The periodic table stares down from the walls of just about every chemistry lab. But the periodic table didn’t actually start with Mendeleev. Even more amazingly, he accurately predicted the properties of the missing elements. To celebrate the 150th anniversary of this pivotal moment in science, the UN has proclaimed 2019 to be the International year of the Periodic Table. Theodor Benfey’s spira table, created in 1964. It shows the names of the groups in which the elements are. The credit for its creation generally goes to Dimitri Mendeleev, a Russian chemist who in 1869 wrote out the known elements (of which there were 63 at the time) on cards and then arranged them in columns and rows according to their chemical and physical properties. Mendeleev’s genius was in what he left out of his table. Notice the question marks in his table above? For example, next to Al (aluminium) there’s space for an unknown metal. Decades before, chemist John Dalton tried to create a table as well as some rather interesting symbols for the elements (they didn’t catch on). So where Dalton, Newlands and others had laid out what was known, Mendeleev left space for the unknown. Many had tinkered with arranging the elements. And just a few years before Mendeleev sat down with his deck of homemade cards, John Newlands also created a table sorting the elements by their properties. He recognised that certain elements were missing, yet to be discovered.

At first glance Mendeleev’s table doesn’t look much like the one we are familiar with.

Mendeleev foretold it would have an atomic mass of 68, a density of six grams per cubic centimetre and a very low melting point. Six years later Paul Émile Lecoq de Boisbaudran, isolated gallium and sure enough it slotted right into the gap with an atomic mass of 69.7, a density of 5.9g/cm³ and a melting point so low that it becomes liquid in your hand. Mendeleev did the same for scandium, germanium and technetium (which wasn’t discovered until 1937, 30 years after his death). For one thing, the modern table has a bunch of elements that Mendeleev overlooked (and failed to leave room for), most notably the noble gases (such as helium, neon, argon). And the table is oriented differently to our modern version, with elements we now place together in columns arranged in rows. For example, the halogens – fluorine (F), chlorine (Cl), bromine (Br), and Iodine (I) (the J symbol in Mendeleev’s table) – all appear next to one another. Today they are arranged in the table’s 17th column (or group 17 as chemists prefer to call it). But once you give Mendeleev’s table a 90-degree turn, the similarity to the modern version becomes apparent. Dimitry Mendeleev’s table complete with missing elements. At first glance Mendeleev’s table doesn’t look much like the one we are familiar with. The credit for the modern periodic table creation generally goes to Mendeleev, a Russian chemist who in 1869 wrote out the known elements (of which there were 63 at the time) on cards and then arranged them in columns and rows according to their chemical and physical properties.

It may seem a small leap from this to the familiar diagram but, years after Mendeleev’s publications, there was plenty of experimentation with alternative layouts for the elements. Charles Janet’s left-step table. The elements that fall on each of the wheel’s spokes share common properties just as those in a column (group) do so in today’s table. A 3D ‘Mendeleev flower’ version of the table. For the first time, the noble gases appeared in their now familiar position on the far right of the table. Particularly influential was Charles Janet’s version. Werner also tried to take a leaf out of Mendeleev’s book by leaving gaps, although he rather overdid the guess work with suggestions for elements lighter than hydrogen and another sitting between hydrogen and helium (none of which exist). Interestingly, Janet also provided space for elements right up to number 120 despite only 92 being known at the time (we’re only at 118 now). Despite this rather modern looking table, there was still a bit of rearranging to be done. The resulting ‘left step’ table is still preferred by many physicists. He took a physicist’s approach to the table and used a newly discovered quantum theory to create a layout based on electron configurations. One particularly striking example is Heinrich Baumhauer’s spiral, published in 1870, with hydrogen at its centre and elements with increasing atomic mass spiraling outwards. III. Spiral, helical, and miscellaneous charts, G. N. Quam, Mary Battell Quam. Even before the table got its permanent right-angle flip, folks suggested some weird and wonderful twists. He took a physicist’s approach to the table and used a newly discovered quantum theory to create a layout based on electron configurations. Heinrich Baumhauer’s spiral. Reprinted (adapted) with permission from Types of graphic classifications of the elements.

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