2.4.3 The building up principle and the electronic configurations I.The building up principle (for ground state) a. Pauli exclusion principle Every orbital may contain only two electrons of opposite spins b.The principle of minimum energy. Whilst being compatible with the Pauli principle, electrons occupy the orbital with the lowest energy first.2.4.3 The building up principle and the electronic configurations a. Pauli exclusion principle. Every orbital may contain only two electrons of opposite spins. b. The principle of minimum energy. Whilst being compatible with the Pauli principle, electrons occupy the orbital with the lowest energy first. I. The building up principle (for ground state)