Lanthanide contraction-definition-causes-consequences in chemistry

KAKALI GHOSH , Teacher,blogger. M.Sc chemistry.

4 min readApr 24, 2020

Lanthanide contraction definition in chemistry

In case of lanthanide elements , it has been experimentally found that with increasing atomic number , the atomic and ionic size of lanthanide elements gradually decreases from lanthanum to lutetium.

It is very much regular for lanthanide ions ( Ln 3+) . It will be seen that on going from La 3+ to Lu 3+, the ionic radius shrinks from 115 pm to 93 pm .

The steady decrease in ionic radius all along the series amounting in all to 22 pm , is called lanthanide contraction .

The lanthanide contraction is also known as 4f or f contraction .

Lanthanide contraction causes

In case of lanthanide series elements, the differentiating electron entered into the 4f-orbital one at each step .

Now, the shielding or screening effect of ‘f’-orbital is very much less than ‘s’ , ‘p’ even than that of ‘d’-orbital . This is due to the shape of ‘f’-orbital.

Hence the effective nuclear charge increases by one at each step . Consequently , the inward pull experienced by the 4f electrons increases.

As a result , the size of the entire 4f n shell reduces . The sum of the successive reductions gives the lanthanide contraction .

Lanthanide contraction consequences

( I ) Change of basic properties of lanthanide metal hydroxide compounds .

Due to lanthanide contraction , the ionic size from La 3+ to Lu 3+ along lanthanide series gradually decreases.

As a result, the covalent properties of lanthanide metal hydroxide compounds regularly increases [according to Fajan’s rule,]from lanthanum to lutetium .

That is ,Lu ( OH )3 is more covalent than La ( OH )3 . Now , with increasing covalent character, their basic property decreases .

Consequently, the most basic hydroxide among the lanthanide elements is La ( OH )3 and the least basic hydroxide is Lu ( OH )3 .

( II ) The properties of metal ions are determined by their size and charge .Since all the lanthanide ions are of about the same size and also carry the same charge due to lanthanide contraction, their properties almost identical.

Hence separation of lanthanides from one another is very much difficult.

( III) The difference of density between the second and third transition metal series .

The density of second and third transition metal series are much more higher due to lanthanide contraction .

Because the atomic volume of two different elements of 4d and 5d series with same group is near about same , but atomic mass of 5d series elements becomes much higher.

( IV )The decrease in chemical reactivity of next transition elements on the lanthanide .

Due to lanthanide contraction, the ionization energy of the next on the lanthanide elements increases. Hence , their chemical reactivity decreases.

For examples, the chemical reactivity of the element which belong to sixth period such as, gold ( Au ), platinum ( Pt ), mercury ( Hg ) etc are very much less.

( V ) Similarities in atomic size of second and third transition series elements .

Due to lanthanide contraction, the atomic and ionic radius of the next and before on the lanthanide elements with same group, are about same.

For examples, the atomic radius of Zr [ group — 4 , period — 5] and Hf [ group — 4 , period — 6] are 160 pm and 159 pm respectively .

( VI ) Separation difficulties between second and third transition series elements .

The atomic and ionic radius of second and third transition series elements are identical to each other due to lanthanide contraction.

As a result, their physical and chemical properties are similar to each other . Hence , separation of second and third transition series elements from each other are much difficult.

For example, separation of ‘ Zr’ from ‘Hf’ is very much difficult due to lanthanide contraction.

Why is La ( OH )3 more basic than Lu ( OH )3 ?

It has been experimentally found that with increasing atomic number , the atomic and ionic size of lanthanide elements gradually decreases from lanthanum to lutetium due to lanthanide contraction.

As a result, the covalent properties of lanthanide metal hydroxide compounds regularly increases [according to Fajan’s rule,]from lanthanum to lutetium .