Bonding in Heteronuclear Diatomic Molecules

The P2 molecule contains phosphorus atoms from the third row of the periodic table. We will assume that the diatomic molecules of the Period 3 elements can be treated In a way very similar to that which we have used so far. Thus we will draw the MO diagram for P2 analogous to that for N2. The only change will be that the molecular orbitals will be formed from 35 and 2p atomic orbitals. The P2 model has 10 valence electrons (5 from each phosphorus atom). The resulting molecular orbital diagram is 

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The molecular orbital energy-level diagram for both the NO+ and CN ions. 

Use the molecular orbital model to predict the magnetism and bond order of the NO and CN ions. 

The NO ion has 10 valence electrons (5 + 6 - 1). The CN ion also has 10 valence electrons (4 + 5 + 1). Both ions are therefore diamagnetic and have a bond order derived from the equation 

The MO energy-level diagram for the NO molecule. We assume that orbital order is the same as that for Nj. The bond order is 2.5. 

Experimentally, nitric oxide is indeed found to be paramagnetic. Note that this odd-electron molecule is described very naturally by the MO model. In contrast, the LE model, in the simple form used in this text, does not readily describe such molecules. 

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The effects of electronegativity differences on the bonding in heteronuclear diatomic molecules... 

Bonding in Heteronuclear Diatomic Molecules Bond properties... 

In Chapter 7 we saw that the unequal sharing of electrons between two atoms with different electronegativities, A(EN) > 0, results in a polar bond. For heteronuclear diatomic molecules such as HE, this bond polarity results in a polar molecule. Then the entire molecule acts as a dipole, and we would find that the molecule has a measurable dipole moment, that is, greater than zero. 

The three 2p orbitals from each atom can be combined to give o bonding and antibonding orbitals and n bonding and antibonding orbitals. In heteronuclear diatomic molecules, these orbitals are simply labelled c or n and do not have subscripts. The subscripts g and u refer to behaviour under inversion through the... 

The bond in a heteronuclear diatomic molecule, a diatomic molecule built from atoms of two different elements, is polar, with the electrons shared unequally by the two atoms. We therefore rewrite Eq. I as... 

The limitation of the above analysis to the case of homonuclear diatomic molecules was made by imposing the relation Haa = Hbb> as in this case the two nuclei are identical. More generally, Haa and for heteronuclear diatomic molecules Eq. (134) cannot be simplified (see problem 25). However, the polarity of the bond can be estimated in this case. The reader is referred to specialized texts on molecular orbital theory for a development of this application. 

An HC1 molecule is a heteronuclear diatomic molecule composed of H (EN = 2.1) and Cl (EN = 3.0). Because the electronegativities of the elements are different, the pull on the electrons in the covalent bond between them is unequal. Hence HC1 is a polar molecule. 

Up to now we have been discussing the local properties of the exchange-correlation potential as a function of the spatial coordinate r. However there are also important proi rtira of the exchange-correlation potential as a function of the particle number. In fact there are close connections between the properties as a function of the particle number and the local properties of the exchange-correlation potential. For instance the bumps in the exchange-correlation potential are closely related to the discontinuity properties of the potential as a function of the orbital occupation number [38]. For heteronuclear diatomic molecules for example there are also similar connections between the bond midpoint shape of the potential and the behavior of the potential as a function of the number of electrons transferred from one atomic fragment to another when... 

Fig. 3.2 The bonding and antibonding states for (a) the homonuclear and (b) the heteronuclear diatomic molecule. The shift in the energy levels due to overlap repulsion has not been shown.

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