Orbitals Hund’s rule

The two 2p electrons are unpaired in each of two p orbitals (Hund s rule). 

RULE 3 If two or more orbitals of equal energy are available, one electron occupies each until all orbitals are half-full. Only then does a second electron occupy one of the orbitals Hund s rule). The electrons in the half-filled orbitals all have the same spin. 

Hund s rule The fact that negatively charged electrons repel each other has an important impact on the distribution of electrons in equal-energy orbitals. Hund s rule states that single electrons with the same spin must occupy each equal-energy orbital before additional electrons with opposite spins can occupy the same orbitals. For example, let the boxes below represent the 2p orbitals. One electron enters each of the three 2p orbitals before a second electron enters any of the orbitals. The sequence in which six electrons occupy three p orbitals is shown below. 

The Pauli Exclusion Principle (two electrons per orbital), Hund s Rule (put one electron into each orbital with the same energy before pairing electrons) and the Aufbau Principle (fill orbitals with electrons in order from lowest energy to highest) all apply to MOs. We can draw an MO energy... 

Orbitals of equal energy are called degenerate orbitals. Hund s Rule for filling degenerate orbitals was discussed in Section 5-17. 

FIGURE 14.7 The tt molecular orbitals that cyclooctatetraene would have if it were planar, Notice that, unlike benzene, this molecule is predicted to have two nonbonding orbitals, and because it has eight tt electrons, it would have an unpaired electron in each of the two nonbonding orbitals (Hund s rule. Section 1.11). Such a system would not be expected to be aromatic. 

That electrons are indistinguishable from one another The aujbau principle and the order of filling the available atomic orbitals Hund s rules... 

Analogies for orbitals, Hund s rule, and the four quantum numbers are included in this reference. 

Is n 2s n 2p n The 2p eiectrons should occupy different orbitals. Hund s rule has been violated. 

Electron spins are not addressed explicitly by MO theory, but they are treated with respect to the Pauli principle just as atomic orbitals are Only two electrons can occupy any one orbital, and their spins must be opposite. Just as in atoms, electrons in molecules fill MOs starting with the lowest-energy MO, and in order of increasing energy. If two or more MOs are degenerate, one electron fills each MO before pairing of electrons in orbitals (Hund s rule). 

Fig. 13.16 The polarization mechanism for /hh spin-spin coupling. The spin information is transmitted from one bond to the next by a version of the mechanism that accoimts for the lower energy of electrons with parallel spins in different atomic orbitals (Hund s rule of maximum multiplicity). In this case, 7 < 0, corresponding to a lower energy when the nuclear spins are paraUeL...

Z = 6, C. A second electron also goes into the 2p subshell, but into one of the remaining empty p orbitals (Hund s rule) with a spin parallel to the first 2p electron, ( e figure to the right.)... 

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