Covalent bond radii

As in the case of ions we can assign values to covalent bond lengths and covalent bond radii. Interatomic distances can be measured by, for example. X-ray and electron diffraction methods. By halving the interatomic distances obtained for diatomic elements, covalent bond radii can be obtained. Other covalent bond radii can be determined by measurements of bond lengths in other covalently bonded compounds. By this method, tables of multiple as well as single covalent bond radii can be determined. A number of single covalent bond radii in nm are at the top of the next page. 

As a result of the development of the x-ray method of studying the structure of crystals and the band-spectroscopic method and especially the electron-diffraction method of studying gas molecules, a large amount of information about interatomic distances in molecules and crystals has been collected. It has been found that the values of interatomic distances corresponding to covalent bonds can be correlated in a simple way in terms of a set of values of covalent bond radii of atoms, as described below.1... 

From the covalent bond radii (Table 8.1) we can estimate that a bond between a nitrogen atom and an atom of the first Iransition series should be about 200 pm long. The size of the "hole" in the center of the porphvnn ring is ideal for accommodating metals of the first transition series (Fig. I9.lb>. The porphyrin system is fairly rigid. 

The interatomic distance at the bottom of the potential well, the most favorable distance of separation, is known as the van der Waals contact distance. A particular atom has a characteristic van der Waals radius (Table 11.3). These radii are additive, so that the optimal distance of contact between two atoms may be found by the addition of their two van der Waals radii. The van der Waals radii are not as sharply defined as covalent bond radii. This is because the potential energy wells are so shallow that contact distances may vary by 0.1 A (0.01 nm) or so... 

Since the molecule HgO (g) has the same number of valence electrons as NHg(g), a pyramidal molecular configuration (C ) is assumed for HgO (g) in the tabulation. Clifton (7) has obtained a bond distance O-H of 0.95 A from an application of covalent bond radii and the effect of electric charge, and a bond angle H-O-H of 109 by extrapolating a plot of bond length versus bond... 

In the crystal and in the liquid, the hydrogen molecules may be described as in contact with one another. The observed distance between the centers of two nonbonded atoms in contact is given reasonably well by the sum of the packing radii of the two atoms. Distances between the centers (the nuclei) of two atoms connected to one another by a chemical bond are given reasonably well by the sum of their covalent-bond radii. (See the tables following plate 57.)... 

It has been found that the values of interatomic distances corresponding to covalent bonds can be correlated in a simple way in terms of a set of values of covalent bond radii of atoms. ... 

The sum of the covalent bond radii for the Al-0 pair is 1.84 A and the values observed should be compared with this value. Indeed, observed covalent A1 -O bonds are close to it and slightly longer bonds can be regarded as dative. 

The foregoing discussion has been based on the relative sizes of cations and anions. A closely parallel discussion could be presented based on covalent bond radii and van der Waals radii of atoms (Section 6-15). The van der Waals radius of the fluorine atom, 135 pm (Table 6-6), is nearly equal to the ionic radius of the fluoride ion, 136 pm, and the sums of covalent radii are approximately equal to the corresponding sums of ionic radii. 

Electronegativity, covalent bond radii, bond length of M-C and M-H and dissociation energies of M-C, M-H and M-M of group 14 elements are shown in Table... 

As the sum of the covalent bond radii of CrC is 2.21 A [52,53] (CrC in r/ compounds is 2.250 A in Table 13.2), these compounds form double bonds which are slightly shorter than that of the CrC single bond [53]. 

Table 7 Covalent bonding radii (in pm) for calculations of single bond distances in Lewis-valent compounds with the modified Schmnakcr-Stevenson rule (Eq. 8)...

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