How will the rate of diffusion of an interstitial i Impurity atom Increases? CIRCLE THE BEST ANSWER Increase Decrease The rate of diffusion will be unaffected by changes In the size of the impurity species. impurity species change as the size of the impurity species. Solution The rate of diffusion will decrease as the size of the impurity atoms increases because greater the size of impurity atoms greater will be the difficulty in accomodating the impurity atoms at interstitial sites and also larger size will decrease its mobility and provide hindrance during its movement from one site to another. all these factors contribute in decrease in rate of diffusion..