3S1

3s1 is a shorthand notation used in atomic physics and chemistry to indicate that an atom or ion has one electron in the 3s subshell, corresponding to principal quantum number n=3 and angular momentum l=0. The superscript denotes the occupancy of that subshell; 3s1 means the 3s subshell contains a single electron, with the rest of the atom's electrons arranged in lower shells or other subshells as specified elsewhere in the full configuration.

In practice, 3s1 appears in the standard ground-state configuration of sodium, written as [Ne] 3s1. This reflects that sodium has ten electrons filling the neon core and a single valence electron in the 3s subshell. In ions such as Na+, the 3s electron is removed, giving the noble-gas core [Ne].

From a spectroscopic perspective, a single electron in an s orbital leads to a simple term symbol, typically a doublet S1/2 for the valence electron in an isolated atom. However, in multi-electron atoms, electron–electron interactions can mix configurations, and the overall term symbol depends on coupling schemes.

3s1 is a partial notation; it's frequently used alongside additional occupancy indicators to describe the complete electronic structure. It is most informative when discussed within the context of a full configuration or period in the periodic table, such as the third period alkali metal sodium.