Worked example - how many electrons fit in the n=3 shell? Step 1: l can be 0, 1, 2 (s, p, d). Step 2: count orbitals - 3s has 1 orbital, 3p has 3, 3d has 5; total 9 orbitals. Step 3: each orbital holds 2 electrons (Pauli), so 18 electrons. Check with the formula 2n^2 = 2 x 9 = 18. Now write oxygen (Z=8): 1s2 2s2 2p4. The 2p subshell has 3 orbitals; the 4 electrons go one each into all three orbitals first (Hund's rule), then the fourth pairs up - leaving 2 unpaired electrons, which is why O2 is magnetic.