 2019 was dedicated as the International Year of the Periodic Table by UNESCO to celebrate 150 years since Dmitri Mendelev first formulated the modern table of chemical elements. Of course, science isn't done by lone geniuses no matter what the movies tell us, and Mendelev wasn't truly the first to create a table. Organised lists of elements had been drawn up by alchemists for centuries, even if their content was a little off. Then, in the early 1800s John Dalton published a list of chemical elements, although his symbols never really caught on, these were certainly a more complete list of actual elements and their properties than what came before. And more notably, German chemist Lothar Meyer published a number of actual periodic tables as early as the 1850s, including tables that left gaps for potentially undiscovered elements, an action that's normally credited only to Mendelev. But Mendelev went one further than Meyer and started making predictions about those then unknown elements, and for that he usually gets the main credit for the periodic table. While most people probably know what the periodic table looks like because it's on the walls of so many school classrooms and labs, they might be surprised to see that Mendelev's 1869 version looks like, well, it doesn't really look like something we'd recognise today, it looks like just a simple list with some mild attempts at formatting, there's very little to it at first glance. So this is the thing we're celebrating 150 years later? Well, the lack of detail here is understandable. In 1869 Mendelev and others didn't have that much information to go on. If you compare to information often seen on a modern periodic table, we didn't know about electrons and protons, so the atomic number was out. Quantum mechanics and good quality spectroscopy was another half century away, so electron configuration couldn't be on there. And since isotopes weren't known and mass spectrometry was also a few decades away, the masses were very approximate, so lack of decimal places seen in most tables used by working chemists. All there was to add to the table was the shorthand letter for the element's name, and its un-precise and possibly miscalculated mass. So it looks like just a list, but you do turn it sideways and flip it around and maybe squint a bit, you can start to see some of the overlap with the modern version, which is actually a mid-20th century invention often credited through the American chemist Glenn Seaborg. By 1870 Mendelev's system was starting to take better shape, and his natural system of elements has more information on it, although mostly just the simple compounds that the elements were known to form. And a side note truncated to just the letters and their relative position starts to look a little like something we'd see today if you were to draw some boxes around it. In 1871 the table was much more complete, and this is the type of layout that hung around the longest. I have a similar copy in a textbook dated about 1920, and you can even see it in the background of photos of Glenn Seaborg from the 1950s, though that contains much more modern data. If we look closely at the rows of Mendelev's original though, we can begin to see that this does look more like the periodic table than you might think from just glancing at it. You can see the main group elements are in the right columns and in the right order, and what Mendelev has done differently to our modern version is to interleave the D-block transition elements into the rows between them. But if you know nothing about atomic structure, then it does make sense to do something like this. A lot of early transition metal elements like Scandium do exhibit similar valency and oxidation states to the P-block elements like Aluminium. So putting them all in the same column does feel right knowing nothing else, even if grouping manganese with the halogens as a big red flag that something might not be quite right with the layout. Now with some modern aesthetic interpretation and color coding, what's going on becomes a little more obvious. This means if we do a little bit of resizing of this table, then slide all of those main group elements and their rows to the right, the picture becomes a little clearer. The fact that copper, silver and gold appear twice on Mendelev's table helps demonstrate that you had the right idea, but was possibly just reluctant to generate 17 separate columns of data and create duplicate groups just for these metals. If we then compress that together, merging those twinned coinage metals, we start to see the periodic table take the form we already know. At this point Mendelev has left lots of gaps and it almost looks like he's been very overzealous with them, but again this is understandable because the mass of the elements makes a very big jump this far down on the table and this jump would eventually be explained by the f-block elements. But the majority of those remained undiscovered in Mendelev's time, so we can just delete those spaces or pretend we've shifted them away into their own block. Now everything begins to snap neatly into place. All that's left is to delete dedinium, which turned out to not be an element but a mixture of neodymium and prasodymium. Delete erbium as that got confused with terbium for a long time and then shuffle lanthanum into group three that's your preference for depicting where the f-block should go. And that's it. We're still missing the noble gases, which were another decade or so away from being discovered, but it turns out Mendelev's simple-looking list really was our modern periodic table in disguise.