in chemistry, crystallography, and materials science, the coordination number, also called ligancy, of a central atom in a molecule or crystal is the number of atoms, molecules or ions bonded to it. the ion/molecule/atom surrounding the central ion/molecule/atom is called a ligand. this number is determined somewhat differently for molecules than for crystals.
for molecules and polyatomic ions the coordination number of an atom is determined by simply counting the other atoms to which it is bonded (by either single or multiple bonds). for example, [cr(nh3)2cl2br2]− has cr3+ as its central cation, which has a coordination number of 6 and is described as hexacoordinate.