Lanthanoid Contraction

Contents

1 Lanthanoid Contraction

In the lanthanoid series, with increasing atomic number, the atomic and ionic radii decrease from one element to another but the decrease is very small.

Far example : On moving from Ce to Lu, the atomic radii decrease from 183 pm to 173 pm and the decrease is only 10 pm. The ionic radii decrease from 103 pm to 85 pm on moving from Ce³⁺ to Lu³⁺ ions and the decrease is only 18 pm.

The steady decrease in atomic and ionic sizes of lanthanide elements with increasing atomic number is called lanthanoid contraction.

Cause of Lanthanoid Contraction

In the lanthanoid series, as we move from one element to another, the nuclear charge increases by one unit and one electron is added. The new electrons are added to the same inner 4f-subshell. However, the 4f-electrons shield each other from the nuclear charge quite poorly because of the very diffused shapes of the f-orbitals. The nuclear charge, however, increases by one at each step. Hence, with increasing atomic number and nuclear charge, the effective nuclear charge experienced by each 4f-electron increases. As a result, the whole of 4f-electron shell contracts at each successive element, though the decrease is very small. This results in gradual decrease in size of lanthanoids with increase in atomic number.

Consequences of lanthanoid contraction

1) Resemblance of second and third transition series – It has significant effect on the relative properties of the elements before and after the lanthanoids in the periodic table. There is a regular increase in size from Sc to Y to La.

After the lanthanoids contraction , the increase in radii from second to third transition series almost vanishes. The pairs of elements: Zr-Hf, Nb-Ta, Mo-W, etc., possess almost the same size. The properties of these elements are also very similar. As a result of lanthanoid contraction, the elements of second and third transition series resemble each other much more than the elements of first and second transition series.

2) Similarity among lanthanoids- Because of very small change in radii of lanthanoids, their chemical properties are quite similar. Thus, it is very difficult to separate the elements in pure state.

Methods based on repeated fractional crystallization or ion exchange techniques, which take the advantage of slight differences, in their properties arising from very slight size differences of their trivalent ions have been used.

3) Basicity differences- Due to lanthanoid contraction, the size of lanthanoid ions decreases regularly with increase in atomic number. As a result of decrease in size, their covalent character between lanthanide ion and OH‾ ions increases from La³⁺ to Lu³⁺. Therefore, the basic strength of the hydroxides decreases with increase in atomic number.

Thus, La(OH)₃ is most basic while Lu(OH)₃ is the least basic.

Uses of Lanthanoids

Lanthanoids find special use for the production of alloy steels for plates and pipes. These are also called mischmetals. A well known alloy is mischmetal which consists of a lanthanoid metal (about 95%), iron (about 5%) and traces of S, C, Ca, Al etc. It is used in magnesium based alloy to produce bullets, shells and lighter flint.

(1) Cerium constitutes about 30-50% of the alloys of lanthanides. They are used for scavenging oxygen and sulphur from other metals.

(2) Addition of about 3% misch metal to magnesium increases its strength and is used in making jet engine parts.

(3) Steel mixed with La, Ce, Pr and Nd is used in the manufacture of flame throwing tanks.

(4) Lanthanide oxides are used for polishing glass. Neodymium and praseodymium oxides are used for making coloured glasses for goggles. These are particularly useful for glass blowers as they absorb the bright yellow light.

(5) Mixed oxides of lanthanoids are used as catalysts in petroleum cracking.

(6) Some lanthanoids are used as phosphors in television screens and similar fluorescing surfaces.

(7) Cerium salts are used in dying cotton. They are also used as catalysts.

(8) Lanthanide compounds are used as catalysts for hydrogenation, dehydrogenation, oxidation and petroleum cracking. They are also used in magnetic and electronic devices for their paramagnetic and ferromagnetic properties.

(9) Gadolinium sulphate hepta hydrate has been used to produce very low temperature.

(10) Recently lanthanides have been used in lasers. For example, neodymium oxide dissolved in selenium oxychloride has been used as a powerful liquid laser.

(11) Cerium-magnesium alloys are used in flash light powders.