The other case of special importance is when a molecule D dissociates into two atoms upon adsorption. Here, the following assumptions would be held to be valid:
# D completely dissociates to two molecules of D upon adsorption.
# The D atoms adsorb onto distinct sites on the surface of the solid and then move around and equilibrate.
# All sites are equivalent.
# Each site can hold at most one atom of D.
# There are no interactions between adsorbate molecules on adjacent sites.
Using similar kinetic considerations, we get
The 1/2 exponent on p</sub> arises because one gas phase molecule produces two adsorbed species. Applying the site balance as done above,