David Orme Masson, the Periodic Classification of the Elements and His ‘Flap’ Model of the Periodic Table

In the early 1890s, David Orme Masson, Professor of Chemistry at the University of Melbourne, invented a new way to display the periodic table of the elements, in which the transition elements were arranged on a flap that projected from the plane containing the main group elements. He shared the idea with his mentor, Sir William Ramsay, at University College London, who published a similar model in his 1896 book. The ‘flap' arrangement was an outcome of Masson's research interest in the periodic classification of the elements, to which he also made contributions in the 1890s about the placement of hydrogen and suggested to Ramsay that a new main group was needed to accommodate the rare gases such as helium and argon then being discovered in London. Although it was not widely adopted elsewhere, Masson's ‘flap' model was a research and a teaching tool that was used at the University of Melbourne and in school chemistry teaching in Victoria for over half a century.