On the 6th March 1869, Russian chemist Dmitri Mendeleev presented his periodic table to the Russian Chemical Society. While not the first to attempt to classify the elements, Mendeleev’s system was the one to gain universal acceptance.

Mendeleev was both a teacher and an academic chemist, and in the early 1860s published a prize-winning textbook that placed him at the forefront of chemical education. He was, however, frustrated by the apparent disorder of the chemical elements so set about developing a way to classify them.

Other scientists had tried to find ways to arrange the elements since the 18th Century. Just a few years before Mendeleev’s periodic table, the English chemist John Newlands published his ‘Law of Octaves’ which classified the known elements into eight groups in order of their relative atomic masses. However, Newlands’ work was ridiculed at the time and was only formally recognised in 1887.

Mendeleev, meanwhile, continued his work to classify the elements. Tradition dictates that he wrote the characteristics of the known elements on cards, and played a game of ‘chemical solitaire’ with them. He apparently fell asleep during one of these card games, and had a dream in which he saw the cards “fall into place”. Mendeleev realised that the features of the elements repeated in a predictable pattern, based on their atomic weight. While this led to there being some gaps or spaces in ‘periods’ where he believed an element should exist, he was able to calculate the missing element’s atomic mass and properties. His predications later turned out to be correct when missing elements such as gallium were discovered.

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