A mechanism suggested to explain the reaction between such metal atoms as sodium or potassium and such halogen molecules as bromine, e.g. K + Br₂ → KBr + Br. As the alkali atom approaches the bromine molecule its valence electron moves to the bromine molecule (thus providing a ‘harpoon’). There are then two ions with a Coulomb attraction between them. As a result the ions move together and the reaction takes place. This mechanism, which has been worked out quantitatively, explains why the reaction occurs far more readily than might be expected taking into account only mechanical collisions between the alkali-metal atoms and halogen molecules.