For an atom, coordination number can be determined by counting all those other atoms which are bonded to it. Bonding may be single or multiple bonding.
Let’s take an example to understand it.
Coordination number of Pt and Ni in [PtCl 6] 2− and [Ni(NH 3 ) 4] 2+ can be calculated using steps shown below which gives coordination number 6 and 4 respectively for Pt and Ni.
However there are less clearly defined bonds in Solid- state structures of crystals and hence we use a simpler Mode in which touching spheres shows atoms. In this simpler model, coordination number of an atom is number of other atoms which are touching it.
When an atom is situated in interior of crystal lattice then coordination number is total number of atoms that touches given atom and is called as bulk coordination number.
If given atom is located at surface of crystal lattice then coordination number is called as surface coordination number.
Process which describes how to find the coordination number is shown below:
1) First write down the chemical formula for coordination complex whose coordination number is to be calculated.
2) Name must be broken into smaller parts if question is given in form of a name. The ligands with name are arranged in alphabetical order
3) If a chemical formula is given then in chemical formula find out the atoms which are ligands.
4) Find sum of all ligands which we found in name or chemical formula of coordination complex.
5) Coordination number is the summation of total number of ligands