The coordination number of a central atom in a molecule or crystal is the number of its near neighbors. 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, and has a coordination number of 6.
The forces that hold the atoms together in these complexes and that lead to the observed coordination numbers are of different kinds. The bonds to the highly electronegative fluorine atoms in the fluoride complexes are essentially ionic; and the increase in coordination number, with fluoride ion, from 4 to 6 to 7 for B3+, Fe3+, and Zr4+ is the result mainly of the increase in size of the cation, which permits a successively larger number of fluoride ions to be packed about the central ion.
Coordination Number Examples
Carbon has a coordination number of 4 in a methane (CH4) molecule since it has four hydrogen atoms bonded to it.
In ethylene (H2C=CH2), the coordination number of each carbon is 3, where each C is bonded to 2H + 1C for a total of 3 atoms.