To consider the structure of a liquid, it is useful to define the radial distribution function g(r) which is the representation of the probability of finding a given atom or molecule in a shell of thickness dr from a central atom or molecule. The quantity n(r)dr is then the number of atoms or molecules whose distance from the origin is between r and r+dr and is given by
This type of distribution function is characteristic of liquids. Even for a liquid as complex as water, we can see in Figure 3.2 that the order does not extend above three molecular layers. We can therefore conclude that a liquid differs from a solid by the value of the structural order. In a solid, this order extends over all the molecules with a few faults spread through the network. In a liquid, the structural order extends only to subensembles, whose size varies constantly. Thus the coordination number of a molecule in a liquid varies perpetually (between 4 and 5 in the case of water), whilst it is fixed in a solid.