrons can couple to give total orbital angular momentum L = 1 (P) and total spin S = 0 (singlet) or S = 1 (triplet). Thus the 2s2p configuration gives rise to the terms 1P1 and 3P0,1,2. The exact fine-structure ordering depends on the element, electron repulsion, and spin-orbit interaction. Often, the triplet 3P_J levels lie near or below the corresponding singlet 1P_J levels in light atoms.

Transitionally, 2s2p states contribute to spectral lines in the ultraviolet and visible regions of atoms and ions, particularly in Be-like and B-like species where excited configurations with two electrons in the n=2 shell are common. Energies and wavelengths are calculated in atomic-structure studies and used to identify lines in laboratory plasmas and astrophysical sources.

Relation to other n=2 configurations: The other two-electron configurations in the same shell are 2s^2 and 2p^2, providing a complete set of low-lying states for the n=2 shell. Together they form part of the ladder of excited configurations in two-electron systems.