First of all, make the exact formula of the substance. For example, take the well-known from the school course of chemistry of yellow blood salt. Its formula K3[Fe(CN)6]. What is the coordination number of iron ion in this compound? The formula is easy to understand, that first of all iron is associated with the cyanide ions, CN-, therefore, its coordination number is 6.
The concept of "coordination number" is used not only in chemistry of complex compounds, but also in crystallography. Let's consider the more familiar you cook a mole of sodium chloride. Its formula is NaCl. It would seem simple enough to me – and the coordination number of sodium and chlorine is 1. But do not rush to conclusions.
Remember: in a regular, solid sodium chloride has a cubic crystal lattice. In the nodes alternately alternate ions of sodium and chloride associated with "neighbors". And how many "neighbors" each ion? It is easy to calculate that the 6 of them. (Four "horizontally", two "vertical"). It turns out: the coordination number and of sodium and chlorine in the substance is equal to 6.
And what about, for example, with the most famous precious stone – a diamond? What is the coordination number of carbon is part of it? Remember that a diamond is a carbon crystal lattice, the so-called "tetragonal" shapes. Each carbon atom is associated in it with four other atoms, thus the coordination number is 4.
Where else applied the concept of "coordination number"? It can be used when describing the chemical properties of liquid and amorphous substances, in cases where the actual number of chemical bonds of the Central atom does not coincide with its valence. Consider, for example, the widely used compound is nitric acid. Its empirical formula is HNO3 and it follows from this that the valence of nitrogen is clearly more than 3.
Writing structural formula you will see that the nitrogen atom linked to only three oxygen atoms, hence its coordination number is 3.