IV.

History

A number of chemists contributed to the development of the periodic table during the 19th century; however, Mendeleyev is regarded as the primary discoverer. His version of the table was the first to accommodate all the known elements and also successfully predicted the existence of several elements that had not yet been discovered. These undiscovered elements included gallium, scandium, and germanium.

At first, the ordering of the elements was based on atomic weight—the weight of a single atom of the element. This method of ordering broke down in a few cases, such as tellurium and iodine. The atomic weight of tellurium is actually higher than that of iodine, suggesting that tellurium belongs in group 17 while iodine belongs in group 16. Judging by their chemical properties, however, tellurium belongs in group 16 and iodine in 17. This anomaly was resolved in 1914, when it was discovered that atomic number provides a better basis than atomic weight for ordering the elements. Tellurium has an atomic number of 52 and iodine’s is 53.

Elements with similar properties are placed in the same group of the periodic table, but for many years it was a mystery why these elements behaved similarly. At the beginning of the 20th century, when theories of physics changed rapidly, an approximate explanation for the repeating patterns within the periodic table was found. Scientists discovered that the elements within a single group of the periodic table possess the same number of outer-shell electrons, particles that had not even been discovered when scientists produced the first periodic tables.