Periodic Table

Mendeleev was a Russian chemist who reportedly developed his system after writing each element's properties on individual cards, then arranging and rearranging them almost like a game of solitaire during a long train journey. What he landed on, in St. Petersburg in 1869, was a grid where elements lined up in order of increasing atomic weight, yet elements with similar chemical behavior kept landing in the same vertical columns.

Mendeleev deliberately left gaps in his table for elements that hadn't been discovered yet, correctly predicting their existence and even estimating their properties based on the empty spaces surrounding them. This predictive power is what set his version apart from earlier attempts by other scientists to organize the elements, and it's largely why he's credited as the table's inventor rather than just one contributor among many.

The table we use today has been refined since Mendeleev's time - elements are now ordered by atomic number (introduced by Henry Mosely, in 1913) rather than atomic weight, a switch that resolved a few inconsistencies in the original layout. It has grown to include 118 confirmed elements, and is now maintained and updated by the International Union of Pure and Applied Chemistry, or IUPAC. Despite over 150 years of new discoveries and lab-made superheavy elements, the basic logic Mendeleev devised - rows called periods, columns called groups, with similar chemistry recurring periodically - holds the whole system together still.