The growth of islands during thin film deposition is described in terms of
capture zones that determine the islands' growth rates. The evolution of th
e capture zones themselves is modelled as a fragmentation process when new
islands nucleate, which leads to a derivation of the joint probability dist
ribution of island and capture zone sizes. The model shows robust scaling f
or spontaneous nucleation where the critical island size i = 0, and only a
weak time dependence for i = i. The solutions of the model correspond well
to the joint probability distributions found in thin film deposition simula
tions, and yield good quantitative predictions of the island and capture zo
ne size distributions for the first time.