The Nobel Prize in Chemistry 1956 was awarded jointly to Sir Cyril Norman Hinshelwood and Nikolay Nikolaevich Semenov "for their researches into the mechanism of chemical reactions".
 

Van 't Hoff and the Swede Svante Arrhenius had already in the 1880's disclosed that when molecules of two substances collide, the collision must be sufficiently violent if the initial molecules are to break down and their atoms to rearrange into new molecules, that is, for a chemical reaction to take place. In 1900 Max Planck had found that light was composed of discrete quanta. In 1913 the German chemist Max Bodenstein put forth an idea which proved to be extremely fertile, the idea of chain reactions. Danish and a Dutch scientist, Christiansen and Kramers, in 1923 pointed out that such a chain reaction need not start with a molecule excited by light, but could also start with two molecules colliding violently in the way van 't Hoff had thought of.