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 5.9 g/cm 3 and a melting point so low that it becomes liquid in your hand. Mendeleev foretold it would have an atomic mass of 68, a density of 6 g/cm 3 and a very low melting point. Notice the question marks in his table, shown in figure 2? For example, next to aluminium (Al), there is space for an unknown metal. Even more amazingly, he accurately predicted the properties of the missing elements. So where Dalton, Newlands and others had laid out what was known, Mendeleev left space for the unknown.
He recognised that certain elements were missing, yet to be discovered. Mendeleev’s genius, however, was in what he left out of his table. And just a few years before Mendeleev sat down with his deck of homemade cards, John Newlands created a table sorting the elements by their properties. Decades before, chemist John Dalton tried to create a table as well as some rather interesting symbols for the elements (figure 1), but they didn’t catch on. Many had tinkered with arranging the elements. In 1869, he wrote out the known elements (of which there were 63 at the time) on cards and arranged them in columns and rows according to their chemical and physical properties.īut the periodic table didn’t actually start with Mendeleev. The credit for the creation of the periodic table generally goes to the chemist Dmitri Mendeleev. But its now-iconic design could have looked very different. The periodic table hangs on the wall in just about every chemistry classroom.
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