Helium-radon clusters will migrate upwards provided the clusters are made up of one radon atom and more than 8 helium atoms. With less than 8 the atoms, the specific gravity of the helium-radon clusters will be heavier than their buoyancy and the clusters will migrate downwards. Hence the average number of atoms will be greater 1a upwards-migrating clusters. The mathematical calculation has been done to obtain the average atomicity of clusters from experimental radon data. The results show that the number of α particles provided by uranium ore is insufficient, if not considering the diffusion of radon, because one radon atom needs to combine with 7.5 helium atoms. But if the diffusion of radon is taken into account, there will have enough number of α particles because one radon atom needs to combine with just 4.26—5.57 helium atoms. Therefore, the diffusion of radon cannot be ignored when we apply the migration mechanism of helium-radon clusters to explain the vertical migration experiment data of radon and its daughters.