Cause of Lanthanide contraction

In lanthanides the additional electron enters into (n−2)f orbital. The mutual shielding effect of (n−2)f electrons is very little because the shape of f−subshell is very much diffused. Thus the effective nuclear charge increases in comparison to the mutual shielding effect of (n−2) f electrons. The outer electrons are attracted more by the nucleus. Consequently the atomic and ionic radii decreases from La (57) to Lu (71)

This type of contraction also occurs in actinides. The jump in contraction between the consecutive elements in the actinides is greater than lanthanides. This is due to the lesser shielding of 5f−electrons which are therefore pulled more strongly by the nucleus.

In a group
(i) The atomic radius of elements increases moving from first transition series (3d) to second transition series (4d). This is due to the increase in number of shells with the increase in atomic number.

(ii) The atomic radii of second (4d) and third (5d) transition series in a group is almost same except Y(39) and La (57)

In third transition series, there are fourteen lanthanides in between La (57) of III B and Hf (72) of IV B groups, so the atomic radius of Hf(72) decreases much due to lanthanide contraction in lanthanides. The difference in the nuclear charge in the elements of a group in first and second transition series is + 18 units while this difference in second and third transition series is + 32 units except Y (39) -> La(57). Due to the increase of + 32 units in the nuclear charge there is a sizable decrease in the atomic radius which balances the increase in size due to the increase in number of shells.

So in a group moving from second to third transition series, the atomic radii of the elements almost remain the same except IIIB. The difference is about 0.02A°.

Illustration : The radii of Ar is greater than the radii of chlorine

Solution: In chlorine, the radii means the atomic or covalent radii which are actually half the inter − nuclear distance between 2 atoms whereas in Argon the radii means the Vander Waal’s radii as Argon is not a diatomic molecule. Vander Waal’s radii is actually half the distance between adjacent molecule. So Vander Waal’s radii being larger than atomic radii, Argon, has got a larger radii than chlorine

Illustration : Berilium and Al are placed in different periods and groups but they show the similar properties.

Solution: On moving across a period the charge on the ions increases and the size decreases, causing the polarising power to increase. On moving down a group the size increases and polarising power decreases. On moving diagonally i.e., from Be to Al these two effects partly cancel each other and so there is no marked change in properties.

Next Page →

← Back Page

Leave a Reply