f-Block Elements (inner
transition elements)
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The
elements in which the last electron enters in the anti-penultimate f-subshell
are called f-block elements. They include lanthanides of the 6th
period and actinides of the 7th period.
ü The
Lanthanoids: Elements’ filling up of 4f - orbitals is
called Lanthanoids series. The 14 elements after lanthanum of the 6th
period are called lanthanides or lanthanoids or lanthanones or rare earths.
They include elements from 58Ce to 71Lu. They are
generally represented as Ln.
Ø Atomic
and Ionic Radii: The overall decrease in atomic and ionic
radii from Lanthanum to Lutetium in a unique feature in the chemistry of the
lanthanoids. This regular decrease is known as lanthanoid contraction. In lanthanides, as the atomic
number increases, the nuclear charge increases one by one and the electrons are added to the
anti-penultimate f subshell. Due to its diffused shape, f orbitals have poor
shielding effect. So the nucleus can attract the outer most electrons strongly
and as a result the radii decreases.
Consequences of Lanthanoid Contraction:
a)
It results in slight variation in their chemical properties which helps in their separation by ion exchange
methods.
b)
Each element beyond lanthanum has same atomic radius as that of the element
lying above it in the same group (e.g., Zr 145 pm, Hf 144 pm); Nb 134 pm, Ta
134 pm; Mo 129 pm, W 130 pm).
c)
The covalent character of
hydroxides of lanthanoids increases as the size decreases from La3+
to Lu3+. Hence, the basic strength decreases. Thus, La(OH)3 is most basic
whereas Lu(OH)3 is least basic. Similarly, the basicity of
oxides also decreases in the order from La3+ to Lu3+.
d)
Tendency to form stable
complexes from La3+ to Lu3+ increases as the size
decreases in that order.
Ø Oxidation
State: In lanthanoids, the most common oxidation state is +3. However, +2 and +4 ions in solution or in
solid compounds are also obtained. This irregularity arises mainly from the extra stability of empty,
half-filled or filled f subshells. Cerium shows the oxidation state +4
due to its noble gas configuration. Pr, Nd, Tb and Dy also exhibit +4 state but
only in oxides, MO2. Eu and Yb shows +2 oxidation state because of
the stable f7 or f14 configuration. Sm shows +2 oxidation
state also.
Ø Colour:
Most of the trivalent
lanthanoid ions are coloured both in the solid state and in aqueous solution.
This is due to the partly filled f-orbitals which permit f-transition.
La3+
(colourless) Lu3+
(colourless)
Ce3+
(colourless) Yb3+
(colourless)
Pr3+ (yellow green) Tm3+
(green)
Nd3+ (red) Er3+ (pink)
Pm3+
(uncertain) Ho3+
(Yellow)
Sm3+ (yellow) Dy3+
(yellow)
Eu3+ (pink) Tb3+ (pink)
Gd3+ (pink)
Ø Complex
Formation: Although
the lanthanoid ions have a high charge (+3) yet the size of their ions is very
large yielding small charge to size ratio, i.e., low charge density. As
a consequence, they have poor tendency to form complexes.
Ø Reducing
Character: The E° values for M3+; M3+(aq)
+ 3e− → M(s) lie in the range of - 2.2 to -2.4 V (exception being Eu, E° = -
2.0V) indicating thereby that
they are highly electropositive, readily lose electrons and thus are good
reducing agents.
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