The Development of Periodic Table

The development of Periodic Table
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 properties appeared under each other. In Mendeleev's periodic table, there were 17 columns with two partial periods of seven elements each followed by two nearly complete periods.
In 1871, Mendeleev produced a new table contained 17-group table with eight columns. This table showed similarities in an entire network of vertical, horizontal, and diagonal relationships. There were some gaps left for undiscovered elements. He predicted the existence and properties of those elements which he called eka-aluminum, eka-boron, and eka-silicon. The elements gallium, scandium and germanium were found later to fit his predictions quite well.
Also in 1869, Lothar Meyer produced a periodic table of 56 elements based on a regular repeating pattern of physical properties such as molar volume. The elements were also arranged in order of increasing atomic weights which was similar to Mendeleev’s table.
Later, Lord Rayleigh and William Ramsey discovered the ‘inert gases’ which are called ‘noble gases’ nowadays. In 1894, Rayleigh discovered a new gaseous element named argon which was chemically inert and did not fit into the periodic groups at that time. Ramsey later discovered more inert gases and represented them in a new group in the periodic table.
In 1914, through the work with X-rays, Henry Moseley determined the atomic number (the actual nuclear charge) of each of the known elements. He rearranged the elements in order of increasing atomic number rather than atomic weight.
In 1940, Glenn Seaborg discovered 10 new elements such as neptunium, their atomic numbers were 94 to 102. These new elements were called ‘actinides’. Seaborg reproduced the periodic table by placing the lanthanide and actinide series of elements under the rest of the periodic table. These elements technically should be placed between the alkaline earth metals and the transition metals. However, since this would make the periodic table too wide, they were placed below the rest of the elements.