 In the 1800s, people knew that each element is a pure substance and that it's made up of a single type of atom. They also knew that for any element, the atoms in that element would have a specific mass. We call the mass of an atom its atomic mass. Although scientists had not yet discovered every element, they had discovered a lot of them. But in spite of all of this knowledge, scientists didn't have a great way to organize these elements, something that became increasingly important as more and more elements were discovered. Scientists tried things like organizing elements from smallest atomic mass to largest atomic mass. They also tried organizing elements that had similar chemical and physical properties into groups. But there was no system of organization that really worked well. Until Dimitri Mendeleev had the idea to try to organize the elements using both atomic mass and the properties of elements at the same time. Legend has it that Dimitri Mendeleev was on a long train ride. Inspired by a card game called Solitaire, he made a bunch of cards with atomic masses and the properties of each element on the card. For three days and nights, he sorted through these cards, trying to look for patterns. After falling asleep from exhaustion, he had a dream in which the elements all fell into place. He noticed that when he organized the elements from smallest to largest, there was a pattern of properties that would repeat in a cycle. Rather than lining all the elements up in a row or sorting them into separate piles, he laid them out in a table with rows and columns. Mendeleev's table looked like this. Mendeleev's table had some gaps in it where there were no known elements that went into the space. But Mendeleev was sure something must go there, and he was even able to describe what these missing elements would behave like because of the patterns he had shown. When these elements were finally discovered years later, their physical and chemical properties turned out to be what Mendeleev predicted they would be. Over the years, as scientists discovered more elements and learned more about the structure of atoms, the periodic table changed a bit. But the principles behind it are still pretty much the same.