Simple cross-section model for elementary reactions

A simple reaction cross-section model has been obtained. It consists of hard spheres where the mass of the products has been introduced via angular momentum conservation. For exothermic reactions the cross section shows a maximum, providing that a threshold energy is present. The model has been applied to the alkali-atom—alkyl-oxide reactions, giving a simple understanding of the systematic trend of the cross section and the energy dependence of its maximum.