Phosphorus

• An amateur alchemist named Hennig Brand discovered phosphorus in 1649. Although elements such as copper, lead, mercury, gold, silver, and tin had been known for centuries, phosphorus was the first element to be discovered via scientific experiment. In subsequent years more elements would be discovered, totaling 63 by the time Mendeleev would develop his periodic table.
  
  

Triads

• During a twelve-year period in the nineteenth century (1817 to 1829), Johan Dobereiner arranged elements in groups of three, which he called triads. He did so because he had noticed that calcium, barium, and strontium were chemically alike and that strontium's atomic weight was the average of calcium and barium. Further research on elements revealed that relationships between elements went beyond groups of three. Dobereiner's discovery, however, would prove to be the foundation for the arrangement of elements by relationship.
  
  

Precursors to the Periodic Table

• While John Dalton first discovered atomic weight in the early 1800s, the first person to arrange elements according to atomic weight was A.E. Buguyer de Chancourtois in 1862. He represented his findings as a spiral wrapped around a cylinder. A year later John Newlands came up with a theory, the Law of Octaves, proposing that elements repeated regularly in intervals of eight. His arranging of elements in groups of 11 was a precursor to "periods," the horizontal rows on Mendeleev's periodic table.
  
  

Mendeleev

• Dmitri Mendeleev's periodic table arranged the 63 elements known at that time in a manner that showed their relationships horizontally (periods), according to atomic weight, and vertically (groups), according to similarity. Mendeleev's findings were published in his article "On the Relationships of the Elements to their Atomic Weights" in 1869. Although his table contained gaps, Mendeleev predicted that there were elements that would fill in these gaps, many of which, such as gallium and germanium, were eventually discovered by the scientists that followed him.
  
  

After Mendeleev

• In the late nineteenth century, William Ramsay discovered the first noble (inert) gases, including argon, helium, and neon, requiring additions to the periodic table, which were followed by additional discoveries and and revisions to the table. Glenn Seaborg's contribution to the periodic table was particularly significant, as he was responsible for discovering a succession of elements from plutonium (94) in 1940 all the way to nobelium (102). Continued discoveries would bring the number of elements on the table to 118, with ununoctium, discovered in 2002.