s orbital

Meaning: An s orbital is the electron probability distribution with angular momentum quantum number ℓ = 0. It is spherically symmetric, with no angular nodes. Since ℓ = 0 forces mℓ = 0, each s subshell has one orbital, holding at most 2 electrons of opposite spin. It gets larger and higher in energy as n increases (1s < 2s < 3s). In multielectron atoms, s electrons penetrate closer to the nucleus, so 2s lies below 2p.

Equation: Radial nodes = n − ℓ − 1, so for an s orbital, radial nodes = n − 1. Here n is the principal quantum number and ℓ is the angular momentum quantum number. Both are dimensionless integers, so the node count is a pure number.

On the MCAT: Expect electron configurations (1s² 2s² 2p⁶), groups 1 and 2 filling s subshells, and quantum-number sets judged valid or invalid (n = 2, ℓ = 0, mℓ = 1 is invalid). Size and energy comparisons across different n also appear.

Common confusion: Spherical does not mean node-free. Only 1s has no nodes. The 2s orbital has one radial node and 3s has two, as n − 1 shows. Also, an orbital is a probability cloud, not a circular path the electron follows.