To determine the order of subshells from lowest energy to highest energy (Aufbau Principle), we construct the following table. 145 OP 5P 2R 5d 4d 3d give order of sequence: subshells have orbitals: s = 1, p = 3, d = 5, f = 7 each orbital can hold two electrons (Pauli Exclusion Principle): s = 2e, p = 6e, d = 10e, f = 14e