Surface atoms are activated as result of replacement collisional sequences in solids by incident ions. In dependence on theirs threshold (displacement) energy and angular distribution they may be sputtered (removed) or laterally relocated on the surface. The relocation length distribution depends on the energy and angular distributions of activated atoms and interaction atom-solid potential. The process of lateral relocation of surface atoms is considered as a sequence of stochastic removal and adsorption processes. The rate equations describing processes of sputtering, relocation and thermal diffusion are built and steady state solutions are presented for multielemental solids. The mechanisms of stochastic mixing of atoms and roughening of surface is discussed.