Homonuclear diatomic molecules properties

As can be seen from its ground-state molecular orbital diagram in Figure 4.11, dioxygen has a paramagnetic ground state. It is the only stable homonuclear diatomic molecule with this property. 

In general, it is advantageous to use the symmetry elements of a molecule in dealing with the molecular orbitals. For example, consider the symmetry properties of a homonuclear diatomic molecule... 

The symmetry operations E, C, and av (reflection in a plane that contains the axis A-B) are present. All molecules that possess these symmetry properties have the point-group symmetry Coov The orbitals are characterized by symbols similar to those used for a homonuclear diatomic molecule, such as a, n, etc. The character table for CMV is given in Table 2-2. 

Table 3.3.2 summarizes the various properties of second-row homonuclear diatomic molecules. In the last column of the table, we list the bond order between atoms A and B in the molecule AB. Simply put, the bond order is a number that gives an indication of its strength relative to that of a two-electron single bond. Thus the bond order ofHf (cr ) is 1/2, while that of H2 (afs) is 1. For a system with antibonding electrons, we take the simplistic view that one antibonding electron cancels out one bonding electron. Thus the bond orders in lief (ofs o-j 1) and He2 (ofs aj s2) are 1 /2 and 0, respectively, and helium is not expected to form a diatomic molecule. 

The reliability of MO calculations for metal atoms can be judged by application to homonuclear diatomic molecules. Experimental electronic properties have been measured using mass spectrometers for many such molecules. Dimers... 

TABLE 10.5 Properties of Homonuclear Diatomic Molecules of the Second-period Elements ... 

A homonuclear diatomic molecule is one in which both nuclei are the same, for example H2 and N2. In the first row of the Periodic Table, H2 is the only example. From the second row we have N2, 02 and F2, which are stable under normal conditions of temperature and pressure. We looked at N2 in the previous Section. Here we shall consider the molecular orbital description of 02, and use it as an example of how we can use the theory to explain and/or predict properties of molecules. 

Figure 11.20 MO occupancy and molecular properties for B2 through Ne2- The sequence of MOs and their electron populations are shown for the homonuclear diatomic molecules in the p block of Period 2 [Groups 3A(13) to 8A(18)]. The bond energy, bond length, bond order, magnetic properties, and outer (valence) electron configuration appear below the orbital diagrams. Note the correlation between bond order and bond energy, both of which are inversely related to bond length.

Two branches are found. Dissociation energies are less than limyv oo 7V> total number of electrons is uneven. They are larger than lim y oo 7V > total number of electrons is even. The first property is due to the occupation of a nonbonding orbital, the second property corresponds with the experimental observation that the bond energies of homonuclear diatomic molecules are usually stronger than the energy of an atom-atom bond in the bulk. The atom-atom distance also usually proves smaller in a homonuclear molecule than in the bulk. Both properties arise from the weaker metal-metal bond in the bulk of a metal. This is due to delocalized nature of metal electrons, as we will explain in more detail later. 

While the area is less well developed than that of atomic ions, some results are derived, also based on scaling properties, for diatomic systems. In particular, Eq. (5.8) appears to be worthy of further study as an approximate representation of chemical hardness for neutral homonuclear diatomic molecules. 

With these concepts and Figure 10.27, which shows the order of increasing energies for 2p molecular orbitals, we can write the electron configurations and predict the magnetic properties and bond orders of second-period homonuclear diatomic molecules. We will consider a few examples. 

Solution From Table 10.5 we can deduce the properties of ions generated from the homonuclear diatomic molecules. 

SECOND-ROW HOMONUCLEAR DIATOMIC MOLECULES Let US proceed now to the atoms in the second row of the periodic table, namely, Li, Be, B, C, N, O, F, and Ne. These atoms have 2s, Ipx, 2py, and Ip valence orbitals. We first need to specify a coordinate system for the general homonuclear diatomic molecule A2, since the Ip orbitals have directional properties. The z axis is customarily assigned to be the unique molecular axis, as shown in Fig. 2-10. The molecular orbitals are obtained by adding and subtracting those atomic orbitals that overlap. 

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