Question

What is lanthanoid contraction?

Answer 1

The atomic and ionic radii of lanthanoids gradually decrease with an increase in atomic number. This process is known as lanthanoid contraction.

Answer 2

In the lanthanoid series, as the atomic number increases, the atomic and ionic radii decrease from one element to the next, but this decrease is very small. For example – on going from Ce to Lu the atomic radius decreases from 183 pm to 173 pm and this decrease is only 10 pm. Similarly, on going from Ce³⁺ to Lu³⁺ ion the ionic radius decreases from 103 pm to 85 pm, which is only 18 pm. Hence, for an increase of 14 in atomic number, the decrease in atomic and ionic radii is much less. This decrease is much less than the elements of other groups and periods.

Table - Variation in atomic and ionic radii of lanthanum and lanthanoids (pm)

Element	La	Ce	Pr	Nd	Pm	Sm	Eu	Gd	Tb	Dy	Ho	Er	Tm	Yb	Lu
Radius (Ln)	187	183	182	181	181	180	199	180	178	177	176	175	174	173	-
Radius (Ln3+)	106	103	101	99	98	96	95	94	92	91	89	88	87	85	-

The steady decrease in atomic and ionic sizes of lanthanoid elements with an increase in atomic number is called 'lanthanoid contraction'.

The decrease in ionic radii of trivalent lanthanoids (Ln³⁺) is shown in.

Causes of Lanthanide Contraction: In the lanthanoid series, on going from one element to the next, the nuclear charge increases by one unit and an electron is added. These new electrons are added to the parallel 4f-subshells. However, the shielding effect of one 4f-electron on the other 4f-electron (from the nuclear charge) is less due to the very large size of the f-orbitals. Although the nuclear charge increases by one unit at each step, the effective nuclear charge experienced by each 4f-electron increases with the increase in atomic number and nuclear charge. As a result, the entire 4f-electron shell contracts with the addition of each element, although this reduction is minimal. As a result, a regular decrease in the size of lanthanoids is observed with increasing atomic number. The sum of successive reductions gives the total lanthanoid contraction.