For most of us, the periodic table is a thing of beauty in the abstract, but for chemists it established an immediate orderliness and clarity that can hardly be overstated. "Without a doubt, the Periodic Table of the Chemical Elements is the most elegant organizational chart ever devised," wrote Robert E. Krebs in The History and Use of Our Earth's Chemical Elements, and you can find similar sentiments in virtually every history of chemistry in print. Today we have "120 or so" known elements—ninety-two naturally occurring ones plus a couple of dozen that have been created in labs. The actual number is slightly contentious because the heavy, synthesized elements exist for only millionths of seconds and chemists sometimes argue over whether they have really been detected or not. In Mendeleyev's day just sixty-three elements were known, but part of his cleverness was to realize that the elements as then known didn't make a complete picture, that many pieces were missing. His table predicted, with pleasing accuracy, where new elements would slot in when they were found.

No one knows, incidentally, how high the number of elements might go, though anything beyond 168 as an atomic weight is considered "purely speculative," but what is certain is that anything that is found will fit neatly into Mendeleyev's great scheme.