The two-body and three-body interactions of atoms with one valence electron have been studied. The potentials for the ¹∑ and ³∑ ground-state interactions of alkali metal atoms have been obtained by a semi-mpirical scaling procedure from those of hydrogen atoms. The second virial coefficients of alkali metal vapors have been calculated by numerical integration using both the interaction potentials obtained by scaling and the analytical potentials （Rydberg and antibonding Morse potential functions） with the statistical weight factors of 1/4 and 3/4 for the ¹∑ and ³∑ potentials, respectively. It has been shown that there is good agreement between the calculated and experimental values over the experimental range and also that the potentials obtained by scaling are better representative of the true potentials than the analytical potential functions. The additive third virial coefficients have also been calculated by numerical integration as a weighted combination of the two types of the three-body interactions, which are the singlet-singlet-triplet and triplet-triplet-triplet interactions, with the statistical weight factors of 7/16 and 9/16, respectively. It has been seen that the nonadditive(correction) terms of the third virial coefficients may be rather small for alkali metal vapors.