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In the early 19th Century, Johann Döbereiner made the first attempts to classify elements using their atomic weights and identified a number of 'triads'. In 1863, John Newlands discovered the Law of Octaves, where by he realised that when the elements were written in order of increasing weights every 8th element had similar properties. However, he had no knowledge of the noble gases and so therefore there were only seven elements in each period. Also, some of the atomic weights had been measured inaccurately and so he had difficulties placing elements beyond the first 20. In 1869, Dimitri Mendeleev revised some atomic weight values and left gaps for undiscovered elements to produce a more accurate version of the modern periodic table. He predicted properties for five of these undiscovered elements and three of these predictions were very accurate. Mendeleev's ideas of the periodic table were supported by the discovery of gallium in...

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