The Periodic Table

The periodic table orders the chemical elements so that elements sharing similar chemical properties fall into the same column, an arrangement first published by Dmitri Mendeleev in 1869 based on atomic weight. Mendeleev's version left deliberate gaps for elements not yet discovered and predicted their properties from the surrounding pattern, a forecast later confirmed when gallium, scandium and germanium were isolated in 1875, 1879 and 1886 with properties matching his predictions closely; that predictive record is the table's strongest claim to being a real theoretical pattern in nature rather than a convenient filing scheme, and the reason it is treated here as a theory rather than only a classification. The table was independently approached at almost the same time by Julius Lothar Meyer, who published a similar arrangement in 1870 emphasizing atomic volume rather than making Mendeleev's bold predictions. The modern table orders elements by atomic number rather than atomic weight, a correction made possible by Henry Moseley's 1913 X-ray spectroscopy work after Mendeleev's death, which resolved the handful of places where strict weight ordering had put chemically similar elements out of step. The table's predictive record has one honest complication: the entire noble gas column was not among Mendeleev's predictions. Argon, identified in 1894 by Lord Rayleigh and William Ramsay from a small unexplained density discrepancy in atmospheric nitrogen, was followed within four years by Ramsay's discovery of helium, neon, krypton and xenon, revealing a family of chemically unreactive elements with no place in Mendeleev's existing columns. Ramsay proposed fitting them into a column of their own, group zero, now group 18, and the table absorbed the addition without disrupting its existing structure; but the completion came from observation finding room the pattern had left, not from the pattern itself predicting the gases the way it had predicted gallium, scandium and germanium.