The dynamic behaviour of small atomic clusters consisting of eight atoms and ten atoms on an Ir(001) surface has been studied using a field ion microscope. It has been found that eight-atom and ten-atom clusters are energetically favoured to form ring configurations with central lattice vacancies in the clusters. The fact that the atomic-ring configurations with fewer nearest-neighbour bonds are more thermally stable than those dense cluster structures with more nearest-neighbour bonds demonstrates convincingly the general believed non-additivity of bond energies at the metal surface.