I have a question in my book where five electrons are placed in a infinite square well and Im supposed to calculate the lowest energies. My problem is with the electron configuration. I think that the first two shall be placed in n=1 and then the rest shall be placed in n=2. This since l=0,1 and m=-1,0,1 (plus the spin). However my book gives me a different solution. Two in n=1, two in n=2 and one in n=3, due to Pauli exclusion. But three electrons can have different quantum numbers in n=2. So why shall one of the electroons be placed in n=3?