1S

1s refers to the first electron shell or subshell in atoms. It corresponds to the principal quantum number n = 1 and azimuthal quantum number l = 0, making the 1s orbital spherically symmetric. The 1s orbital can hold up to two electrons with opposite spins, in accordance with the Pauli exclusion principle. Because it lies closest to the nucleus, electrons in the 1s level are the most tightly bound and typically form the atom’s core.

In hydrogen and hydrogen-like ions, the 1s state is the ground state, with energy determined primarily by the Coulomb attraction to the nucleus. In multi-electron atoms, inner electrons shield outer electrons but the 1s level remains the deepest occupied shell and is highly localized near the nucleus. Transitions involving the 1s level can produce characteristic X-ray emissions and other spectral lines, informing studies of atomic structure and material composition.

Notation and examples are common in atomic physics and chemistry. Electron configurations use 1s to denote occupancy of the first shell, such as 1s^1 for hydrogen and 1s^2 for helium. In many-electron atoms, the 1s electrons are regarded as core electrons that are largely nonreactive in chemical bonding, while outer shells determine chemical properties. The concept of the 1s shell is foundational to understanding atomic structure, spectroscopy, and the ordering of electron shells in elements.