Determination of Ionic Radii
Ionic radii can be obtained from the dimensions of the unit cell which are obtained from x-ray studies.
It has been noticed that the interionic distances obtained from the calculated value are in good agreement with the observed values (X-ray studies).Ionic radii of common ions
Calculated sum of ionic radius and observed inter ionic distance
Variation of Ionic Radii
Along a group
Ionic radii increases down a group.
Na+ < K+ < Rb+ < Cs+F- < Cl- < Br- < I-
This is because- the number of shells increase.
- screening effect increases.
Along a period
For isoelectronic ions (ions with the same number of electrons) belonging to the same period greater the nuclear charge smaller is the ionic size.
Al3+ < Mg2+ < Na+Sizes of cations and anions versus corresponding atoms
- Anions are larger than the neutral atoms. The effective nuclear charge decreases and the electron cloud expands.
- Cations are smaller than the neutral atom because the effective nuclear charge increases.


