Question

question number 15 please tell past

Answer 1

Ans 15
a) Because electrons revolving in a shell are associated with certain fixed amount of energy. To jump to the next energy level electrons need to certain fixed energy & when it comes down it releases certain energy. 

Thus shells are called or known as energy levels or shells.

Answer 2

they are called energy shells because electron revolve around the nucleus in a certain circular paths .Shells name are K L M N .M shell contain 18 electrons