Diatomic Molecules of the First and Second Periods

The electron distributions for the homonuclear diatomic molecules of the first and second periods are shown in Table 9-1 together with their bond orders, bond lengths, and bond energies. 

The simplest molecule formed is that produced by the overlap of the Is orbitals of two hydrogen atoms (Figure 14.12). Later in the chapter molecular orbital theory is used to describe the bonding in the diatomic molecules in the first and second periods of the periodic table. MO theory was mentioned briefly in Chapter 13 as a description of bonding in complex ions and complexes. An alternative model of covalent bonding is provided by the valence bond (VB) theory. VB theory was used in Chapter 13 to describe the bonding in complex ions, although it is not a very satisfactory model. 

New graphical representations of the exact molecular orbitals for Hj that make it easier to visualize these orbitals and interpret their meanings. These images provide a foundation for developing MO theory for the first- and second-period diatomic molecules. 

Figure 9-5 shows molecular orbital energy level diagrams for homonuclear diatomic molecules of elements in the first and second periods. Each diagram is an extension of the... 

There are only a few heteronuclear diatomic molecules that are formed from elements of the first and second rows of the Periodic Table and are stable as diatomic molecules in the gas phase at normal temperatures and pressures. These are HF, CO and NO. Others have been observed at high temperatures, in discharge lamps, in flames or in space. Examples are LiH, LiF, OH, BeH, BeO, BF, BH, CH, CN and NH. Some of the molecules in this second list will be stable with respect to the two separate atoms but not at normal temperatures and pressures with respect to other forms of the compound. LiH, LiF and BeO are normally found as ionic solids. The other molecules are unstable with respect to covalent compounds in which the atoms have their normal valencies H20, BeH2, BF3, B2H6, CH4, (CN)2 and NH3. 

The validity of molecular orbital theory is supported by its ability, unlike valence bond theory, to correctly predict certain properties of homonuclear diatomic molecules of elements in the first and second periods. What prediction would valence bond theory make about the paramagnetism of these molecules For which molecules does molecular orbital theory make a different prediction ... 

A A For which of the homonuclear diatomic molecules or ions of elements in the first and second periods would the paramagnetism or the bond order actually be different if the incorrect energy level diagram (Figure 9-5a vs. Figure 9-5 b) were used (Hint There are four or more correct answers.)... 

Describing molecular orbital configurations Given the formula of a diatomic molecule obtained from first- or second-period elements, deduce the molecular orbital configuration, the bond order, and whether the molecular substance... 

Figure 9-5 Energy level diagrams for first- and second-period homonuclear diatomic molecules and ions (not drawn to scale). The sohd lines represent the relative energies of the indicated atomic and molecular orbitals, (a) The diagram for H2, Hc2, Li2, Bc2, B2, C2, and N2 molecules and their ions, (b) The diagram for O2, F2 and Nc2 molecules and their...

In the last fifteen years, concerning the first and second row elements of the periodic table, the following diatomic molecules containing the Sc atom have been examined by ab initio post-HF methods ScH [4], ScH [5], ScHe [6], ScLi... 

Figure 9-5 Energy level diagrams for first- and second-period homonuclear diatomic molecules and Ions (not drawn to scale). The solid lines represent the relative energies of the indicated atomic and molecular orbitals.

The main scientific prospect for the SAU systems is the publication of atlases, the first of which contains forecasted inter-nuclear separations for diatomic molecules (Hefferlin, Davis and Ileto 2003) and the second of which presents vibration frequencies (Davis and Hefferlin, in preparation). The prospect for public acceptance of this very general periodic system could improve if the public develops a taste for multiple dimensions One way in which this might occur is by exposure to written (Abbott 1952 Burger 1983) or filmed expositions (Banchoff and Strauss Productions 1979),... 

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 10.26 shows the molecular orbital energy level diagram for the first member of the second period, Li2. These molecular orbitals are formed by the overlap of li and 2s orbitals. We will use this diagram to build up all the diatomic molecules, as we will see shortly. 

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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