The Second-Row Diatomic Molecules

FIGURE 7.20 Molecular orbital diagrams for the second-row diatomic molecules (a) N2, (b) 02, and (c) F2. The 02 molecule has two unpaired electrons in its two degenerate 77 2p orbitals and is therefore paramagnetic. 

FIGURE 6.18 Trends in bond order, bond length, bond energy, and force constant with the number of valence electrons in the second-row diatomic molecules. 

Given the energy ordering of the molecular orbitals, it is a simple matter to determine the electron configurations for the second-row diatomic molecules B2 through Ne2. For example, a boron atom has three valence electrons. (Remember that we need to consider the inner-shell Is electrons.) Thus, for B2 we must place six electrons in MOs. Four of these fully occupy the (T2s and molecular... 

Figure 9.2. Correlation diagram for second-row diatomic molecules. The column on the left shows orbitals for the united atom, those on the right those of the separated atoms.

Now that we ve looked at bonding in the H2 molecule, let s move up a level in complexity by looking at the bonding in several second-row diatomic molecules— N2,02, and F2. The valence bond model developed in Section 7.10 predicts that the nitrogen atoms in N2 are triply bonded and have one lone pair each, that the oxygen atoms in 02 are doubly bonded and have two lone pairs each, and that the fluorine atoms in F2 are singly bonded and have three lone pairs each ... 

One minor complication that you should be aware of is that the relative energies of the a and n bonding molecular orbitals are reversed in some of the second-row diatomics. However, the order in which these two orbitals are filled has no effect on the predicted bond orders, so there is ordinarily no need to know which molecules follow which scheme. 

Studies on molecular charge distributions and chemical binding due to Bader and co-workers include the first-row homonuclear diatomics (Bader et al., 1967a), the first-row diatomic hydrides (Bader et al., 1976b), the first-row 12- and 14-electron diatomic series (Bader and Bandrauk, 1968a), the second-row diatomic hydrides (Cade et al., 1969), and the excited, ionized, and electron-attached states of several diatomic molecules (Cade et al., 1971). Bader (1970, 1975,1981), Deb (1973), and Mulli-ken and Ermler (1977) review their works in some detail. 

PERIOD 2 DIATOMIC MOLECULES We extend the concepts of molecular orbital theory to construct energy-level diagrams for second-row diatomic molecules. 

The molecular orbital description of second-row diatomic molecules leads to bond orders in accord with the Lewis structures of these molecules. Further, the model predicts correctly that O2 should exhibit paramagnetism, an attraction of a molecule by a magnetic field due to unpaired electrons. Those molecules in which all the electrons are paired exhibit diamagnetism, a weak repulsion from a magnetic field. 

Table 7.9 The reducible representations for the 2p valence orbitals of a second-row diatomic molecule. The px and py orbitals (perpendicular to the molecular axis) are degenerate forming rig and riu combinations.

As mentioned above, predicted properties can be sensitive, in some cases, to the value of the range separation parameter. " In this context, it is worth noting that the original suggestion to use ft = 0.33 a comes not from fitting anion VDEs but rather was optimized to reproduce bond lengths for second-row diatomic molecules. " In other LRC functionals, the ft (or co) parameter has been optimized to reproduce various experimental A less empirical approach has been advocated " " and... 

The data show that bond energies for these three diatomic molecules increase moving across the second row of the periodic table. We must construct molecular orbital diagrams for the three molecules and use the results to interpret the trend. 

The basic principles dealing with the molecular orbital description of the bonding in diatomic molecules have been presented in the previous section. However, somewhat different considerations are involved when second-row elements are involved in the bonding because of the differences between s and p orbitals. When the orbitals being combined are p orbitals, the lobes can combine in such a way that the overlap is symmetric around the intemuclear axis. Overlap in this way gives rise to a a bond. This type of overlap involves p orbitals for which the overlap is essentially "end on" as shown in Figure 3.5. For reasons that will become clear later, it will be assumed that the pz orbital is the one used in this type of combination. 

Abstract A brief introduction deals with the time period from Dalton to the discovery of isotopes by Soddy and Fajans in the early twentieth century which was soon followed by the invention of the mass spectrograph (1922). The next section covers the period from 1922 to the discovery of deuterium by Urey and his colleagues. It includes a discussion of isotope effects in spectroscopy, particularly band spectra of diatomic molecules, and also discusses the discovery of the important stable isotopes in the second row of the periodic table. It ends with the discovery of deuterium, probably the most popular isotope for isotope effect studies. The chapter ends with a short description of the apparatus of theory and experimentation available for isotope effect work at the time of the discovery of deuterium. 

Oxygen, fluorine, and man. These three molecules can be treated with the same energy diagram that we have been using for other diatomic molecules of the second-row elements. As we shall see shortly, the intervening molecules, B, C-. and N2. require additional considerations, which lead to an alteration in (he relative energies of the molecular orbitals. 

Which of the homonuclear diatomic molecules of the second row of the periodic table (Li2 to Nc2) are predicted by MO theory to be paramagnetic Which ones are predicted to have a bond order of one Which ones are predicted to have a bond order of two Which one is predicted to have the highest bond order ... 

Larger molecules have greater values of b. For instance, H2, a first-row diatomic molecule, has a greater b value than the first-row monatomic He. The b value for CO2, which contains three second-row atoms, is greater than that for N2, which contains only two second-row atoms. 

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. 

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