Question

Question 21________________

Answer 1

Answer:

Explanation: 1] because we more study about elements

2] because he know more element will come in future

3]he  don't know name of noble gases

Answer 2

Answer:

Explanation:

D) because at that time noble gases were not discovered.

C) because he knolew that there are many more elements yet to be discovered as at that time only 63 elements were discovered. He also discovered some metals like eka boron eka silicon etc

B) on increasing order of atomic mass

On the basis of chemical properties like oxide and hydride formation.

A) we classify elements by grouping to make study convenient and easy