Dmitri Mendeleev not only created the first periodic table, but also predicted elements that didn't exist at that time. At first, just like every great mind in history the scientific society didn't want to accept his great discovery. But, he was so good at calculations that soon after, no one dared to doubt his Periodic Law. For instance, he questioned Thorium's atomic weight and claimed that it was calculated wrong. Because, he predicted that it was different than what it was known and he was right !
When no element fitted to his table, he left an empty space. He calculated those undiscovered elements' atomic weights and some other properties. Two of these are Gallium and Germanium. When gallium was discovered, Lecoq de Boisbaudran (the discoverer) reported a different atomic weight for it. Mendeleev sent a letter to him and asked him to repeat his experiments and check his calculations. In the end, he was right ! Germanium's discovery came after many years and again confirmed Mendeleev's law.
He was a man of science. He worked with Bunsen in Heidelberg Laboratories and left the work after an argument with Bunsen. He attended lectures of Kirchhoff. His doctoral thesis was on solubility of alcohol in water. He spent many years in teaching chemistry, he helped to develop the oil industry in Azerbaijan wrote an inorganic chemistry book.
He deeply believed in science. He didn't like literature "We could live at the present day without a Plato, but a double number of Newtons is required to discover the secrets of nature, and to bring life into harmony with the laws of nature."
I think what made him different was his bravery and determined mind. Even if he had doubts, he defended his great discovery. He believed it to be true and science has proven he is right.
"...although I had my doubts about some obscure points, yet I have never doubted the universality of this law, because it could not possibly be the result of chance."
Happy Birthday Mendeleev !
Here is a good reading on Mendeleev and his great discovery by Eric Scerri ( @ericscerri ): http://blog.oup.com/2012/08/how-exactly-did-mendeleev-discover-his-periodic-table-of-1869/
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