Chemical reactivity and lanthanoid contraction

d and f Block Elements

Chemical reactivity and lanthanoid contraction and its consequences

The lanthanides have low charge density due to their larger size in spite of having a high charge (+3). Hence, they do not cause much polarization of the ligands and have a weak tendency for complex formation. If lanthanide elements are ignited in air or O₂, they readily form the oxides of Ln2O3 type except for Ce which gives a dioxide, CeO₂. The oxides are ionic and basic. The basic nature of oxides decreases along with the series with decreasing ionic size. Ytterbium resists the action of air even at 10000C due to the formation of a protective layer of oxide on its surface. The lanthanides react slowly with cold water but readily with hot water.

2 Ln + 6 H₂O → 2 Ln(OH)₃ + 3H₂

On adding aqueous ammonia to this aqueous solution, hydroxides are precipitated as a gelatinous precipitate. These hydroxides are also ionic and basic, the basic nature decreasing with increasing atomic number. La(OH)₃ is most basic and Lu(OH)₃ is the least basic. Their basic character is more than that of Al(OH)₃ but less than that of Ca(OH)₂.

Lanthanides form oxo-salts such as nitrates, sulphates, perchlorates and salts of oxoacids which are soluble in water but carbonates and oxalates are insoluble. The difference in basicity is responsible for the difference in thermal stability of the oxosalts which decreases along with the series. Thus, La(NO₃)₃ is more stable than Lu(NO₃)₃.

The lanthanides also burn in halogens to produce LnX₃ type halides and combine with H₂ at high temperature to give stable MH₂ or MH₃ type hydrides. Among halides, fluorides are insoluble but other halides are soluble in water.

It has been observed that the atomic, as well as the ionic radii of lanthanides, decrease steadily as we move from Ce to Lu. The ionic radii have been listed below(for Ln³⁺ ions): The size of Lanthanoids and its trivalent ion decreases from La to Lu due to poor shielding of 4f electrons.

The ionic radii decrease steadily all along with the series amounting in all to 18 pm. This shrinking in the ionic size of the Ln³⁺ ions with increasing atomic number is called lanthanide contraction. The term steadily decreases means the values decrease regularly and with a very small difference though the nuclear charge increases by +14 units from the first to the last element. It may be concluded that the lanthanide contraction among the 4f-series elements and their ions takes place due to the poor shielding effect of 4f-electrons and gradual increase in the nuclear charge.

Some important consequences of lanthanides contraction are listed below:

1. The high density of post lanthanide elements: Because of lanthanide contraction the atomic sizes of the post lanthanide elements become very small. The arrangement of atoms in metallic lattice is much compact that the densities are very high.
2. The basic character of oxides, Ln₂O₃ and hydroxides, Ln(OH)₃: There is a decrease in basic strength of oxides and hydroxides of lanthanides with an increase in atomic number. The basic strength of hydroxide decreases from La(OH)₃ TO Lu(OH)₃. The basicity decreases as ionic radii decreases.
3. A small change in the properties due to Lanthanide contraction allows the separation of Lanthanides by the difficult methods based on fractional crystallization and basicity differences.
4. The pair of elements i.e. Zr-Hf, Nb-Ta, have almost similar size and they are much closer to one another in properties than the pairs of elements of 1st and 2nd transition series, e.g. solubilities of their salts are very much like one another.
5. The occurrence of Y with heavy Lanthanides: The crystal radii of Y⁺³ and Er⁺³ are equal (Y⁺³=0.93 A0 and Er⁺³ =0.96 A0 ). This similarities in atomic size of these two cations coupled with the equality in ionic charge (= +3 in both the ions) account for the invariable occurrence of Y with heavier Lanthanides.