2. Atomic and metallic rays

The electron cloud of an atom has no precise boundary. The radius of an atom can therefore only be determined indirectly, by measuring the distance between the nuclei of two covalently bonded atoms. This distance, which represents the bond length, is equal to the sum of the atomic radii (also called covalent radii) of the two atoms. The values of the atomic radii are given in table 2 . It should be stressed that the values that can be assigned to atomic radii are often not very precise, as they depend somewhat on their molecular environment. Hydrogen is an example of extreme imprecision.

In the case of rare gases, which only exceptionally form bonds, special methods must be used, and results differ significantly depending on the method