Mendeleev Draws the Periodic Table: 1869
1869 · event
In the winter of 1869 a chemistry professor in Saint Petersburg was writing a textbook and could not find a sensible order in which to present the elements. Sixty three were known. Dmitri Mendeleev wrote their names and properties on cards and moved them around his desk until a pattern appeared, arranging them by increasing atomic weight and finding that similar chemical behaviour returned at regular intervals. On 6 March 1869 his table was presented to the Russian Chemical Society.
The arrangement was not the first attempt. Several chemists had noticed periodic relationships, and Lothar Meyer in Germany produced a similar table at almost the same time. What set Mendeleev's work apart was his willingness to trust the pattern more than the data. Where the sequence did not fit, he left empty spaces, and he described in detail the properties that the missing elements would have when somebody found them.
They were found. Gallium was isolated in 1875, scandium in 1879 and germanium in 1886, and each matched the predicted density, weight and chemistry closely enough to convince the profession that the table was describing something real about matter itself.
The reason behind the pattern arrived later. In 1913 Henry Moseley showed that the proper ordering is by the number of protons in the nucleus rather than by weight, and quantum mechanics then explained why the periods repeat. A whole new column, the noble gases, was added at the end of the nineteenth century without disturbing the structure. The periodic table now hangs in every school laboratory in the world, and it is one of the few scientific images that a person who never studied science can still recognise at a glance.