a. The relative content of radioactive atoms in the gas is small. Their number per unit volume increases linearly with height: n = αh. The mass of an atom is m, its free path is λ, and its temperature is T. Estimate the density of these atoms on earth.
b. Estimate the diffusion coefficient of water vapor in the air at 20◦C. The radius of water molecules is 0.21 nm. The radius of nitrogen and oxygen molecules is 0.18 nm.