LANTHANIDES AND ACTINIDES
UNIT 3 LANTHANIDES AND ACTINIDES
LESSON STRUCTURE
3.0 Introduction
3.1 Chemistry of 'f' Block Elements
3.2 Position in Periodic Table
3.3 Oxidation States and their Stability
3.4 Lanthanide and Actinide Contraction
3.5 Magnetic Properties
3.6 Spectral Properties
3.7 Seperation Technologies in Lanthanides
(a) Ion Exchange
(b) Solvent Extraction
3.8 Transuranic elements
3.9 Synthesis and Chemistry of Np and Pu
3.10 Summary
3.11 Questions for Exercise
3.12 Suggested Readings
3.0 Introduction
The two series Lanthanides and Actinides constitute ‘f’ block elements.
They are group of 15 elements each. The most common oxidation state for the
lanthanides and some actinides is +3. They are similar to each other in properties.
The filling of 4f orbital is known as lanthanides and filling of 5f orbital is known
as actinides.
The lanthanides were known as rare earths earlier but it was not appropriate
as many of the lanthanides are not particularly rare.
The fourteen elements from actinium are called actinides. The actinides do
not show chemical uniformity as in the case of lanthanides. All the actinides are
radioactive but the most abundant isotopes of Thorium and Uranium have very
long half lives. Except Actinium, Thorium, Protoactinium and Uranium, all the
elements are synthetic elements or man made elements or transuranic elements.
( 71 )
,LANTHANIDES AND ACTINIDES
3.1 Chemistry of ‘f’ Block Elements
Lanthanum, the first member of lanthanides is a true member of Gr III,
with atomic number 57. The forteen members which follow lanthanum e.g. from
atomic number 58 to 71, in which the 4f electrons are succesively added to the
La configuration are the rest. The term lanthanide is taken to include La, as this
element is the prototype for the succeeding fourteen elements.
Actinium is similarly, though being an element of Group III with atomic
number 89 is the first member of actinide series. The fourteen elements which
follow actinium with atomic number 90 to 103 constitute actinide series.
Both lanthanides(Ln) and actinides(An) have variable valencies. The principal
oxidation state of all the elements is +3. In the case of Ln the other valencies are
+2 and +4. In An there are a number of oxidation states varying from +2 to +7.
The Ln+3 ion show much less tendency to form complexes than ‘d’block
elements, because of their larger size and lower electronegativity which do not
promote covalent bond formation, same is the case of actinide ions.
The colour in La3+ and An3+ ions are due to f–f transitions, which are very
sharp in contrast to d–d transitions. In the case of La+3, crystal field effects are
much less pronounced than spin–orbit coupling but in An+3 ions, the 5f orbitals
show significant crystal field stabilization effects.
The deep seated nature of 4f orbitals in Ln gives rise to less quenching of
orbital contributions to magnetic moments. The values are mostly in agreement
with L–S coupling scheme rather than only spin values. The magnetic properties
of An show overall similarity to those of the corresponding Ln ions but are
somewhat lower due to quenching of orbital contribution by crystal field effects.
The periodic trends of ‘d’ and ‘f’ block elements are different from ‘s’ and
‘p’ block elements. In the sixth period, the 4‘f’ orbitals are very deeply burried
under the core and valence orbitals, Thus there is no double bonding. Again ‘d’
and ‘f’ block elements can form bonds. Maximum two bonds can be formed per
pair of atoms using dxy and d x 2 y 2 orbitals.
f block elements show spectra with pale colours due to Laporte’s forbidden
f–f transitions. However, charge transfer spectra of MLCT and LMCT show intense
colours.
Ruby lasers and phosphors are based on ‘f’ block elements. These phosphors
are present in colour televisions. ‘f’ block elements are also substituted
isomorphorously for Ca2+ ions in enzymes.
( 72 )
, LANTHANIDES AND ACTINIDES
3.2 Position in Periodic Table
The fifteen elements of Ln having atomic weights between those of Ba (at.
no. 56) and Hf (at. no. 72) must be placed betwen these two elements.
