ON A WINTER’S day in 1869, Russian scientist Dmitri Mendeleev sat at his desk trying to write a textbook on chemistry. Almost from the start, he had had difficulty structuring the material. Sixty-three elements were known at the time, a mixture of solids, liquids and gases, metals and non-metals. Which should he discuss first? Were some more important than others? There was no obvious order in their composition or properties.

Taking a break from his work, Mendeleev pulled out a pack of playing cards and began a game of solitaire. As he laid out the rows it suddenly occurred to him that perhaps elements could be organised in a similar way. He picked up the cards, added some others and then on the back of each wrote down the name of an element and its chemical properties. Shuffling the cards in his hands, Mendeleev realised that one way of ordering the elements might be by their atomic weight. Following this principle, he dealt the cards until he reached sodium. In terms of its chemical properties, sodium was quite similar to the first element that Mendeleev had set down, lithium. On a hunch, he placed sodium under lithium and began a new row. As he continued to place cards, Mendeleev suddenly realised that each element was similar to the one above it, while also having a place between those on either side.