4. Introduction
Lanthanides are elements in which differentiating electrons enter (n-2) f -orbital.
Since (n-2) f orbital lies comparatively with the inner to the penultimate shell, these
elements are also called inner transition elements.
Atomic number of lanthanum is 57, electronic configuration of lanthanum
is [Xe]4f05d16s2[Xe]4f05d16s2.
Lanthanides have general electronic configuration [Xe]4f1−145d0−16s2[Xe]4f1−145d0−16s2,
orbitals can accommodate 14 electrons.
Cerium having atomic number 58 to Lutetium having atomic number 71 are lanthanoids.
Lanthanum itself is a d-block element, elements following lanthanum are called lanthanides.
In Lanthanide, the last electron enters into a 6s orbital, so lanthanides are placed in period 6
and Group 3 in periodic table.
5. Introduction
In lanthanide, differentiating electrons enter in a 4f orbital. fourteen elements following
actinium are called Actinides.
Actinoids include fourteen elements from Thorium having atomic number 90 to Lawrencium
having atomic number 103.
General electronic configuration of Actinides is [Rn]5f1−146d0−17s2[Rn]5f1−146d0−17s2
As the valence electrons enter in 7s orbital, Actinides occupy position in period 7 in periodic
table.
6. General Characteristics of lanthanides and actinides
The chemistry of the lanthanides differs from main group elements and
transition metals because of the nature of the 4f orbitals.
These orbitals are “buried” inside the atom and are shielded from the atom’s
environment by the 4d and 5p electrons.
As a consequence, the chemistry of the elements is largely determined by their
size, which decreases gradually with increasing atomic number. This
phenomenon is known as the lanthanide contraction.
All the lanthanide elements exhibit the oxidation state +3. Actinides are typical
metals. All of them are soft, have a silvery color (but tarnish in air), and have
relatively high density and plasticity.
Some of them can be cut with a knife. The hardness of thorium is similar to that
of soft steel, so heated pure thorium can be rolled in sheets and pulled into wire.
Thorium is nearly half as dense as uranium and plutonium but is harder than
both of them.
7. Unlike the lanthanides, most elements of the actinide series have the same properties as
the d block.
Members of the actinide series can lose multiple electrons to form a variety of different
ions. All actinides are radioactive, paramagnetic, and, with the exception of actinium, have
several crystalline phases.
All actinides are pyrophoric, especially when finely divided (i.e., they spontaneously
ignite upon exposure to air).The melting point of actinides does not have a clear dependence
on the number of f electrons.
The unusually low melting point of neptunium and plutonium (~640 °C) is explained by
hybridization of 5f and 6d orbitals and the formation of directional bonds in these metals.
Like the lanthanides, all actinides are highly reactive with halogens and chalcogens;
however, the actinides react more easily.
Actinides, especially those with a small number of 5f electrons, are prone to
hybridization.
This is explained by the similarity of the electron energies at the 5f, 7s, and 6d subshells.
Most actinides exhibit a larger variety of valence states.
8. What is Lanthanide Contraction?
Lanthanide contraction is the decrease in the size of atoms with the increasing atomic
number in the lanthanide series.
It is a steady decrease in the atomic radius and the ionic radius of chemical elements in
the lanthanide series. Further, this happens because of the filling of 4f orbitals with
electrons before filling up the 5d orbital.
Here, the 4f electrons show a poor shielding towards the nuclear charge, which in turn
cause the 6s electrons to move towards the nucleus of the atom, resulting in a small
radius.
Moreover, this contraction is quite regular. The atomic numbers in the lanthanide
series included in this contraction phenomena are from 57 to 71.
The element having atomic number 71 is Lutetium, which has a smaller ionic radius
than the chemical element having atomic number 72 in the subsequent series of
elements.
9. Actinide contraction is the decrease in size of atoms with the increasing atomic
number in the actinide series.
The contraction here is a result of imperfect shielding of one 5f electron by
another 5f electron of the same orbital.
Thus, due to this poor shielding of the nuclear charge by 5f electrons, the
effective nuclear charge increases, which leads to the contraction of the atom or
decrease in the atomic size.
What is Actinide Contraction?
10.
11. Occurance and extraction
Except promethium which is unstable and occurs only in traces, all the
lanthanides occur in nature to a considerable extent, cerium being the most
abundant of all the elements.
There are more than hundred minerals known to contain lanthanides but very
few are of commercial importance.