Barium has the same outer electronic structure as Calcium and Strontium
as well as resemble them in properties. Thus Barium must be placed below
Strontium (Group II). Similarly Hafnium must be placed below Zirconium
(Group IV). This leaves only one place for Ln below Yttrium in Group III.
Since all the fifteen elements of Ln resemble each other in many ways,
they must be placed in the same group. Ln elements resemble Yttrium in a
number of points.
(a) Due to Ln contraction the ionic radius of Y3+ is same as Er3+ (Y3+ =
0.93 Å and Er3+ = 0.96Å)
(b) Yttrium occurs in nature as Ytterbite, Xenotime etc which are also
the ores of heavier lanthanides.
Thus all the Lanthanide elements should be accomodated together. This is
done by placing La below Y and the remaining fourteen Ln have been placed
seperately in the f block, in the lower part of the modern periodic table.
Lanthanides are known as first inner transition metal series. They are first
series of ‘f’ block elements.
The position of An in the periodic table can be under two heads.
(a) Prior to the discovery of the transuranic elements (before 1940)—The
knowledge of Ln series helped the scientists to predict another series of elements
resulting from the filling up of electrons to (n–2)f shell (in 5f shell). Before the
discovery of transuranic elements, the naturally occuring Ac (At. no. 89), Th (At.
no. 90), Pa (at no. 91) and U (at. no. 92) were placed in GrIII, IV, V and VI of the
periodic table as they had +3, +4, +5 and +6 oxidation states and resembled La,
Hf, Ta and W, the earlier elements in their properties. The undiscovered transuranic
elements were expected to occupy the corresponding positions in the periodic
table.
(b) After the discovery of transuranic elements Np (at. no. 93) and Pu
(at. no. 94) which were discovered in 1940 and 1941, they did not resemble
Rhenium and Osmium but resembled U in many ways. Thus Seaborg in 1944
suggested the idea of second series of inner transition elements, similar to Ln.
Thus these elements of at. no. 89 to 103 were known as actinides and was given
a place in the f block.
The periodic table with ‘f’ block is shown below.
( 73 )
UNIT 3 LANTHANIDES AND ACTINIDES
LESSON STRUCTURE
3.0 Introduction
3.1 Chemistry of 'f' Block Elements
3.2 Position in Periodic Table
3.3 Oxidation States and their Stability
3.4 Lanthanide and Actinide Contraction
3.5 Magnetic Properties
3.6 Spectral Properties
3.7 Seperation Technologies in Lanthanides
(a) Ion Exchange
(b) Solvent Extraction
3.8 Transuranic elements
3.9 Synthesis and Chemistry of Np and Pu
3.10 Summary
3.11 Questions for Exercise
3.12 Suggested Readings
3.0 Introduction
The two series Lanthanides and Actinides constitute ‘f’ block elements.
They are group of 15 elements each. The most common oxidation state for the
lanthanides and some actinides is +3. They are similar to each other in properties.
The filling of 4f orbital is known as lanthanides and filling of 5f orbital is known
as actinides.
The lanthanides were known as rare earths earlier but it was not appropriate
as many of the lanthanides are not particularly rare.
The fourteen elements from actinium are called actinides. The actinides do
not show chemical uniformity as in the case of lanthanides. All the actinides are
radioactive but the most abundant isotopes of Thorium and Uranium have very
long half lives. Except Actinium, Thorium, Protoactinium and Uranium, all the
elements are synthetic elements or man made elements or transuranic elements.
( 71 )
,LANTHANIDES AND ACTINIDES
3.1 Chemistry of ‘f’ Block Elements
Lanthanum, the first member of lanthanides is a true member of Gr III,
with atomic number 57. The forteen members which follow lanthanum e.g. from
atomic number 58 to 71, in which the 4f electrons are succesively added to the
La configuration are the rest. The term lanthanide is taken to include La, as this
element is the prototype for the succeeding fourteen elements.
Actinium is similarly, though being an element of Group III with atomic
number 89 is the first member of actinide series. The fourteen elements which
follow actinium with atomic number 90 to 103 constitute actinide series.
