Diatomic molecules of the second-period elements

Among the diatomic molecules of the second period elements are three familiar ones, N2,02, and F2. The molecules Li2, B2, and C2 are less common but have been observed and studied in the gas phase. In contrast, the molecules Be2 and Ne2 are either highly unstable or nonexistent. Let us see what molecular orbital theory predicts about the structure and stability of these molecules. We start by considering how the atomic orbitals containing the valence electrons (2s and 2p) are used to form molecular orbitals. 

Which supposed homonuclear diatomic molecules of the second-period elements should have zero bond order ... 

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

Electron Configurations of Diatomic Molecules of the Second-Period Elements 403... 

For the molecular orbitals in O2 and F2, the situation is as expected because the energy difference between the 2s and 2p orbitals is large, and little s and p mixing takes place that is, the (T2s and a2p orbitals are not modified as described above. For other diatomic molecules of the second-period elements (for example, C2 and N2), the TT2p orbitals are at a lower energy than a2p because the energy difference between the 2s and 2p orbitals is smaller, and 2s-2p orbital interactions affect the way in which atomic orbitals combine. This leads to the modified a2s and (T2p orbitals described above. 

Here is how we assign electrons to the molecular orbitals of the diatomic molecules of the second-period elements We start with the and a g orbitals filled. Then we add electrons, in order of increasing energy, to the available molecular orbitals of the second principal shell. Figure 11-26 shows the electron assignments for the homonuclear diatomic molecules of the second-period elements. Some molecular properties are also listed in the figure. 

To obtain the MO structure of the diatomic molecules of the elements in the second period, we fill the available molecular orbitals in order of increasing energy. The results are... 

In addition to the homonudear molecules, the elements of the second period form numerous important and interesting heteronudear species, both neutral molecules and diatomic ions. The molecular orbital diagrams for several of these species are shown in Figure 3.9. Keep in mind that the energies of the molecular orbitals having the same designations are not equal for these species. The diagrams are only qualitatively correct. 

The electronic configurations of the homonuclear diatomic molecules of the elements of the second period, and some of their ions, are given in Table 4.1. 

The experimental data for homonuclear diatomic molecules arc summarized in Table XXX and Figure /, where it is seen that amongst elements of the second period there is a progressive change in the values of the dissociation energy, ue. the bond energy, and the... 

The variation in the energy levels and electron occupancy for homonu-clear diatomic molecules of the elements of the second period is shown in Figure 8.13. It can be seen that the energy of the 2pjt orbitals remains approximately constant in going from Li2 to Fj, whereas the energy of the 2p.Cg orbital drops considerably, and finally becomes less than that of the 2p7t orbital towards the end of the period. 

Molecular orbital theory explains the formation of multiple bonds both in the case of transition metal complexes as well as diatomic molecules of main group elements and compounds of these elements. There is an analogy between E2 molecules of the elements of the second period of the Periodic Chart (and their compounds containing multiple bonds) and M2 molecules as well as dimeric transition metal complexes which possess metal-metal bonds. The molecules C2, N2, and O2 have ttV , and elec-... 

To obtain the MO structure of the diatomic molecules of the elements in the second period, we fill the available molecular orbitals in order of increasing energy. The results are shown in Table 1. Note the agreement between MO theory and the properties of these molecules. In particular, the number of unpaired electrons predicted agrees with experiment. There is also a general correlation between the predicted bond order ... 

Fig. 5.12 Simplified molecular orbital energy levels for diatomic molecules of elements in the second period, assuming no mixing of s and p orbitals. The three 2p orbitals are degenerate, that is, they all have the same energy and might also be...

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... 

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. 

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. 

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... 

---