By Bella Eagles
In 1817, Dobereiner noticed that certain elements that were chemically similar could be grouped together in threes. In each group of three, the atomic weight of one element fell halfway between the atomic weights of the other two elements. In 1829, Dobereiner proposed the Law of Triads, based on his findings.
In 1864, English chemist John Newlands noticed that, if the elements were arranged in order of atomic weight, there was a periodic similarity every 8 elements. He proposed his ‘law of octaves’ on this.
In 1869, Lothar Meyer created a periodic table of 56 elements based on a regular repeating pattern of physical properties such as molar volume. The elements were arranged in order of increasing atomic weights.
In 1869, Russian chemist Dmitri Mendeleev produced a periodic table based on atomic weights but arranged ‘periodically’. Elements with similar properties appeared under each other. Gaps were left for yet to be discovered elements.
In 1894, William Ramsay discovered the noble gases and realised that they represented a new group in the periodic table.
In 1914, Henry Moseley determined the atomic number of each of the known elements. He realised that, if the elements were arranged in order of increasing atomic number rather than atomic weight, they gave a better fit within the periodic table.
In 1940, Glenn Seaborg artificially produced heavy mass elements such as neptunium. These new elements were part of a new block of the periodic table called actinides.