Monazite sand is the best known and most important mineral of lanthanide
elements which is essentially a mixture of orthophosphates, LnPO4 containing
upto 12% thorium, the element of 5fseries, small amounts of Zr, Fe and Ti as
silicates, lanthanum and about 3% yttrium.
Among lanthanides contained in monazite, the bulk is of Ce, Nd, Pr and others
occur in minute quantities.
12. Occurance and extraction
Extraction of lanthanide metals After conventional mineral dressing which gives minerals
of more than 90 percent purity, the mineral is broken down by either acidic or alkaline
attack.
By making use of different solubilites of double salts: Ln2(SO4)3.Na2SO4.xH2O for light
and heavy lanthanides and low solubility of hydrated oxide of thorium, the lanthanide
fractions and thorium containing portions are separated in acidic medium.
Monazite is treated with hot conc. H2SO4 when thorium, lanthanum and lanthanons
dissolve as sulphates and are separated from insoluble material (impurities).
On partial neutralisaion by NH4OH, thorium is precipitated as ThO2. Then Na2SO4 is
added to the solution.
13. Occurance and extraction
Lanthanum and light lanthanides are precipitated as sulphates leaving behind the heavy
lanthanides in solution.
To the precipitate obtained as above, is added hot conc. NaOH. The resulting hydroxides of
light lanthanides are dried in air at 1000C to convert the hydroxides to oxides
. The oxide mixture is treated with dil. HNO3. This brings CeO2 as precipitate and other
lanthanides in solution.
From the solutions obtained as above for heavy and light lanthanides, individual members of
lanthanide series are isolated by the following methods:
Isolation of Individual Lanthanide Elements: All the lanthanides have the same size and
charge (of +3 unit).
The chemical properties of these elements which depend on the size and charge are,
therefore, almost identical.
Hence, their isolation from one another is quite difficult.
However, the following methods have been used to separate them from one another.
14. Valency change Method:
This method is based on the change of chemical properties by changing the oxidation
state of the lanthanide elements.
The most important application of this method is made in the separation of cerium and
europium elements from mixture of lanthanides.
(i) The mixture containing Ln3+ ions if treated with a strong oxidising agent such as
alkaline KMnO4, only Ce3+ ion is oxidized to Ce4+ while other Ln3+ ions remain
unaffected.
To this solution alkali is added to precipitate Ce(OH)4 only, which can be filtered off
from the solution.
(ii) Eu2+ can be separated almost completely from Ln3+ ions from a solution by
reducing it with zinc-amalgam and then precipitating as EuSO4 on adding H2SO4 which is
insoluble in water and hence can be separated.
The sulphates of other Ln3+ ions are soluble and remain in solution.
15. Occurance and extraction
Modern Ion-Exchange Method:
This is the most rapid and most effective method for the isolation of individual
lanthanide elements from the mixture.
An aqueous solution of the mixture of lanthanide ions (Ln3+aq) is introduced
into a column containing a synthetic cation exchange resin such as DOWAX-50
[abbreviated as HR (solid)].
The resin is the sulphonated polystyrene containing-SO3H as the functional
group.
As the solution of mixture moves through the column, Ln3+aq ions replace H+
ions of the resin and get themselves fixed on it:
Ln3+aq + 3H(resin) → Ln(resin)3 + 3H+ aq
The H+ aq ions are washed through the column.
The Ln3+aq. ions are fixed at different positions on the column. Since, Lu3+aq.
is largest (Lu3+ anhyd. is smallest and is hydrated to the maximum extent) and
Ce3+aq. is the smallest, Lu3+aq. ion is attached to the column with minimum
firmness remaining at the bottom and Ce3+aq. ion with maximum firmness
remaining at the top of the resin column.
In order to move these Ln3+aq. ions down the column and recover them, a
solution of anionic ligand such as citrate or 2-hydroxy butyrate is passed slowly
through the column (called elution).
The anionic ligands form complexes with the lanthanides which possess lower
positive charge than the
16. Occurance and extraction
initial Ln3+aq ions. These ions are thus displaced from the resin and moved to the surrounding
solutions as eluant- Ln complexes.
For example, if the citrate solution (a mixture of citric acid and ammonium citrate) is used as
the eluant, during elution process, NH4 + ions are attached to the resins replacing Ln3+aq. ions
which form Ln-citrate complexes:
Ln (resin)3 + 3NH4 + → 3NH4- resin + Ln3+aq Ln3+aq + citrate ions → Ln-citrate complex
As the citrate solution (buffer) runs down the coloumn, the metal ions get attached alternately
with the resin and citrate ions (in solution) many times and travel gradually down the column and
finally pass out of the bottom of the column as the citrate complex.
