Although the actual size of an atom cannot be measured exactly, it is possible to measure
the distance between the nuclei of two atoms. For example, the ‘covalent radius’ of the Cl
atom is assumed to be half of the distance between the nuclei in a Cl 2 molecule. Similarly,
the ‘metallic radius’ is half of the distance between two metal atoms in the crystal lattice of a
metal. These two types of radius are generally known as ‘atomic radii’.
The table below contains the resulting atomic radii for the elements of period three of the
Periodic Table, Na to Cl.
(i) Explain qualitatively this variation in atomic radius.