Actinide contraction-definition-causes-consequences.

Actinide Contraction Definition In Chemistry

The atomic and ionic radii of the actinide elements gradually decreases as we move from thorium to lawrencium along actinide series .

Thus,on going from Th 3+to Lr 3+ ion ,the ionic radius decrease from 109 pm to 98 pm . There is also a steady decrease in the radii of An 4+ ion [ An = actinide ] .

This steady decrease in atomic and ionic radii with increasing atomic number of actinide elements that is, positive nuclear charge , is called actinide contraction and it is analogous to lanthanide contraction.

Although ,in case of actinide elements, the contraction of volume from one element to another is higher than lanthanide element.

Because of the poor shielding or screening effect of 5f-orbital than 4f-orbital. The actinide contraction is also known as f or 5f- contraction .

causes of actinide contraction

The common electronic configuration of actinide elements is 5f 1–14 6d 0–1 7s 2 .

In actinide series elements , the differentiating electrons entered into the inner 5f –orbital by one at each step from actinium to lawrencium along actinide series .

Now , the shielding or screening effect of ‘5f’-orbital is very much less than ‘s’ , ‘p’ even than that of ‘d’-orbital . This is due to the shape of ‘5f’-orbital.

Hence the effective nuclear charge increases by one at each step . Consequently , the inward pull experienced by the 5f electrons increases.

As a result , the size of the entire 5f n shell reduces . The sum of the successive reductions gives the actinide contraction .

Consequences of actinide contraction

Most of the actinide elements are man made, except Ac, Th , Pa , U. They are formed by the nuclear reaction . These elements are called trans uranic elements.

However, due to actinide contraction , the ionic size of the actinide elements are almost same . For this reason, separation of actinide elements from each other is difficult .

For example, separation of Ac, Th , Pa and U elements to each other from uranium ores are not so easy .

Again, due to actinide contraction, the ionic size from Ac 3+ to Lr 3+ along actinide series gradually decreases.

As a result, the covalency of the compounds of actinide metal elements increases from left to right along actinide series .

That is, the compounds of lawrencium are most covalent and the compounds of actinium are least covalent .

Why is Ac ( OH )3 more basic than Cf ( OH )3 ?

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

As a result, the covalent properties of actinide metal hydroxide compounds regularly increases [according to Fajan’s rule,]from actinium to lawrencium.

That is , Cf ( OH )3 is more covalent than Ac ( OH )3 . Now , with increasing covalent character, their basic property decreases . Consequently, Ac ( OH )3 is more basic than Cf ( OH )3 .