The covalent bonds are formed from the overlap of the orbitals in the region between the centres of two atoms. As a result of it the nuclei of bonded atoms approach each other more closely than do the nuclei of non-bonded atoms. The covalent radius of an atom is taken as one half the distance between the nuclei of two identical atoms forming a single covalent bond.
Thus we can say that the bond distance between the two atoms A — B should be the arithmetic mean of the bond lengths A — A and B — B. Take the case of covalent radius of carbon which is one half the experimentally determined distance between C — C single bond. This gives the value of 77 pm. Similarly, for the Si — Si linkage the covalent radius comes to be 117 pm. Now if we consider the bond distance between carbon and silicon, we should expect a bond length of 194 pm. This is in very good agreement with the experimentally determined C — Si distance of 193 pm in carborundum (silicon carbide). Covalent radius of C — Si bond is half of the bond length.
The covalent radii decrease with increase in bond order because there is corresponding decrease in internuclear distance. Thus in carbon, the internuclear distance and atomic radii decrease with increase in bond.
Bond Order Inter Nuclear Distance (pm) Atomic Radius (pm) C – C 154 77 C=C 134 67
C =- C 120 60
Although the above rule works in many simple diatomic molecule, but this is not generally the case. Very often there is a considerable deviation from the expected result. This deviation can be attributed to many factors like multiple.