Homonuclear diatomic molecules bond dissociation energies

Figure 13.18 Bond dissociation energies for gaseous, homonuclear diatomic molecules (from J. A. Kerr in Handbook of Chemistry and Physics, 73rd edn., 1992-3, CRC Press, Boca Raton, Florida), pp. 9.129-9.137.

Table 3.13. Calculated spin multiplicity, bond length Re, and dissociation energy De of first-row homonuclear diatomic molecules, with comparison experimental valued1 in parentheses...

Starting from ionization radii, r o, and using experimentally measured values of dissociation energy and interatomic distance for homonuclear diatomic molecules, a self-consistent set of characteristic radii, suitable for the point-charge calculation of bonding parameters, of both homonuclear and... 

In order to test the point-charge method experimentally measured dissociation energy and interatomic distance are required for each chemical bond. Dissociation energies for most homonuclear diatomic molecules have been measured spectroscopically and/or thermochemically. Interatomic distances for a large number of these are also known. However, for a large number of, especially metallic diatomic molecules, equilibrium interatomic distances have not been measured spectroscopically. In order to include these elements in the sample it is noted that for those metals with measured re, it is found to be related, on average, to 5, the distance of closest approach in the metal, by re = 0.78(5. On this assumption reference values of interatomic distance (d) become available for virtually all elements, as shown in the data appendix. In some special cases well-characterized dimetal bond lengths have also been taken into account for final assessment of interatomic distance. 

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... 

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 isoelectronic equivalence is the simplest procedure for estimating electron affinities. It was applied to H2 and I2 and to the atomic electron affinities. Species with the same outer electronic configuration should have similar electron affinities and bond dissociation energies. This results in the relative constancy of the electron affinities of a given family of atoms. The equivalence of the bond dissociation energies for the X2( ) and Rg2(+) ions is also based on this principle. The systematic variation of the electron affinities of the homonuclear diatomic molecules is another example. 

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 9.1 Electron affinities of the elements, electron affinities, and bond dissociation energies of the homonuclear diatomic molecules in the form of a Periodic Table [1],...

Two branches are found. Dissociation energies are less than limyv oo 7V> total number of electrons is uneven. They are larger than lim y oo 7V > total number of electrons is even. The first property is due to the occupation of a nonbonding orbital, the second property corresponds with the experimental observation that the bond energies of homonuclear diatomic molecules are usually stronger than the energy of an atom-atom bond in the bulk. The atom-atom distance also usually proves smaller in a homonuclear molecule than in the bulk. Both properties arise from the weaker metal-metal bond in the bulk of a metal. This is due to delocalized nature of metal electrons, as we will explain in more detail later. 

The nature of the molecular ionization and the final state of the dissociated cation must be understood for quantitative applications of these principles. In Table I, the bond dissociation energies for several homonuclear diatomic molecules are calculated using Equation 4 and are compared to the dissociation energies found by spectroscopic means. The dissociation energy determined from ionization energies decreases from... 

Table 4.1. Gaseous homonuclear diatomic molecules, A2(g) standard dissociation energies at 298 K, D298 equilibrium bond distances, R vibrational wavenumbers, a> dissociation energies at zero K, Dq reduced masses, /u, and force constants,/r.

Fig. 4.1. Standard dissociation energies of homonuclear diatomic molecules at 298 K (in kJ mol ) as a function of the equilibrium bond distance (in pm).

In Fig. 4.1 we present a plot of the dissociation energies of the homonuclear diatomic molecules H2, X2 and M2 as a function of the bond distances. The bond distances reflect the sizes of the atoms involved. Indeed, one might define the bonding radius of the atom A as half the single bond distance A-A. It is seen that - except for the rule-breaker F2 - the bond energy decreases with increasing bond distance large atoms form weak bonds. 

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