The periodic table

Finally, in 1869 and 1870, two scientists, Dmitri Mendeléev and Julius Lothar Meyer, published clear versions of the idea. Not only was Mendeléev the first to publish, but he also announced in 1871 that the gaps in his periodic table would be filled as new elements were discovered. He specified three gaps, all of which were filled by discoveries between 1875 and 1885. As a consequence, Mendeléev gets most of the credit for the periodic table.

No one knew, however, why the properties of elements were periodic. After electrons and protons were discovered, Henry Moseley showed in 1914 that each element had a definite number of protons that normally corresponded to the same definite number of electrons. This atomic number, not the atomic weight at all, was the basis of the periodic table. With Moseley's work, it was clear that gaps existed between whole numbers of protons. These gaps have since all been filled, with the new elements falling into appropriate periodic table slots, although there has to be some restructuring from the original idea. The number of protons in an atom determines, for a neutral atom, the number of electrons, which fall into several somewhat concentric shells about the nucleus of the atom. The electrons in the outermost shell determine the chemical properties of the element. For certain elements, adding a proton to the nucleus adds an electron that is not in that outer shell, so those elements have to be grouped together on one cell of the table--the rare earths are all in the cell for atomic number 57 and the actinide series is all in the cell for atomic number 89.

Mendeléev and the others were able to define early forms of the periodic table because to a large extent the number of protons correlates with the atomic weight, especially for lighter elements. It is not until one reaches element 28, nickel, that an atomic weight is even slightly out of order. Nickel and element 27, cobalt, both have atomic weights that are close, but nickel at 58.7 is a bit less