‘Bridging matter’ introduces statistical thermodynamics, which provides the link between the notional insides and outsides of atoms and molecules. It identifies the bulk properties of a sample with the average behaviour of all the molecules that constitute it. A key concept is the Boltzmann distribution, which shows the exponentially decaying function of the energy of molecules as temperature is increased. It captures two aspects of chemistry: stability and reactivity. Statistical thermodynamics is used by physical chemists to understand the composition of chemical reaction mixtures that have reached equilibrium. It also provides an explanation of Le Chatelier's principle, which states that a system at equilibrium responds to a disturbance by tending to oppose its effect.