Dimitri Mendeleev 1869, Professor if Chemistry at the University of Saint Petersburg (Leningrad). Mendeleev stated that the elements vary periodically (in cycles) according to their atomic masses.
Mendeleev separated his elements and left spaces on his table in order for the periodicity to continue. He then predicted that elements would be discovered to fill these "gaps" in the table. Mendeleev even accurately stated the properties of these elements. Scandium (ekaboron), galluim (ekaaluminum), and germanium (ekasilicon). By 1886 all of the elements predicted by Mendeleev had been isolated.
When Mendeleev's notes show that the periodic system was created in a single day, February 17, 1869. He arrived at his system by puzzling over cards containing the names of the 63 known elements along with their atomic weights and important chemical and physical properties.
Lothar Meyer-1886, also developed a periodic table based on atomic masses, independently of Mendeleev. Meyer had several inaccuracies and some elements were not included. Meyer was the first scientist to introduce the concept of valence as a periodic property. Both Mendeleev and Meyer were awarded the Royal Societies Davy Medal.
Mendeleev is given credit because of his accurate property prediction of yet undiscovered elements.
Meyer and Mendeleev were both students of Robert Bunsen. Bunsen never accepted the connection between spectral lines (his "contribution" and periodicity). Bunsen remarked (in reference to Newlands) that one could just as well classify the stock exchange reports in a system.
Henry Moseley-1914, student of Rutherford. Moseley was studying X-ray formation by high energy electron bombardment. He graphed the square root of the X-ray frequency vs atomic mass. This plot gave a nearly linear line except for three atomic pairs. Ar(39.95)/K(39.10), Co(58.93)/Ni(58.69), Te(127.60)/I(126.90).
When the atoms were plotted according to atomic number, then a linear relationship was established. Moseley stated, "There is every reason to suppose that the integer that controls the X-ray spectrum is the charge on the nucleus."
Periodic Law - The properties of the chemical elements are a periodic function of atomic number.
Why is Mendeleev is given Credit in Modern Text Books
Mendeleev's Table allowed for and was capable of adjusting to future discoveries:
This page was made by Erik Epp.