These reactive H atoms then combine more rapidly with other molecules that collide with the surface. A particularly simple reaction involving H2 is isotope exchange with D2. The probable series of events for the reaction
 

is shown in the panels opposite(fig 15).

In the first panel, an H2 molecule is pulled apart as it binds to two metal atoms on the catalytic surface. The energy that is required to dissociate the H2 molecule is gained from the energy of the two H-metal bonds that are formed.

In panel two, a D2 molecule approaches the catalytic surface, and in panel three, one end of the molecule binds to another site on the surface. By forming a weak D-metal bond, the molecule weakens its own internal D-D bond, thereby making it more susceptible to attack by the nearby bound H atom.

In panel four the central D atom is shared equally between H and D in a configuration that is analogous to the activated complex in the uncatalyzed reaction. This activated complex could decompose equally well in two ways: on to panels five and six, or back through panels three and two.

In about half the cases it will continue on as in panel five, breaking the D-D bond entirely and leading in panel six to a released H-D molecule and a deuterium atom bound to the surface.