Give Reason. Two electrons in helium are placed in only one shell while three electrons in Lithium occupy two shells.

1) first shell can have a max of two electrons only n = 1 , So, 2n² = 2

2) Helium has 2 electrons so they can be accomodated in the first shell itself. Lithium has three electrons. So, once the first shell is filled completely by two electrons, the third electron goes to the 2nd shell.