Interactive Periodic System of the Elements (PSE) for your iPhoneMerck’s interactive “Periodic System of the Elements for iPhone” app puts a wealth of information on the chemical elements right into your pocket, presented in a sleek, elegant and user-friendly interface. Find just the fact you need in a matter of seconds – and let yourself be surprised how much more there is to know: from relative atomic mass, density or appearance to melting and boiling points to electron configuration.