Elements such as gold and silver have been known since the earliest times. In 1649, a German alchemist, Hennig Brand made the first scientific discovery of an element, phosphorous. In the next 200 years, knowledge about the properties of elements and their compounds was gained. By 1869, 63 elements had been discovered in total. Scientists began to categorise those elements according to the similarities of their physical and chemical properties, which become the modern Period table we are using today.
Between 1817 – 1829, a German chemist Johann Dobereiner classified some elements into groups of three, he called them triads. In 1817, Dobereiner noticed that the atomic weight of strontium, Sr, was halfway between the weights of calcium and barium, which these elements possess similar chemical properties. He also noticed the same pattern for the alkali metal triad (Li/Na/K) and the halogen triad (Cl/Br/I). In 1829 Dobereiner proposed the Law of Triads which Middle element in the triad had atomic weight that was the average of the other two elements. Later, other scientists found other triads and recognised that elements could be grouped into set larger than three. However, as the measurement of atomic weights was not accurate at that time, it was hard to grouping more elements. In 1862, French geologist Alexandre-Emile Béguyer de Chancourtois listed the elements on paper tape and wound them around a cylinder. Certain ‘threes’ of elements with similar properties came together down the cylinder. He called that model the ‘telluric screw’.
In 1864, English chemist John Newlands noticed that by arranging the elements in order of increasing atomic weights, similar physical or chemical properties repeated every eight elements. He proposed it as ‘law of octaves’.
In 1869, Russian chemist Dmitri Mendeleev constructed a periodic table by arranging elements based on atomic weights and properties which elements with similar...