Early Attempts

• Chemists tried to organize elements that shared similar properties into groups of threes called triads, but, as more elements were discovered, many did not seem to fit this pattern. In 1862, a French geologist tried to sort all then-known elements by atomic weight and find trends, but not all elements seemed to fit the pattern. It was not until 1869 that Russian scientist Dmitri Mendeleev came up with a solution.
  
  

Mendeleev's Table

• Mendeleev's idea was to organize the elements based not only on atomic weight but on properties. To complete the pattern, he left gaps for elements that had not yet been found but that he believed would be discovered eventually. His predictions were proven right; germanium, for example, was predicted by Mendeleev before it was discovered. Despite Mendeleev's success in finding a better way to arrange the elements, not all of them seemed to "fit" in the right place in his table.
  
  

Atomic Number

• In the early 20th century, British scientist Henry Moseley found a way to determine the atomic number of an element (the number of protons in its nucleus), adding the last missing piece to the puzzle. By organizing the elements in Mendeleev's arrangement but using atomic number rather than atomic weight, chemists were able to classify the elements in a way that made sense of their observed properties.