Covalent Radius Definition

Nitrogen molecule

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The covalent radius refers to the size of an atom that forms part of a single covalent bond. Covalent radius is expressed in terms of picometers (pm) or angstroms (Å). In theory, the sum of two covalent radii should equal the covalent bond length between two atoms, but in practice the length of the bond depends on the chemical environment. Charts are also compiled for the covalent radius for double and triple covalent chemical bonds.

Covalent Radius vs Atomic Radius

There are other methods of gauging the size of atoms. Technically, they are all estimates of the atomic radius. However, data tables of the atomic radius are for the distance between the centers of the nuclei of atoms that are just touching each other. In this context, "touching" means the outermost electron shells are in contact with each other. The ionic radius is another method of estimating atom size. The ionic radius is half the distance between two atoms touching each other in a crystal lattice (atoms forming an ionic bond).

The covalent radius and ionic radius may be larger or smaller than the atomic radius of an atom of an element. In general, atomic radius follows a trend in the periodic table in which the radius increases moving down an element group and decreases moving left to right across a period.

Sources

  • Pyykkö, P.; Atsumi, M. (2009). "Molecular Single-Bond Covalent Radii for Elements 1-118." Chemistry: A European Journal. 15: 186–197. doi:10.1002/chem.200800987
  • Sanderson, R. T. (1983). "Electronegativity and Bond Energy." Journal of the American Chemical Society. 105 (8): 2259–2261. doi:10.1021/ja00346a026