1S2

1S2 is a designation that can refer to different concepts depending on the domain, and there is no single universally assigned meaning. The most common scientific usage is as a shorthand related to electron configurations in atomic structure.

In atomic physics and chemistry, 1s2 (often written as 1s^2) denotes two electrons occupying the 1s orbital, the first shell of an atom. This configuration is characteristic of helium and of helium-like ions (ions with only two electrons, such as Be2+, Li+, and others in their ground states). The 1s orbital can hold at most two electrons, which must have opposite spins due to the Pauli exclusion principle. The 1s^2 configuration represents the ground state for many light elements and serves as the starting point for describing excited configurations such as 1s2s or 1s2p in spectroscopy.

In spectroscopy, term symbols provide a standardized way to label atomic energy levels using the format 2S+1LJ, where S is total spin, L is orbital angular momentum (S, P, D, F, etc.), and J is total angular momentum. A typical example is 1S0 or 3P2. The label “1S2” is not a standard term symbol, and if encountered in literature or data, it is likely a nonstandard notation, a typographical error, or a context-specific shorthand that requires additional information to interpret correctly.

Beyond physics and chemistry, entities such as product codes, project designations, or catalog numbers may also use alphanumeric strings like 1S2, but their meanings are entirely context-dependent.