https://doi.org/10.1351/goldbook.C01170
Various collision theories, dealing with the frequency of collision between between the centres of two molecules was equal to the sum of their radii. For a gas containing only one type of molecule, A, the Here is the is the mean molecular speed, given by kinetic theory to be , where is the molecular mass, and . Thus: The corresponding expression for the for two unlike molecules A and B, of masses and is: where is the , and . For the where is the
reactant
molecules, have been put forward. In the earliest theories reactant molecules were regarded as hard spheres, and a collision was considered to occur when the distance collision density
is given by simple collision theory as: number density
of molecules and collision density
reduced mass
collision frequency
factor these formulations lead to the following expression: Avogadro constant
. More advanced collision theories, not involving the assumption that molecules behave as hard spheres, are known as generalized kinetic theories.