Mathematical expression, called a wave function , that describes properties characteristic of no more than two electrons near an atomic nucleus or molecule.
An orbital can be considered a three-dimensional region in which there is a 95% probability of finding an electron. Atomic orbitals are designated by a combination of numerals and letters (e.g., 1s, 2p, 3d, 4f). The numerals are the principal quantum number and are related to the atomic energy level and distance from the nucleus; the letters indicate the orbital's angular momentum and hence its shape. An s orbital with zero net orbital angular momentum is spherical. A p orbital with one fundamental unit of angular momentum, ħ, is shaped somewhat like a dumbbell (ħ is Planck's constant, h , divided by 2π). The shapes of the other orbitals are more complicated. Molecular orbitals have geometries determined by the overlap of two or more atomic orbitals and are designated by Greek symbols, e.g., σ and π.