text: The reaction H + Br → 2 HBr proceeds by the following mechanism:
  * Initiation 
  : Br → 2 Br• (thermal) or Br + hν → 2 Br• (photochemical)
  : each Br atom is a free radical, indicated by the symbol "•" representing an unpaired electron.
  * Propagation (here a cycle of two steps)
  : Br• + H → HBr + H•
  : H• + Br → HBr + Br•
  : the sum of these two steps corresponds to the overall reaction H + Br → 2 HBr, with catalysis by Br• which participates in the first step and is regenerated in the second step.
  * Retardation (inhibition)
  : H• + HBr → H + Br•
  : this step is specific to this example, and corresponds to the first propagation step in reverse.
  * Termination 2 Br• → Br
  : recombination of two radicals, corresponding in this example to initiation in reverse.
  As can be explained using the steady-state approximation, the thermal  reaction has an initial rate of fractional order (3/2), and a complete rate equation with a two-term denominator (mixed-order kinetics).