Question

How many elements were known at the time of Mendeleev’s classification of elements?

Answer 1

Hint: Dimitriv Mendeleev is a Russian chemist. In 1869, he arranged 63 elements in the periodic table. That periodic table is called the "Mendeleev periodic table". He arranged the elements in the periodic table based on their atomic weight.

Complete step by step answer:
Several scientists tried to arrange the elements in the periodic table. In 1829, John Dobereiner classified elements into a group of three based on their increasing atomic mass. According to his law, the atomic mass of the middle element is the arithmetic mean of the other two elements. And the group in three elements is called a triad.
In 1865, John Newland observed that if an element is arranged in an order of increasing energies, then the eighth element should possess similar properties with the first element which is similar to note in octave music. Thus, this law is known as the Law of octaves.
In 1869, J.Lother-Meyer obtained the curve by plotting atomic volumes vs atomic weights of the elements. He thus concluded that elements occupying a similar position in the curve should possess similar properties.
Later in 1869, Dimitriv Mendeleev arranged 63 elements in the periodic table. He introduced the law known as, "Mendeleev periodic law". The law states that the properties of the elements are the periodic function of their atomic weights. He arranged elements based on their atomic weight. Mendeleev also observed some irregularities between two succeeding elements. So, he left some space between such elements since he believed that new elements may be discovered in the future.
Mendeleev's periodic table consists of 9 groups and 7 periods. The horizontal rows are called periods and vertical columns are called groups.
Thus, at Mendeleev’s time, 63 elements were discovered.

Note: In 1913, British physicist Henry Mosley arranged elements in the periodic table based on their atomic number. The periodic table that he arranged is called the "Modern periodic table". We all followed the modern periodic table. Currently, the periodic table has a total of 118 elements.