Question

How to measure the covalent radii and the metallic radii?

Answer 1

The covalent radius is measured when the atomic clouds overlap forming a single bond. When this happens the nuclei are pulled together because the electrons that make up the single bond are positioned between the nuclei. This negative charge positioned between two nuclei will tend to cause the nuclei to contract and make the covalent radius smaller. The measurement of the distance between the nuclei divided by two is then called the covalent radius of that atom.

The metallic radii are measured in a different way. In general, it is very difficult to find a molecule where there are just two metal atoms bonded with a single bond. The metal - metal bond is then measured from larger samples of metals consisting of many metal atoms in a chunk of metal. The metal ions that make up this chunk are positively charged centers that are surrounded by a sea of electrons. In this picture the metal ions are just touching their electron clouds, not overlapping as in covalent bonds. Because of this, the metallic radii of atoms will tend to be greater than the corresponding covalent radii of those metals.

Finally, another problem in comparing covalent radii and metallic radii is that the molecules that form single covalent bonds are usually nonmetals. The nonmetals are not metals so they have no metallic radii. The metallic elements rarely form single covalent bonds, like in the $Hg{2}^{2+}$ ion, so it is difficult to compare the metallic bonds that are easily measured and the corresponding covalent bonds that are generally not available for measurement. The best that can be done is to determine the covalent radii for metals by estimating their sizes.