Homonuclear diatomic molecule second-period atoms

Homonuclear Diatomic Molecules Second-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. 

Note The bond energy is the energy required to dissociate the gas-phase diatomic molecule Into its constituent atoms. Bond energies for the second-period homonuclear diatomic molecules are given in Table 6.3. 

The establishment of accurate and precise Ea for molecules is the ultimate goal of experimental and theoretical studies. The Ea and bond dissociation energies for the Group IA, IB, and IIIA-VIIA homonuclear diatomic molecules are shown in the form of a Periodic Table in Figure 9.1. These are the second entry in each block below the Ea for the atoms [1—3]. The specific references for these data are given in Appendix I, where the Ea of the homonuclear diatomic molecules are listed. The values for the rare gases are 0+ because they only result from the polarization attractions of the dimers. 

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...

We are now ready to study the ground-state electron configuration of molecules containing second-period elements. We will consider only the simplest case, that of homonuclear diatomic molecules, or diatomic molecules containing atoms of the same elements. 

Just as we treated the bonding in H2 by using molecular orbital theory, we can consider the MO description of otiier diatomic molecules. Initially we will restrict our discussion to homonuclear diatomic molecules (those composed of two identical atoms) of elements in flie second row of the periodic table. As we will see, die procedure for determining the distribution of electrons in these molecules closely follows the one we used for H2. 

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.

Figure 10.34 shows the relative energies of the molecular orbitals obtained from 2s and 2p atomic orbitals. This order of molecular orbitals reproduces the known electron configurations of homonuclear diatomic molecules composed of elements in the second row of the periodic table. The order of filling is... 

Homonuclear diatomic molecules (molecules made up of two atoms of the same kind) formed from second-period elements have between 2 and 16 valence electrons. To explain bonding in these molecules, we must consider the next set of higher energy molecular orbitals, which can be approximated by linear combinations of the valence atomic orbitals of the period 2 elements. 

Usually the electronic structure of diatomic molecules is discussed in terms of the canonical molecular orbitals. In the case of homonuclear diatomics formed from atoms of the second period, these are the symmetry orbitals 1 og, 1 ou, 2ag,... 

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