Covalent radius of fluorine

The first attempt at trying to find the covalent radius of fluorine was in 1937, by Brockway. Brockway prepared a vapour of F2 molecules by means of the electrolysis of potassium bifluoride (KHF2) in a fluorine generator, which was constructed of Monel metal. Then, the product was passed over potassium fluoride so as to remove any hydrogen fluoride (HF) and to condense the product into a liquid. A sample was collected by evaporating the condensed liquid into a Pyrex flask. Finally, using electron diffraction, it was determined that the bond length between the two fluorine atoms was about 145 pm. He therefore assumed that the covalent radius of fluorine was half this value, or 73 pm. This value, however, is inaccurate due to the large electronegativity and small radius of fluorine atom. ==Schomaker and Stevenson==

In 1941, Schomaker and Stevenson proposed an empirical equation to determine the bond length of an atom based on the differences in electronegativities of the two bonded atoms. :dAB = rA + rB – C|xA – xB| :(where dAB is the predicted bond length or distance between two atoms, rA and rB are the covalent radii (in picometers) of the two atoms, and |xA – xB| is the absolute difference in the electronegativities of elements A and B. C is a constant which Schomaker and Stevenson took as 9 pm.) This equation predicts a bond length which closer to the experimental value. Its major weakness is the use of the covalent radius of fluorine that is known as being too large. ==Pauling==

In 1960, Linus Pauling proposed an additional effect called "back bonding" to account for the smaller experimental values compared to the theory. His model predicts that F donates electrons into a vacant atomic orbital in the atom it is bonded to, giving the bonds a certain amount of sigma bond character. In addition, the fluorine atom also receives a certain amount of pi electron density back from the central atom giving rise to double bond character through (p-p)π or (p-d)π "back bonding". Thus, this model suggests that the observed shortening of the lengths of bonds is due to these double bond characteristics. ==Reed and Schleyer==

Reed and Schleyer, who were skeptical of Pauling's proposition, suggested another model in 1990. They determined that there was no significant back-bonding, but instead proposed that there is extra pi bonding, which arose from the donation of ligand lone pairs into X-F orbitals. Therefore, Reed and Schleyer believed that the observed shortening of bond lengths in fluorine molecules was a direct result of the extra pi bonding originating from the ligand, which brought the atoms closer together. ==Ronald Gillespie==

In 1992, Ronald Gillespie and Edward A. Robinson suggested that the value of 71 pm was too large because of the unusual weakness of the F-F bond in F2. Therefore, they proposed using the value of 54 pm for the covalent radius of fluorine. However, there are two variations on this predicted value: if they have either long bonds or short bonds. • An XFn molecule will have a bond length longer than the predicted value whenever there are one or more lone pairs in a filled valence shell. ==Pekka Pyykkö==

Theoretical chemist Pekka Pyykkö estimated that the covalent radius for a fluorine atom to be 64 pm in a single bond, 59 pm and 53 pm in molecules where the bond to the fluorine atom has a double bond and triple bond character, respectively. ==References==