Question

Give an account of the process adopted by Mendeléev for the classification of elements. How did he arrive at “Periodic Law”?

Answer 1

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.

He 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’.

Further, Mendeléev left some gaps in his Periodic Table. Mendeléev boldly predicted the existence of some elements that had not been discovered at that time. Mendeléev named them by prefixing a sanskrit numeral, Eka (one) to the name of preceding element in the same group. For instance, scandium, gallium and germanium, discovered later, have properties similar to Eka–boron, Eka–aluminium and Eka–silicon, respectively.