In contrast the PES for the reaction H + Cl 2 → HCl + Cl is repulsive (or late-downhill) because R* HCl < R* ClCl and the transition state is reached when the products are separating. For this reaction in which the atom A (here H) is lighter than B and C, the reaction energy is released primarily as translational kinetic energy of the products. For a reaction such as F + H 2 → HF + H in which atom A is heavier than B and C, there is mixed energy release, both vibrational and translational, even though the PES is repulsive.