Covalent radii multiple-bond

Each atom makes a characteristic contribution, called its covalent radius, to the length of a bond (Fig. 2.21). A bond length is approximately the sum of the covalent radii of the two atoms (36). The O—H bond length in ethanol, for example, is the sum of the covalent radii of H and O, 37 + 74 pm = 111 pm. We also see from Fig. 2.21 that the covalent radius of an atom taking part in a multiple bond is smaller than that for a single bond of the same atom. 

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 atomic radius of the atom X is defined as half the length of an X-X single bond. This can be obtained experimentally from the structures of elemental substances containing molecules X where the X-X bond order is believed to be unity, e.g. Cl2, P4, S8. It may also be obtained from the X-X distances found in molecules such as HO—OH, H2N—NH2 etc. for atoms which form multiple bonds in the elemental substance. Such atomic radii may be termed covalent radii. For atoms which form metallic elemental substances, metallic radii are obtained. These are usually standardised for 12-coordination of each atom, which is the most common situation in metals. Corrections can be made in the cases of metals which adopt other structures. 

S The covalent radius depends on the multiplicity of the bond, such that the radius for a double bond is smaller than that for a single bond, etc. 

The covalent radius of an element may be considered to be one half of the covalent bond distance of a molecule such as Cl, (equal to its atomic radius in this case), where the atoms concerned are participating in single bonding. Covalent radii for participation in multiple bonding are also quoted in data books. In the case of a single bond between two different atoms, the bond distance is divided up between the participants by subtracting from it the covalent radius of one of the atoms, whose radius is known. A set of mutually consistent values is now generally accepted and, since the vast majority of the elements take part in some... 

On looking for a relationship between ionization radius and the chemistry of homonuclear covalent interaction, the classification into single and multiple bonds is followed as a first approximation. An immediate observation, valid for most single bonds, is a constant value of the dimensionless distance... 

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