1. The problem statement, all variables and given/known data http://i.imgur.com/GQ9Xk6d.png 2. Relevant equations Quantum mechanical model of atomic structure 3. The attempt at a solution Why all sets are allowed? H atom only got one e- which only one orbital should be there, isn't it? If there aren't second or more e- , no second orbital or higher number of orbital, how comes there is higher number of shells (principle n) ? Hence as do the the number of angular momentum (l) for subshell and magnetic (ml) for the variety of orientation? I am a newbie to QM, who learnt only Bohr's model in high school. Thank you very much.