\(\renewcommand\AA{\text{Å}}\)

# Covalent radius

## `periodictable.covalent_radius`

This module adds the following fields to the periodic table

- covalent_radius

- covalent_radius_uncertainty

- covalent_radius_units = ‘angstrom’

Use `init()` to initialize a private table.

Data is taken from Cordero et al., 2008 [1]. Bond specific values (single, double, or triple) are available from PyykkÃ¶ et al., 2009 [2], but they are generally smaller. The CRC Handbook uses the average of Cordero and PyykkÃ¶. Note that the combined Cordero/PyykkÃ¶ tables are included herein as periodictable.covalent_radius.CorderoPyykko, but are not yet parsed.

The abstract of Cordero reads as follows:

> A new set of covalent atomic radii has been deduced from crystallographic data for most of the elements with atomic numbers up to 96. The proposed radii show a well behaved periodic dependence that allows us to interpolate a few radii for elements for which structural data is lacking, notably the noble gases. The proposed set of radii therefore fills most of the gaps and solves some inconsistencies in currently used covalent radii. The transition metal and lanthanide contractions as well as the differences in covalent atomic radii between low spin and high spin configurations in transition metals are illustrated by the proposed radii set.

Note

1. Values are averages only. The particular radius can be highly dependent on oxidation state and chemical compound.

2. The paper lists values for multiple spin states on select elements. We are using sp3 for carbon and low spin for manganese, iron and cobalt.

3. Elements with zero or one measurements of covalent radius are assigned an uncertainty of 0.00. These are He, Ne, Pm, At, Rn, Fr, Ac, Pa.

4. Elements above 96 are assigned a covalent radius and uncertainty of None.

5. Radii are measured from bonds to C, N or O. The choice of which compound was used is element dependent. Details are available in the references.

[1]

Beatriz Cordero, VerÃ³nica GÃ³mez, Ana E. Platero-Prats, Marc RevÃ©s, Jorge EcheverrÃa, Eduard Cremades, Flavia BarragÃ¡n and Santiago Alvarez. Covalent radii revisited. Dalton Trans., 2008, 2832-2838. doi:10.1039/b801115j

[2]

P. PyykkÃ¶ and M. Atsumi. Molecular Double-Bond Covalent Radii for Elements Li-E112. Chemistry, A European Journal, 15, 2009, 12770-12779. doi:10.1002/chem.200901472

periodictable.covalent_radius.init(table, reload=False)

  Add the covalent radius property to a private table. Use reload = True to replace the covalent radius property on an existing table.