When n=3,l=0,1,2 i.e. there are 3s,3p and 3d-orbitals. If all these orbitals are completely occupied asTotal 18 electrons, 9 electrons with s=+21​ and 9 withs=−21​. Alternatively, In any nth orbit, there can be a maximum of 2n2 electrons. Hence, when n=3, number of maximum electrons = 18. Out of these 18 electrons, 9 can have spin −21​ and remaining nine with spin+21​