1S1

1s1 is a compact notation used in atomic physics and chemistry to indicate that a single electron occupies the 1s subshell, the first and closest atomic orbital to the nucleus. It most commonly refers to hydrogen, whose ground-state electron configuration is 1s1. In this usage, the numbers and letter denote the principal quantum number (n = 1) and the angular momentum type (s, for l = 0), with the final digit indicating occupancy.

In multi-electron atoms, the 1s subshell is filled with two electrons (1s2). Therefore, a 1s1 configuration points to a one-electron system or a hydrogen-like ion—an ion with only one electron orbiting a nucleus with a positive charge. The energy of the 1s orbital in the simple hydrogen model is −13.6 eV, and higher energy levels follow the Rydberg formula E_n = −13.6 eV / n^2. Transitions to the 1s state give rise to spectral lines such as those in the Lyman series (ultraviolet region) in hydrogen spectroscopy.

The term 1s1 is thus primarily used to describe hydrogen and hydrogen-like species, where the electronic structure is the simplest quantum-mechanical system, or to denote the occupancy of the first shell in theoretical discussions. In contrast, neutral atoms with more electrons are described by configurations like 1s2 2s2 2p6, reflecting full occupancy of the inner shells.

See also: electron configuration, hydrogen atom, spectroscopy, hydrogen-like ions.