Dmitri Mendeleev was a 19th century Russian chemist with a brilliant mind, a steely determination and a mane of wild hair (yep, your stereotypical ‘mad scientist’). In 1869, he set out to find a way of organising all of the 63 known elements.

Mendeleev claimed that the answer came to him in a dream: Arrange the elements horizontally in order of their atomic mass, while also vertically grouping elements with similar properties. Mendeleev’s breakthrough resulted in him producing the first Periodic Table of the Elements.

The Periodic Table went on to become an invaluable tool for many scientists, and is still widely used today with over one hundred elements. In honour of it’s 150th anniversary, the United Nations has declared 2019 to be the International Year of the Periodic Table of the Elements.

Mendeleev’s table was not, however, complete – there were gaps in it. Watch this TEDEd video to find out why these gaps were arguably the most brilliant bits of the table.