Molecules are coated with a layer of electrons which occupy bonding and maybe nonbonding orbitals. As a result, the surface of each molecule is negatively charged and by and large molecules repel each other. Reactions can occur only if a pair of molecules have enough energy to overcome this superficial repulsion. If they don’t, they will simply bounce off one another like two balls in pool or snooker, exchanging energy and moving off with new velocities, but remaining chemically unchanged. That minimum energy requirement for reaction—a barrier over which molecules must pass if they are to react—is known as the activation energy. In any sample of a compound, the molecules will have a range of energies, but at least some must have more than the activation energy if they are to react.