The d- and f- block Elements
The d-block elements are known as transition metals.
The f-block elements are known as inner transition
metals.
4f – metals lanthanoids
5f – metals actinoids
The d-block elements (transition metals):
a transition element is defined as the one which has
incompletely filled d orbitals in its ground state or in any
one of its oxidation states.
The d-block elements (group 3 – 12 elements) occupy the
large, middle section of the periodic table.
There are three series of transition metals, corresponding
to the filling of 3d, 4d and 5d orbitals.
In general, the valence shell electronic configuration is
(n – 1)d1–10 ns 1–2.
In case of Cr, 3d5 4s1 instead of 3d44s2
Similarly in case of Cu, the configuration is 3d104s1 and
not 3d94s2.
General Properties of the Transition Elements (dBlock)
Physical Properties
All the transition metals exhibit typical metallic properties
such as
high tensile strength,
ductility,
malleability,
high thermal and electrical conductivity,
very much hard
have low volatility
metallic luster, etc.
The melting and boiling points (Enthalpy of
atomization also)of transition metals are high due to the
involvement of
(n –-1)d electrons in interatomic bonding.
enthalpies of atomization
the metals of the second and third series have greater
enthalpies of atomisation than the corresponding elements
of the first series; Since
the enthalpy of atomisation is an important factor in
determining the
standard electrode potential of a metal, metals with very
high enthalpy
of atomisation (i.e., very high boiling point) tend to be
noble in their
reactions
Variation in Atomic and Ionic Sizes of Transition
Metals
As we move from left to right Progressive decrease in
radius with increasing atomic number.
Reason
Effective nuclear charge increases
Poor shielding effect of d electrons
Towards the end there is an increase in size
Reason
When pairing of electrons started inter electronic
repulsion there this is dominant towards the end and
causes increased atomic size.
the radii of the third (5d) series are virtually the same as
those of the corresponding members of the second series.
This phenomenon is associated with the intervention of
the 4f orbitals which
must be filled before the 5d series of elements begin. The
filling of 4f
before 5d orbital results in a regular decrease in atomic
radii called
Lanthanoid contraction which essentially compensates
for the expectedincrease in atomic size with increasing
atomic number.
Ionisation enthalpy
there is an increase in ionisation enthalpy along each
series of the transition elements from left to right.
However, many small variations occur.
Magnetic Properties
diamagnetism and paramagnetism
Diamagnetic substances are repelled by the applied field
Paramagnetic substances are attracted.
Paramagnetism arises from the presence of unpaired electrons,
the magnetic moment is determined by the number of unpaired electrons
and is calculated by using the ‘spin-only’ formula, i.e.,
where n is the number of unpaired electrons and μ
is the magnetic
moment in units of Bohr magneton (BM)
Formation of Coloured Ions
When an electron from a lower energy d orbital is excited to a higher
energy d orbital, d-d transition.
Formation of Complex Compounds
A few examples are: [Fe(CN)6]3–, [Fe(CN)6]4–, [Cu(NH3)4]2+ and
[PtCl4]2–.
This is due to the comparatively smaller sizes of the metal ions,
their high ionic charges and the
availability of d orbitals for bond formation.
Catalytic Properties
Vanadium(V) oxide (in Contact Process)
finely divided iron (in Haber’s Process)
nickel (in Catalytic Hydrogenation)
because the transition metal ions can change their oxidation states, they
become more effective as catalysts
They can provide a large surface area for the reactant to react.
Formation of Interstitial Compounds
Interstitial compounds are those which are formed when small atoms
like H, C or N are trapped inside the crystal lattices of metals.
The principal physical and
chemical characteristics of these compounds are as follows:
(i) They have high melting points, higher than those of pure metals.
(ii) They are very hard, some borides approach diamond in hardness.
(iii) They retain metallic conductivity.
(iv) They are chemically inert.
Alloy Formation
Alloys of transition metals with non transition metals such as brass
(copper-zinc) and bronze (copper-tin), are also of
considerable industrial importance.
Because of similar radii and other characteristics of transition metals,
alloys are readily formed by these metals.
Potassium dichromate K2Cr2O7
Dichromates are generally prepared from chromate, which in turn are
obtained by the fusion of chromite ore (FeCr2O4) with sodium or
potassium carbonate in free access of air
Chromite
sodium chromate
The yellow solution of sodium chromate is filtered and acidified with
sulphuric acid to give a solution from which orange sodium dichromate,
Na2Cr2O7. 2H2O can be crystallised.
sodium dichromate
Orange crystals of potassium dichromate crystallise out
The chromates and dichromates are interconvertible in aqueous solution
depending upon pH of the solution.
Potassium permanganate KMnO4
2MnO2 + 4KOH + O2 →2K2MnO4 + 2H2O
Pot. Manganate
3MnO4 2– + 4H+ →2MnO4– + MnO2 + 2H2O
Commercially it is prepared by the alkaline oxidative fusion of MnO2
followed by the electrolytic oxidation of manganate (Vl).
In the laboratory, a manganese (II) ion salt is oxidised by
peroxodisulphate to permanganate.
Properties
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dark purple (almost black) crystals
isostructural with those of KClO4.
not very soluble in water
diamagnetic.
when heated it decomposes at 513 K.
Acidified permanganate solution oxidises oxalates to carbon dioxide,
iron(II) to iron(III), nitrites to nitrates and iodides to free iodine.
The f-block elements (inner transition elements):
The lanthanoids –
In general, the outermost electronic configuration is 4f1 – 14 6s2.
Due to lanthanoid contraction, there is a gradual decrease in atomic
and ionic radii with increase in atomic number.
The lanthanoids exhibit mainly +3 oxidation state. However,
sometimes +2 and +4 oxidation states are also exhibited.
Physical properties
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silvery white soft metals
tarnish rapidly in air.
They are hard , samarium being steel hard.
Their melting points range between 1000 to 1200 K
typical metallic structure
good conductors of heat and electricity.
Many trivalent lanthanoid ions are coloured
Uses
 for the production of alloy steels for plates and pipes.
 A well known alloy is mischmetall which consists of a lanthanoid
metal
 (~ 95%) and iron (~ 5%) and traces of S, C, Ca and Al.
 to produce bullets, shell and lighter flint.
 Mixed oxides of lanthanoids are employed as catalysts in
petroleum cracking.
 Some individual Ln oxides are used as phosphors in television
screens and similar fluorescing surfaces.
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Chemical reactions:
The actinoids –
Electronic configuration: 7s2 Stable 5f and 6d Variable
Gradual decrease in atomic and ionic radii with increase in atomic
number, due to actinoid contraction
Actinoids are radioactive.
Actinoids exhibit different oxidation states, so their chemistry is
complex.
Properties
The actinoids are highly reactive metals,
The magnetic properties of the actinoids are more complex
the ionization enthalpies of the early actinoids, though not accurately
known, but are lower than for the early lanthanoids.
The lanthanoid contraction is more important because
the chemistry of elements succeeding the actinoids are much less known
at the present time.