orbital

/awr"bi tl/, adj.

1. of or pertaining to an orbit.

n.

2. Physics, Chem.

a. a wave function describing the state of a single electron in an atom (atomic orbital) or in a molecule (molecular orbital).

b. the electron in that state.

[1535-45; < NL, ML orbitalis; see ORBIT, -AL¹]

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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.

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▪ chemistry and physics

 in chemistry and physics, a mathematical expression, called a wave function, that describes properties characteristic of no more than two electrons in the vicinity of an atomic nucleus or of a system of nuclei as in a molecule. An orbital often is depicted as a three-dimensional region within which there is a 95 percent probability of finding the electron (see illustration—>).

      Atomic orbitals are commonly designated by a combination of numerals and letters that represent specific properties of the electrons associated with the orbitals—for example, 1s, 2p, 3d, 4f. The numerals, called principal quantum numbers, indicate energy levels as well as relative distance from the nucleus. A 1s electron occupies the energy level nearest the nucleus. A 2s electron, less strongly bound, spends most of its time farther away from the nucleus. The letters, s, p, d, and f designate the shape of the orbital. (The shape is a consequence of the magnitude of the electron's angular momentum, resulting from its angular motion.) An s orbital is spherical with its centre at the nucleus. Thus a 1s electron is almost entirely confined to a spherical region close to the nucleus; a 2s electron is restricted to a somewhat larger sphere. A p orbital has the approximate shape of a pair of lobes on opposite sides of the nucleus, or a somewhat dumbbell shape. An electron in a p orbital has equal probability of being in either half. The shapes of the other orbitals are more complicated. The letters s, p, d, f, originally were used to classify spectra descriptively into series called sharp, principal, diffuse, and fundamental, before the relation between spectra and atomic electron configuration was known.

      No p orbitals exist in the first energy level, but there is a set of three in each of the higher levels. These triplets are oriented in space as if they were on three axes at right angles to each other and may be distinguished by subscripts, for example, 2p_x, 2p_y, 2p_z. In all but the first two principal levels, there is a set of five d orbitals and, in all but the first three principal levels, a set of seven f orbitals, all with complicated orientations.

      Only two electrons, because of their spin, can be associated with each orbital. An electron may be thought of as having either a clockwise or a counterclockwise spin about its axis, making each electron a tiny magnet. Electrons in full orbitals are paired off with opposite spins or opposite magnetic polarities.

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