An isolated molecule of HI does not split apart spontaneously into H and I atoms, which wait until two more H and I atoms drift by before becoming and . Far too much energy would be required to pull the HI molecule apart; and once done, the hyper-reactive H and I atoms would promptly recombine. Reaction takes place only when two molecules of HI collide with enough energy to change partners and end as and .

The chance that any one HI molecule will react within the next second depends on how many other HI molecules there are to collide with it, or on [HI]. The overall rate of reaction of all HI molecules depends on how many molecules there are, and on the chance that any one will react:

= (No. of HI to react) x (likelihood of reaction per HI)
= [HI]x [HI]
= [HI] [HI]
= [HI]