2s

2s refers to the second subshell of the principal energy shell in atoms, designated by the quantum numbers n = 2 and l = 0. It is the first excited s orbital and forms part of the second electron shell. The 2s orbital is spherically symmetric and has no angular dependence, with the magnetic quantum number ml equal to zero.

In a hydrogen-like atom, the 2s state has an energy of E = -13.6 eV / n^2, giving E2 = -3.4 eV. The radial part of the 2s wavefunction contains one radial node, located roughly at r ≈ 2a0, where a0 is the Bohr radius (about 0.529 Å). This node reflects the probability distribution of the electron in the 2s orbital.

In multi-electron atoms, electron–electron repulsion and shielding modify energies and spatial distributions. The 2s subshell typically lies below the 2p subshell in energy, so 2s electrons are filled before 2p electrons according to the Aufbau principle. The 2s orbital is relatively penetrating toward the nucleus, contributing to shielding and influencing chemical behavior, especially in the second period.

Chemical context and examples: in the second period elements, Li has configuration [He] 2s^1, Be has [He] 2s^2, and B through Ne add 2p electrons after filling the 2s subshell, e.g., B: [He] 2s^2 2p^1, C: [He] 2s^2 2p^2, and so on. The occupancy of the 2s subshell helps determine valence properties and the onset of bonding in these elements.