Covalent radii single-bond, table

The values of f (l) given in the table for electronegative atoms are their normal covalent single-bond radii28 (except for boron, discussed below). The possibility that the radius 0.74 A. of Schomaker and Stevenson29 should be used for nitrogen in the metallic nitrides should be borne in mind. 

Revised Values of Double-Bond Covalent Radii.—This investigation has led to the value 1.34 A. for the carbon-carbon double-bond distance, 0.04 A. less than the value provided by the table of covalent radii.111 4 Five years ago, when this table was extended to multiple bonds, there were few reliable experimental data on which the selected values for double-bond and triple-bond radii could be based. The single-bond radii were obtained -from the study of a large number of interatomic distances found experimentally by crystal-structure and spectroscopic methods. The spectroscopic value of the triple-bond radius of nitrogen (in N2) was found to bear the ratio 0.79 to the single-bond radius, and this ratio was as-... 

The first values of covalent radii, as given in Table 7-2, were formulated before experimental values were available for F—F, O—O, and N—N single bonds. An electron-diffraction study of F2 by Brockway9 then gave the F—F distance as 1.45 A (a value substantiated by Rogers, Schomaker, and Stevenson, Table 7-3, who found 1.435 0.010 A), whereas the accepted radius of fluorine would require 1.28 A. Similar discrepancies were then reported for O—O, for which... 

It is interesting to note that the van der Waals radii given in Table 7-20 are 0.75 to 0.83 A greater than the corresponding single-bond covalent radii to within their limit of reliability they could be taken as equal to the covalent radius plus 0.80 A. 

Table 8.1 lists covalent radii obtained by dividing homonuclear bond distances by two. In many cases the appropriate homonuclear single bond has not been measured and the assigned covalent radius is obtained indirectly by subtracting the covalenl radius of element B in a heteronuclear bond AB to obtain the radius of atom A. 

The simplest diamine, hydrazine N2H4 (Table 3), is normally available as the monohydrate. An X-ray structure of the crystalline solid has been determined (there is some doubt as to the correct space group ) as well as an electron diffraction study of the vapor.The structxues of the di(hydrogen fluoride), di(hydrogen chloride) and monoperchlorate crown ether salts are also known. The N—N distance in hydrazine (1.499 A) is an important parameter, as one half this distance is used for the covalent radius of the single-bonded nitrogen atom. 

The covalent radius is defined as exactly one-half the intemuclear separation between covalently bonded atoms in a predominantly covalent molecule containing a single bond. This definition can be problematic because not every atom can form a pure covalently bonded molecule containing only single bonds. The lighter noble gas atoms, for instance, do not even form bonds. Selected covalent radii are listed in Table 5.1. 

Values of the single-bond covalent radii of nonmetallic elements are given in Table 6-5. The value for hydrogen is 30 pm for all bonds other than H—H (the H—H bond length, 74 pm, corresponds to a larger radius for hydrogen than the value used for other bonds). 

The atomic radius reported in the table is difficult to define, as there is no precise outer boundary of an atom. Its value is obtained from determinations of the atomic distances in a metal by X-ray diffraction methods. The covalent radius is one-half of the distance between the nuclei of two identical atoms joined by a single covalent bond. The ionic radius is calculated from the distance between the nuclei of atoms joined by a ionic bond. The atomic radius of a metal in a metallic structure is usually much greater than the ionic radius of an ion of the same element in a salt crystal. 

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