The Ln3+aq cations with the largest size are, eluted first (heavier Ln3+aq ions) because they
are held with minimum firmness and lie at the bottom of the column.
The lighter Ln3+aq ions with smaller size are held at the top of the column (with maximum
firmness) and are eluted at last.
The process is repeated several times by careful control of concentration of citrate buffer in
actual practice.
17. Occurance and extraction
Uses of lanthanides and there compounds:
(A) Uses of elements: (i) Lanthanides are used in metallothermic reactions due to their
extraordinary reducing property.
Lanthanide - thermic processes can yield sufficiently pure Nb, Zr, Fe, Co, Ni, Mn, Y, W,
U, B and Si. These metals are also used as de-oxidizing agents in the manufacturing of Cu
and its alloys.
(ii) Uses of mish- methods: Alloys of lanthanides are known as mish- methods. The
major constituents of mish –methods are Ce(45.50%),La(25%),Nd(5%) and small
quantities of other lanthanide metals and Fe and Ca impurities.
Mish-metals are used for the production of different brands of steel like heat resistant,
stainless and instrumental steels.
Mg- alloys containing about 30% mish metal and 1% Zr are useful in making parts of jet
engine.
18. Occurance and extraction
(B) Uses of lanthanide compounds: The uses of the compounds of lanthanides can broadly be classified as
follows: (1) Ceramic applications: CeO2, La2O3, Nd2O3 and Pr2O3 are widely used for decolorizing glass.
Lanthanide oxides can absorb ultra- violet rays, thus these are used as additives in glasses for special
purposes , e.g. for making (i) sun- glasses (by adding Nd2O3 )(ii) goggles for glass blowing and welding
work(Nd2O3 + Pr2O3) (iii) glass protecting eyes from neutron radiation (Gd2O3 + Sm2O3).
The addition of more than 1% CeO2 to a glass gives it a brown colour. Nd2O3 and Pr2O3 give
respectively red and green colours. (Nd2O3 + Pr2O3) gives a blue colour.
(2) Refractories: CeS (m.p. = 20000 C) is used in the manufacture of a special type of crucible which are
used for melting metals in a reducing atmosphere at temperatures upto 18000 C.
Borides, carbides and nitrides of lanthanides are also used as refractories.
(3) Abrasives: lanthanide oxides are used as abrasives for polishing glasses. e.g. the mixture of oxides,
CeO(47%); La2O3 + Nd2O3 + Pr2O3 (51%) + SiO2, CaO, Fe2O3 etc(=2%) which is called polirite has been
used for polishing glasses.
(4) Paints: lanthanide compounds are used in the manufacture of lakes, dyes and paints for porcelain. e.g.
cerium molybdate gives light yellow colour, cerium tungstate gives greenish blue colour and salts of Nd give
red colour.
(5) In textiles and leather industries: Ceric salts are used for dying in textile industries and as tanning
agents in leather industries. Ce(NO3)4 is used as a mordant for alizarin dyes. Chlorides and acetates of
lanthanides make the fabric water proof and acid resistant.
19. (6) In medicine and agriculture:
Dimals which are salicylates of Pr and Nd are used as germicides.
Cerium salts are used for the treatment of vomiting and sea-sickness.
Salts of Er and Ce increase the red blood corpuscles and haemoglobin
content of blood.
In agriculture lanthanide compounds are used as insecto- fungicides and
as trace elements in fertilizers.
(7) In lamps:
salts of La, Ce, Eu and Sm are used as activators of luminophores.
They are used in the manufacture of gas mantles, in coatings of
luminescent lamps and for painting the screens of cathode-ray tubes.
(8) In analytical chemistry:
Ce(SO4)2 is used as an oxidizing agent in volumetric titrations.
Radioisotopes of lanthanides are used in the study of co- precipitation,
chromatographic separations etc.
20. (9) Catalytic applications: Certain compounds of lanthanides are employed for the
hydrogenation, dehydrogenation and oxidation of various organic compounds. Cerium
phosphate is used as a catalyst in petroleum cracking.
(10) Electronic applications: Ferrimagnetic garnets of the type 3Ln2O3∙5Fe2O3 are
employed in microwave devices.
(11) Nuclear applications: Certain elements and compounds of lanthanides used in
nuclear fuel control and shielding and fluxing devices. Pr147 is used in the production
of atomic battery.
