Homonuclear diatomic molecule first-period atoms

Homonuclear Diatomic Molecules First-Period Atoms... 

The two conclusions stated earlier justify the following simplified LCAO MOs for the second-period homonuclear diatomic molecules. As with the first-period atoms, we use the new labels in Table 6.1 to distinguish the approximate MOs from the exact MOs in Figure 6.2 and to indicate their atomic parentage. 

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...

Let us consider an element in the first short Period having 2s and 2p orbitals in its valence shell. When two such atoms are combined into a homonuclear diatomic molecule, the two sets of atomic orbitals may combine into various MO s. Before we can specify the electronic structures of the diatomic molecules of these elements, we must know the relative energies of these MO s. 

Homonuclear diatomic molecules are those composed of two identical atoms. In addition to H2 from Period 1, you re also familiar with several from Period 2—N2, O2, and F2—as the elemental forms under standard conditions. Others in Period 2—U2, Be2, B2, C2, and Ne2—are observed, if at all, only in high-temperature gas-phase experiments. Molecular orbital descriptions of these species provide some interesting tests of the model. Let s look first at the molecules from the block. Groups 1 A( 1) and 2A(2), and then at those from the p block. Groups 3A(13) through 8A(18). 

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. 

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.

---