The lanthanides are soft silvery metals that tarnish in the air due to oxidation. They have high melting and boiling points.
Lanthanide Contraction
Moving from left to right across the lanthanide group (increasing Z), the radius of each lanthanide +III ion steadily decreases. This is referred to as lanthanide contraction. After La (Z= 57), as the atomic number increases, electrons are successively added to the inner 4f shell of the atoms, with the outer 5d and 6s electrons remaining without any change. Therefore, theoretically, no change in atomic size from Ce to Lu is expected due to the presence of additional electrons. The size (radius) decreases from 0.106 nm for La³+ to 0.085 nm for Lu³+. This decrease in atomic radius is called the lanthanide contraction. This contraction is a consequence of (i) increased formal nuclear charge Z and (ii) increased effective nuclear charge acting on the outermost electrons.
Both these effects pull the outer electrons closer to the nucleus and decrease the atomic size. The extra orbital electrons (f electrons) incompletely shield the extranuclear charge; f electrons are the least effective in shielding the nuclear charge (shielding efficiency is in the order f < d < p < s). Decreased shielding of the nuclear charge by the inner electrons causes stronger attraction between the nucleus and the outer electrons, causing size contraction. The contraction in the size of the ions, says Ln³+ ions, parallels that of the atoms.
Consequences of Lanthanide Contraction
The properties of an ion depend on the size and charge of the ion. The Ln³+ ions have the same charge (+3); two adjacent Ln³+ ions do not appreciably differ in their sizes (though from La3+ to Lu³+, there is an overall contraction). Therefore, a lanthanide ion and its following ion have almost identical chemical properties. This chemical resemblance results in difficulty in separating the lanthanides.
As a consequence of the lanthanide contraction, the radii of Dy3+ and Ho³+ are almost the same as that of Y3+, which lies above them in the preceding transition series.
Y3+: 0.0900 nm
Dy³+: 0.0912 nm
Ho3+: 0.0901 nm
Due to such a similarity in size, compounds of these three ions have similar properties.
In nature, Y occurs together with the lanthanides in minerals.
Because of the lanthanide contraction, Hf, which follows the lanthanide series, is considerably smaller than expected. Thus, its size is almost the same as that of Zr, which lies above it.
- Zr: 0.145 nm (in second-row. transition series)
- Hf: 0.144 nm (in third-row transition series)
Therefore, Zr and Hf are chemically close; consequently, their separation from a mixture is very difficult. The contraction effect continues with Ta and W, and thus, Ta resembles Nb, and W resembles Mo very closely in properties,
Nb : 0.134 nm, Ta : 0.134 nm
Mo : 0.120 nm, W : 0.130 nm
Chemical Properties
They are electropositive and, therefore, very reactive.
The heavier metals are less reactive than, the lighter ones because they form an oxide layer on their surface. On heating in O2, they form Ln2O3. However, Ce forms CeO2.
They are predominantly tripositive and form ionic compounds.
They react slowly with cold water but rapidly with boiling water.
Their hydroxides are basic; therefore, they dissolve in dilute acids and form salts.
The hydroxides absorb CO₂,
The basicity of the hydroxides decreases as the ionic radius of Ln3+ decreases from Ce3+ to La3+. Thus Ce(OH)3 is the most basic, and Lu(OH)3 is the least basic. The least basic Lu(OH)3 can dissolve in con. NaOH forms a complex hydroxide.
The metals combine with H2 on heating
Yb forms a nonstoichiometric hydride YbH2.5. The lanthanide hydrides are very stable to heat up to 900oC. They are decomposed by water,
They form halides on heating with X2
Carbides are formed by heating Ln2O3, with C, in an electric furnace.
They combine with N, P, As, Sb and Bi at elevated temperatures.
Lanthanides form several oxo salts. Some of the essential oxo salts are given below.
| La2(SO4) | Lanthanum sulphate |
| Ce(SO4)2 | Ceric sulphate |
| Ce2(ox)3 | Cerous oxalate |
| Ce(NO3)4 | Ceric nitrate |
| EuSO4 | Europium sulphate |
| (NH4)2Ce(SO4)3] | Ceric ammonium sulphate, ammoniumtrisulphatocerate(IV) |
| (NH4)2[Ce(NO3)6] | Ceric ammonium nitrate, ammoniumhexanitrocerate(IV) |
| SmCO3 | Samarium carbonate |
| SmSO4 | Samarium sulphate |
| YbCO3 | Ytterbium carbonate |
The Ln elements are strong reducing agents.
Like transition metals, lanthanide ions form complexes. They show high coordination
numbers, usually greater than 6; coordination numbers of 8,9 and 12 are known.
Many lanthanide compounds fluoresce under ultraviolet light.
Most of their compounds are paramagnetic.
Uses
La is used making for making steel alloys (used in plates and pipes).
Mischmetal (Ce + La + other lanthanides) is used in lighter flints.
Lanthanide-cobalt alloys are used for making permanent magnets.
Mixed lanthanide oxides are employed as catalysts in petroleum cracking.
CeO2 is used to polish glass.
Lanthanide oxides are used as phosphors in television screens.
