The concept of atoms goes back at least as far as Democritus [local], 400 BC.

The Periodic Table of Elements is a marvel of organization. As soon as more than thirty elements were known, scientists began attempts to organize [local] the elements into groups with similar properties. The concept of periodic law [local] emerged [local].

Dimitri Mendeleev [local] (also written Mendeleef, Mendelev) is most closely associated with the development of the periodic table. He showed how a table could be used as a predictive tool. It is difficult to overestimate the impact of that ability on the development of the modern model of atoms. Today we realize the valence of an element is determined by the number of electrons it will typically use in bonding. In Mendeleev's time the electron was unknown, and the concept of chemical bonding had not emerged. Mendeleev used the chemical properties of the elements and compounds of those elements to arrange his table.

He had the correct order for elements even though the atomic masses were not in order. There are three pairs in the periodic table where masses are not in sequence: Ar, K; Te, I; and Co, Ni. Mosely [local] showed a systematic change in the X-ray spectra of elements, and this happened to follow the order used by Mendeleev in establishing his periodic table.

Mendeleev could predict properties of unknown elements and their compounds. His confidence to leave gaps and predict properties of those undiscovered elements, was important in the acceptance of his arrangement and the acceptance of the atomic masses that he used on his table.

Since Mendeleev's organization of the table there have been a number of subtle improvements. Seaborg [local], himself the co-discoverer of many elements, was the last to suggest a major revision in the Periodic Table. The element seaborgium [local] was named in his honor.

In 1894, Ramsay discovered the first of the Nobel gases. Until this time, no member of this important family of elements had been discovered.

Numerous periodic tables are available on the Web. The Iridium project provides a periodic table that is used to display electron configurations of the elements.