Michael Hays, age 13, of Westbrook, Maine, for his question:
Who discovered the Table of Elements?
This chart organizes the elements in rows and columns of square slots to reveal their properties and relationships. It is called the Periodic Table of Elements because it arranges the atomic elements in natural groups or periods. It is based on a natural law that governs all atomic structures: This law was discovered by a remarkable Russian chemistry professor, who used it to organize the first Table of Elements. We call him Dmitri Mendeleev, though his name has several other spellings.
Chemists of the 1800s were in a fever of excitement concerning atoms. When the century began, they had identified only about 30 different ones. By 1817, the list had jumped to about 50 and the researchers understood that a basic chemical element is made from atoms of the same kind. They also knew how to measure atomic weights fairly well. By 1829, it became obvious that the known elements should be organized.
It seemed sensible to list them in order, according to their weights. Several methods organized the basic list in rows, curves or columns. And almost every system revealed family relationships in elements of certain weight groups. This baffled everybody through the next SO years while the list of known elements grew to 63.. Often the charts had to be remodeled to place a new atom in its proper slot.
Meantime, at the University of St. Petersburg, Dmitri Medeleev felt certain that there was a natural law behind the scenes. All the elements must be grouped by this Periodic Law. The breakthrough came in 1869, when he used it to organize the first Table of Elements. True, it had vacant slots. But this great genius knew what sort of atoms should fill them. He predicted that one would be similar to the silicon group, another to the boron group and another to the aluminum group.
Up to then, elements were discovered by chance and nobody dreamed of searching for a special one, with unknown properties. Naturally Mendeleev's prediction was a bold challenge and the search was on for the three unknown elements he described in detail. In 1886, silicon's relative was found and named "germanium". Later the two other predicted elements were discovered and placed in their waiting slots. They were named "gallium" and "scandium." Mendeleev's periodic law was proved. But there still were empty spaces on his chart and later his Table of Elements was given a major remodeling.
Our modern Periodic Table of Elements is based on atomic numbers. These are the positive charges of the protons in the nucleus of each different atom. The 18th century tables were based on atomic weights because the electrical structure of the atom was unknown. They worked fairly well because in most cases, larger atoms with lots of charged particles tend to weigh more. Atomic structure was fathomed early in this century and it became obvious that the Table of Elements should be based on the whole numbers of protons in the atomic nuclei.