The afternoon nap which changed the world of chemistry Updated / Friday, 5 Apr 2019 07:54 The periodic table: "a landmark event that was to transform chemistry from a tangle of disorganised facts into a disciplined science". Photo: iStock Analysis: It's 150 years since Dmitri Mendeleev came up with the Periodic Table of chemical elements and transformed chemistry What's all the fuss about? Dmitri Mendeleev's discovery in St Petersburg in 1869 was a landmark event that was to transform chemistry from a tangle of disorganised facts into a disciplined science. It was to formulate the thinking of generations of scientists to come and to be a feature of every future chemistry textbook from Junior Cycle to undergraduate. His table would arrange 63 elements, (an element is a substance that is made up of identical atoms), into a recognisable and repeating pattern that all the other 58+ elements, yet to be discovered, would follow.