should be compared not with the covalent radii but with the van der Waals radii of other elements. Table . (a) Atomic Radii/pm Across the Periods Atom (Period II) Be B C N O F Atomic radius Atom (Period III) Mg Al Si P S Cl Atomic radius Table . (b) Atomic Radii/pm Down a Family Atom (Group I) Atomic Radius Atom (Group ) Atomic Radius F Cl K Br Rb I Cs At (b) Ionic Radius The removal of an electron from an atom results in the formation of a cation , whereas gain of an electron leads to an anion .
The ionic radii can be estimated by measuring the distances between cations and anions in ionic crystals. In general, the ionic radii of elements exhibit the same trend as the atomic radii. A cation is smaller than its parent atom because it has fewer electrons while its nuclear charge remains the same. The size of an anion will be larger than that of the parent atom because the addition of one or more electrons would result in increased repulsion among the electrons and a decrease in effective nuclear charge.
For example, the ionic radius of fluoride ion (F – ) is pm whereas the atomic radius of fluorine is only pm. On the other hand, the atomic radius of sodium is pm compared to the ionic radius of pm for Na + . When we find some atoms and ions which contain the same number of electrons, we call them isoelectronic species* . For example, O – , F – , Na + and Mg + have the same number of electrons ( ).
Their radii would be different because of their different nuclear charges. The Fig. . (a) Variation of atomic radius with atomic number across the second period Fig.
. (b) Variation of atomic radius with atomic number for alkali metals and halogens cation with the greater positive charge