MENDELEEV’S PERIODIC TABLE

Even after the rejection of Newlands’ Law of Octaves, many scientists continued to search for a pattern that correlated the properties of elements with their atomic masses.

The main credit for classifying elements goes to Dmitri Ivanovich Mendeléev, a Russian chemist. He was the most important contributor to the early development of a Periodic Table of elements wherein the elements were arranged on the basis of their fundamental property, the atomic mass, and also on the similarity of chemical properties.

Dmitri lvanovich Mendeleev (1834-1907)

Dmitri lvanovich Mendeléev was born in Tobolsk in Western Siberia, Russia on 8 February 1834. After his early education, Mendeléev could join a university only due to the efforts of his mother. Dedicating his investigations to his mother he wrote, “She instructed with example, corrected with love and travelled with me to places spending her last resources and strength. She knew that with the aid of science without violence, with love but firmness, all superstitions, untruth and errors can be removed.” The arrangement of elements he proposed is called Mendeléev’s Periodic Table. The Periodic Table proved to be the unifying principle in chemistry. It was the motivation for the discovery of some new elements.

When Mendeléev started his work, 63 elements were known. He examined the relationship between the atomic masses of the elements and their physical and chemical properties. Among chemical properties, Mendeléev concentrated on the compounds formed by elements with oxygen and hydrogen. He selected hydrogen and oxygen as they are very reactive and formed compounds with most elements. The formulae of the hydrides and oxides formed by an element were treated as one of the basic properties of an element for its classification. He then took 63 cards and on each card he wrote down the properties of one element. He sorted out the elements with similar properties and pinned the cards together on a wall. He observed that most of the elements got a place in a Periodic Table and were arranged in the order of their increasing atomic masses. It was also observed that there occurs a periodic recurrence of elements with similar physical and chemical properties. On this basis, Mendeléev formulated a Periodic Law, which states that ‘the properties of elements are the periodic function of their atomic masses’.

Mendeléev’s Periodic Table contains vertical columns called ‘groups’ and horizontal rows called ‘periods’ (Table 5.4).

Mendeléev’s Periodic Table was published in a German journal in 1872. In the formula for oxides and hydrides at the top of the columns, the letter ‘R’ is used to represent any of the elements in the group. Note the way formulae are written. For example, the hydride of carbon, CH₄, is written as RH₄ and the oxide CO₂, as RO₂.