Spin Pairing in Molecules

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The Pauli exclusion principle requires that two electrons in the same spatial standing wave quantum state have opposite spins. This is illustrated for the covalent pair bonding electrons in H2 and Li2, and also for the pairs of electrons in the inner helium-like shells of the Li atoms. Thus, the pairing of electrons of opposite spins allows formation of the strong bonds enabling atoms to join to form molecules, as well as determining the number of electrons contained in inner atomic shells. If the bonding electrons have parallel spins, the bonding will be far weaker, or not even take place. (Unit: 6)