Molecular dynamics (MD) simulations of CF3+ ion bombardment of Si predict the formation of a steady-state fluorocarbosilyl mixing layer that actively participates in the etching of the underlying Si. The active nature of this mixing layer has been characterized by computing atomic residence time distributions (RTDs) for adsorbed fluorine and carbon. The average residence time of carbon in the layers is seen to increase dramatically as ion energy increases, while that of fluorine is not sensitive to ion energy. The overall RTDs compare well with those of an ideal stirred tank. A simple "well-mixed" transient mass balance model is presented. The phenomenology of this model is based on interpretations of the MD results. The model correctly predicts the evolution of atomic concentrations in the mixing layer. Both the MD and model results shed new light on how CF3+ ions etch Si.