(1) Principal (n): 1, 2, 3, ... — determines energy level and average distance from nucleus. Shell: K(1), L(2), M(3), N(4). (2) Azimuthal (l): 0 to n-1 — determines subshell shape and orbital angular momentum = sqrt(l(l+1)) * $\frac{h}{2*pi}$. l=0(s), 1(p), 2(d), 3(f). (3) Magnetic ($m_l$): -l to +l — determines spatial orientation of orbital. 2l+1 values. (4) Spin ($m_s$): +1/2 or -1/2 — intrinsic angular momentum. Pauli exclusion: no two electrons in an atom can have all four quantum numbers identical. Each orbital holds max 2 electrons with opposite spins. Shell n has n subshells, $n^2$ orbitals, 2$n^2$ electrons maximum. Subshell l has 2l+1 orbitals, 2(2l+1) electrons.