The Periodic Table
1869
By the 19th century the concept of the elements had changed. Chemists had observed that groups of chemical elements had similar properties. Dmitri Ivanovich Mendeleev (1834-1907) described the relationships between the elements in what is known as the Periodic Law. The graphic representation of this is familiar to us as the Periodic Table. Using this tool Mendeleev recognized the existence of elements that had not yet been discovered.

Illustration from:
Russkoe Khimicheskoe Obshchestvo. Zhurnal (Journal of the Russian Chemical Society); Volume 1, 1869; pages 60-77, "Sootnoshenie Svoystv s Atomnym Vesom Elementov," by Dmitri Mendeleev.