6 developments, who made them and how they helped.
A French geologist named Alexandre-Emile Béguyer de Chancourtois created the telluric screw, which consisted of elements drawn in a spiral around a cylinder which was divided into 16 parts. Oxygen’s atomic weight was taken as 16 and set as the standard. Although the screw was flawed, he was the first to introduce the concept of periodicity (the tendency to recur at intervals).
John Newlands predicted patterns in the properties of elements and stated that “elements belonging in the same group appear in the same horizontal line”. This describes elements with the same number of electrons in their outer shells. There were flaws in the table though, as Newlands thought these patterns were connected by atomic weight and so that's how the table was set out.
German chemist Lothar Meyer produced his first periodic table of 28 elements in 1964 listed by valency. These were mostly main group elements but in 1968 Meyer expanded the table to 56 elements by incorporating transition metals listed in increasing weight order and elements of the same valence in the same columns. However, his table wasn’t published until 1970, a year after Mendeleev.
Russian chemist Dmitri Mendeleev created the ‘Periodic System’ which showed how the elements could be organised (using atomic weight) and he predicted the existence of eight elements and their properties. He also suggested that the predictions he got wrong had incorrectly measured atomic weights and he was right.
Henry Moseley was an experimental physicist who realised that atomic weight didn’t always work with the predicted chemical properties. A man named Antonius van den Broek made a hypothesis that atomic number could be equal to the charge in the atom’s nucleus. After conducting an experiment where he shot electrons at elements, his results showed that the positive charge increased by one from element to element in the periodic table. This cleared out flaws in previous tables and gave each element a defining feature.
Glenn Seaborg was a nuclear chemist who discovered elements 94-102 and 106, which included elements such as plutonium. He also co-discovered neptunium, einsteinium and three other elements. The discovery and co-discoveries of these elements all happened between 1940 and 1955. In 1944 Seaborg suggested a new row to be added to the periodic table called actinides, placed below the lanthanides. This is now the last row on the periodic table containing elements actinium (89) to lawrencium (103).