Periodicity of ionic radii

These are radii of ions in ionic crystals. Ionic radius may be defined as the effective distance from the center of nucleus of an ion up to which it has an influence on its electron cloud. In ionic compounds the inter nuclear distance may be taken as equal to the sum of the ionic radii of the two ions. The inter nuclear distance in ionic crystals are obtained from X-ray studies.

Fig: 4.6 - Internuclear distance and ionic radii

Radius of the cation

A cation is formed by the loss of one or more electrons from the gaseous atom. Thus, the whole of the outer most shell of electrons is removed resulting in the smaller size in the cation.

For example, in lithium atom, there is only one electron in the outermost '2s' shell. As the lithium atom changes to Li⁺ ion the outer most '2s' shell disappears completely. This disappearance results in the decrease in size.

With the removal of electrons from an atom the magnitude of the nuclear charge remains the same while the number of electrons decreases. As a result the nuclear charge acts on less number of electrons. The effective nuclear charge per electron increases and the electrons are more strongly attracted and pulled towards the nucleus. This causes a decrease in the size of the ion.

Radius of the anion

The negative atom is formed by the gain of one or more electrons in the neutral atom. The number of electrons increases while the magnitude of nuclear charge remains the same. The same nuclear charge acts on larger number of electrons than were present in the neutral atom. The effective nuclear charge per electron is reduced and the electron cloud is held less tightly by the nucleus. This causes an increase in the size of the ion. Thus anions are larger in size than the corresponding atom.

Variation of ionic radii in a group

The ionic radii in a particular group increases in moving from top to bottom because of the increase in the principal quantum number though the number of electrons in the valence shell remains the same.

In isoelectronic series of ions, as the nuclear charge increases the electrons are pulled more and more strongly and the size decreases.

Problem

5. Out of Na⁺ and Na which has the smaller size and why?

Solution

Na⁺ has a smaller size than Na. Na⁺ is formed by the removal of one electron from Na. However both of these posses the same nuclear charge. Therefore electrons in Na⁺ are more tightly held than in Na. The removal of one electron from Na also leads to complete removal of the third shell so that in Na⁺, the outermost shell is second. Hence Na⁺ has a smaller size than Na.