IONIC RADIUS

 INTRODUCTION AND DEFINATION

"The distance between the centre of of the nucleus of an  ion and the point upto which the nucleus has influence on its electron cloud."

  • The limit of electron cloud  around nucleus cannot be determined with certainty.
  • The radius of an ion is determined by determining  the internuclear distance in an ionic compound by X-ray measurements.
  • This distance is then taken as the sum of the radii of the two ions involved.
  • "If we know the radius of one ion, we  can calculate the radius of the other".
"The atoms change into the ions either by the loss or gain of electrons".

"A cation is always smaller than the corresponding atom ,whereas anion is always bigger than the corresponding atom".



VARIATION OF IONIC RADII IN A GROUP:-

Since the atomic size increases as we move down the group , therefore, ionic radius also  increases in moving from top to bottom in a group.

Example:- ionic radii of alkali metals may be represented as:

Alkali metal ion                 Ionic radii 

          Li+                                        0.60A⁰

          Na+                                     0.95A⁰

        K+                                        1.33A⁰

        Rb+                                      1. 48A⁰

       Cs+                                       1.69A⁰


VARIATION OF IONIC RADII IN A PERIOD :-

Since the atomic size decreases in a period, therefore, ionic radi us also decreases.

In a period,, the ionic radii of  cations of elements of groups I-III and also anions  of the elements of groups V-VII decrease with the increase in atomic number.

GROUPS           CATION               IONIC RADII
I                           Na+                        0.95A⁰
II                         Mg²+                      0.65A⁰
III                       Al³+                        0.50A⁰

V                       N³-                          1.71A⁰
VI                     O²-                           1.40A⁰
VII                   F-                            1.36A⁰.


DETERMINATION OF IONIC RADIUS :-

Since the  like atoms do not form ionic bond with each other, hence it is difficult to measure the ionic radius. The following methods have been used to determine the ionic radius:-

  1. Landes method
  2. Bragg method 
  3. Pauling method
out of these, Pauling method is the most widely accepted.

PAULING METHOD:-

The method is used to determine ionic radii of isoelectronic ions. We know that  radius of an ion is inversely proportional to the effective nuclear charge, i.e.

r ion = A/Z-sigma.




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