An example of an expanded octet is the compound . In this compound, phosphorous is acting as a metal in that it shares its five electrons. Since chlorine only needs one electron to complete its outer shell, one phosphorous atom can bond with five chlorine atoms. Phosphorous then has ten electrons in its outer shell.
An example of an incomplete octet is the compound . Boron shares its three outer electrons with three chlorine atoms. The chlorine atoms then have filled outer shells. The boron atom only has six electrons in its outer shell and an empty p orbital. Boron trichloride is stable, but since boron has only six electrons in its outer shell, it is possible for the compound to bond with another compound via the empty p orbital. For example, the nitrogen atom in has a lone pair (unbonded pair) of electrons. The nitrogen atom can share its lone pair with the boron atom such that both atoms have a filled outer shell. In this situation, boron trichloride is a Lewis acid and ammonia is a Lewis base.
A Lewis acid is an atom or molecule that can accept an electron pair. A Lewis base donates an electron pair. The bond between the boron and the nitrogen atoms consists of a pair of electrons donated from the nitrogen atom. This type of bond is a coordinate covalent bond in that all electrons for a given bond come from one source.
