p- and d-orbitals.
The simplest types of orbital: the s-orbital.
An orbital is a wavefunction that describes what an electron with a given energy is doing, and where it is most likely to be found, inside an atom or molecule. Equivalently, an orbital can be thought of as one of the regions in the space around an atomic nucleus in which electrons are allowed. There is a high probability of finding an electron in such an orbital, which can accommodate one or two electrons and has a shape and energy characterized by the atom's quantum numbers. See also electron configuration.
The simplest type of orbital – the s-orbital – is spherical and can be occupied by a maximum of two electrons. p- and d-orbitals are more complex in shape. There are three p-orbitals in each energy level (above the first) set at right-anges to each other (labeled 1, 2, 3, in the diagram above). d-orbitals come in sets of five (C through G in the diagram).
In molecules, the bonding electrons move in the combined electric field of all the nuclei. The atomic orbitals then become molecular orbitals – regions encompassing two nuclei, having a characteristic energy and containing two electrons. These molecular orbitals, which can be thought of as formed by the overlap of atomic orbitals, constitute chemical bonds.