1s2s2p

1s2s2p is a shorthand electronic configuration used in atomic spectroscopy to denote a three-electron arrangement in which the electrons occupy the 1s, 2s, and 2p subshells. The notation is encountered in discussions of light-atom systems such as lithium-like ions and in certain excited or core-excited states, where a single electron is placed in each of these low-lying orbitals. In formal accounting, the full occupancies would be written with superscripts (for example, 1s^1 2s^1 2p^1 in a three-electron case), and the configuration can give rise to multiple term symbols depending on the coupling scheme used (LS or jj coupling).

In practice, 1s2s2p serves as a label for a manifold of states rather than a single level. The possible total angular momenta and parities arise from coupling the individual electron angular momenta, leading to a variety of terms that can be split by spin-orbit interaction. Transitions involving this configuration appear in the spectra of elements with three-electron systems and in highly charged ions, often in the ultraviolet or X-ray regions for heavier elements. The exact energies of the associated levels depend on the nuclear charge and correlation effects among the electrons.

Relation to other notations is common: for neutral lithium and similar systems, ground-state configurations are typically written as 1s^2 2s^1, while excited configurations may be denoted in shorthand forms that emphasize the 1s, 2s, and 2p occupancy. The 1s2s2p label thus functions as a concise descriptor within a broader framework of electronic structure notation. See also electron configuration, term symbol, LS coupling, and Li-like ions.