Both lanthanides(Ln) and actinides(An) have variable valencies. The principal
oxidation state of all the elements is +3. In the case of Ln the other valencies are
+2 and +4. In An there are a number of oxidation states varying from +2 to +7.
The Ln+3 ion show much less tendency to form complexes than ‘d’block
elements, because of their larger size and lower electronegativity which do not
promote covalent bond formation, same is the case of actinide ions.
The colour in La3+ and An3+ ions are due to f–f transitions, which are very
sharp in contrast to d–d transitions. In the case of La+3, crystal field effects are
much less pronounced than spin–orbit coupling but in An+3 ions, the 5f orbitals
show significant crystal field stabilization effects.
The deep seated nature of 4f orbitals in Ln gives rise to less quenching of
orbital contributions to magnetic moments. The values are mostly in agreement
with L–S coupling scheme rather than only spin values. The magnetic properties
of An show overall similarity to those of the corresponding Ln ions but are
somewhat lower due to quenching of orbital contribution by crystal field effects.
The periodic trends of ‘d’ and ‘f’ block elements are different from ‘s’ and
‘p’ block elements. In the sixth period, the 4‘f’ orbitals are very deeply burried
under the core and valence orbitals, Thus there is no double bonding. Again ‘d’
and ‘f’ block elements can form bonds. Maximum two bonds can be formed per
pair of atoms using dxy and d x 2 y 2 orbitals.
f block elements show spectra with pale colours due to Laporte’s forbidden
f–f transitions. However, charge transfer spectra of MLCT and LMCT show intense
colours.
Ruby lasers and phosphors are based on ‘f’ block elements. These phosphors
are present in colour televisions. ‘f’ block elements are also substituted
isomorphorously for Ca2+ ions in enzymes.
( 72 )
, LANTHANIDES AND ACTINIDES
3.2 Position in Periodic Table
The fifteen elements of Ln having atomic weights between those of Ba (at.
no. 56) and Hf (at. no. 72) must be placed betwen these two elements.
Barium has the same outer electronic structure as Calcium and Strontium
as well as resemble them in properties. Thus Barium must be placed below
Strontium (Group II). Similarly Hafnium must be placed below Zirconium
(Group IV). This leaves only one place for Ln below Yttrium in Group III.
Since all the fifteen elements of Ln resemble each other in many ways,
they must be placed in the same group. Ln elements resemble Yttrium in a
number of points.
(a) Due to Ln contraction the ionic radius of Y3+ is same as Er3+ (Y3+ =
0.93 Å and Er3+ = 0.96Å)
(b) Yttrium occurs in nature as Ytterbite, Xenotime etc which are also
the ores of heavier lanthanides.
Thus all the Lanthanide elements should be accomodated together. This is
done by placing La below Y and the remaining fourteen Ln have been placed
seperately in the f block, in the lower part of the modern periodic table.
Lanthanides are known as first inner transition metal series. They are first
series of ‘f’ block elements.
The position of An in the periodic table can be under two heads.
(a) Prior to the discovery of the transuranic elements (before 1940)—The
knowledge of Ln series helped the scientists to predict another series of elements
resulting from the filling up of electrons to (n–2)f shell (in 5f shell). Before the
discovery of transuranic elements, the naturally occuring Ac (At. no. 89), Th (At.
no. 90), Pa (at no. 91) and U (at. no. 92) were placed in GrIII, IV, V and VI of the
periodic table as they had +3, +4, +5 and +6 oxidation states and resembled La,
Hf, Ta and W, the earlier elements in their properties. The undiscovered transuranic
elements were expected to occupy the corresponding positions in the periodic
table.
(b) After the discovery of transuranic elements Np (at. no. 93) and Pu
(at. no. 94) which were discovered in 1940 and 1941, they did not resemble
Rhenium and Osmium but resembled U in many ways. Thus Seaborg in 1944
suggested the idea of second series of inner transition elements, similar to Ln.
Thus these elements of at. no. 89 to 103 were known as actinides and was given
a place in the f block.
The periodic table with ‘f’ block is shown below.
( 73 )
