Ionic Radius

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It is defined as the distance from the centre of the nucleus upto the last shell containing the electrons.

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.

Cl^- > Cl

• Cations are smaller than the neutral atom because the effective nuclear charge increases.

Na > Na⁺