The diffusion of single Pb atoms and the formation of stable clusters containing two and three Pb atoms on a Si(111)-(7 x 7) reconstructed surface was studied with variable temperature scanning tunneling microscopy (STM). The numbers of single Pb atoms and Pb atoms in clusters were monitored as a function of time. A new and simple statistical model describing diffusion was developed. This model enables us to estimate the diffusion coefficient and the activation energy of diffusion. The main advantage is the avoidance of questionable counting of jump events in successive STM images. For substrate temperatures of 308, 311, and 313 K we obtained diffusion coefficients of 8.1 x 10(-18), 1.2 x 10(-17), and 1.9 x 10(-17) cm(2)s(-1), respectively, and the activation energy of diffusion equaled E = 0.56+/-0.07 eV. The energy difference Delta E between the binding energies of single Pb atoms at faulted and unfaulted halves of the Si(111)-(7 x 7) unit cells was estimated to be 27+/-9 meV.