F-block Elements
14
elements from atomic number 58 (Cerium) to 71(Lutetium) are called as
Lanthanides and the another 14 elements from atomic number 90 (Thorium) to
103(Lawrencium) are called as Actinides. These 28 elements are arranged in the
2 series placed below the Modern Periodic Table and the elements in these 2
series are collectively called as f-block elements. These elements have outermost three shells
incomplete . In these elements the last or differentiating electron enters in
to the f-orbital of anti-penultimate shell or the 3rd shell from the outermost
shell,Thus the f-orbital is partially filled and hence they are called as
f-block elements. They are also called as " Inner Transition elements
". They are so called because the last electron enters in the f-orbital of
anti-penultimate shell, which is inner and these elements are present between
strongly electropositive s-block elements and strongly electronegative p-block
elements . The general electronic configuration of the f-block elements is
given as :-
(n-2)
f1-14, (n-1) s2 , p6 , d0-1,
ns2 .
Lanthanides
The
group of 14 elements from Cerium(58) to Lutetium(71) are placed alongwith
lanthanum in IIIB group and in the 6th period of the periodic table in a series
below the periodic table called as Lanthanides.They are also called as
Lanthanoids or Lanthanones and are represented as Ln . In Lanthanoids the last
or differentiating electron enters into 4f orbital of the anti-penultimate
shell and hence the series of 14 elements in lanthanoids is called as 4f
series.
Properties
of Lanthanides :-
1)
Electronic Configuration :- The
General electronic configuration of Lanthanides is [Xe] 4f1–14 5d1
6s2 . The complete electronic configuration of Lanthanides can be
given as 1s2 2s2 2p6 3s2 3p6
3d10 4s2 4p6 4d10 5s2 5p6
. The Lanthanoids have partially filled 4f-orbital.But, However ,the first
member Lanthanum and the last member Lutetium of the lanthanide series have no
partially filled 4f-orbital and they should therefore, be excluded from
Lanthanoids, But, since they posses properties similar to those of Lanthanoids,
for all practical purposes they are placed with Lanthanoids . We know that 4f
orbital is filled only after completing 5s, 5p and 6s orbitals. It is found
that when the f-orbital contains 0, 7 or 14 electrons it is stable. The
electronic configuration of Lanthanum which is followed by 14 lanthanides is
2,8,18,18,9,2 . In Lanthanum 4f subshell is vacant and 5d subshell contains one
electron. Thus in Lanthanides 4f orbitals are successively filled,means from
Cerium(58) to Lutetium(71) ,the Additional electron should occupy vacant 5f
orbital and 5d orbital should remain singly filled up. Thus the electronic
configuration of Lanthanides should be 2,8,18,19-32,9,2 .
But
in observed electronic configuration , 5d1 electron gets shifted to 4f orbital
and 5d orbital remains vacant. It is because energies of 5d and 4f orbitals are
closely similar.
2)
Metallic Character :- Lanthanides
are silvery white metals having tensile strength.
3)
Atomic Density :- Lanthanides
have lighter density values ranging from 6.77 to 9.74 gm/cm3 .
Density increases with increase in atomic number.
4)
Melting and Boiling Points :- These
elements posses high M.P. and B.P. , but no regularity is seen with the
increase in atomic number.
5)
Oxidation state :- All
Lanthanides show a common stable oxidation state +3 . It is because there are
two electrons in the outermost shell and one electron in next to outermost
shell i.e penultimate shell.
Also
some Lanthanides show +2 and +4 oxidation states,these are observed in those
elements , which by losing electrons attain the stable 4f0 ,4f7
and 4f14 configuration.
6)
Atomic and Ionic radii :- In
Lanthanide series ,there is regular decrease in atomic size as well as atomic
radii as the atomic number increases from Cerium to Lutetium. This decrease in
size of atoms and ions is known as Lanthanide Contraction. On moving from
Cerium to Lutetium , the atomic radii decreases from 165 pm to 156 pm and the
ionic radii decreases from 103 pm to 85 pm.
7)
Electrode Potential :- Due
to Lanthanide Contraction the value of standard electrode potential increases
regularly from La to Lu.
8)
Ionisation Energies :- Lanthanides
have very low ionisation energies. Ionisation energy values are similar to
alkaline earth metals , particularly calcium. Due to low ionisation energies ,
Lanthanides are highly electropositive in nature.
9)
Magnetic Properties :- Elements
with paired electrons does not show any magnetism due to cancellation of the
opposite spins due to pairing. Lanthanide ions having unpaired electrons and
hence are paramagnetic ,while those having all the orbitals with paired
electrons are diamagnetic in nature . Lanthanum and Lutetium have all paired
electrons and hence are diamagnetic in nature.
10)
Complex Formation :- Lanthanides
do not have much tendency to form complexes due to their low charge density
because of their large size. However, they form complexes with few chelating
agents such as EDTA. The tendency to form complexes increases with increase in
atomic number.
11)
Chemical Reactivity :- As
all the Lanthanides show a similar electronic configuration and common +3
oxidation state, they show similar Chemical Reactivity.
a)
All lanthanides react readily upon exposure to air and tarnish.
b)
They readily dissolve in hot water liberating hydrogen. They can also dissolve
in cold water.
c)
They react with nitrogen and hydrogen forming the corresponding nitrides and
hydrides.
d)
Lanthanides react with other non-metals such as halogens, sulphur, phosphorus,
carbon and silicon and form corresponding compounds.
e)
The high oxidation potentials indicate their strong electro positive nature to
act as strong reducing agents.
f)
Lanthanides react with acids and liberate hydrogen.
12)
Conductivity :- Lanthanides
are good conductors of heat and electricity.
13)
Colour :- All
Lanthanides are silvery white metals . The trivalent lanthanide ions are
coloured both in solid state and in aqueous solution,this colour change is seen
only in case of cations. The colour of a cation depends on the number of
unpaired f electrons.
Lanthanides
with either half-filled or completely filled orbitals are colourless.
Actinides
The
group of 14 elements from Thorium(90) to Lawrencium(103) are placed alongwith
Actinum in IIIB group and in the 7th period of the periodic table in a series
below the periodic table called as Actinides.They are also called as Actinoids
or Actinones and are represented as An . In Actinoids the last or
differentiating electron enters into 5f orbital ,hence the series of 14
elements in Actinoids is called as 5f series. All Actinides are radioactive in
nature.
Elements
beyond Uranium are prepared synthetically in labouratory through nuclear
reactions. The starting element for this purpose is always Uranium. Hence the
elements from Neptunium to Lawrencium are called Transuranic elements ar
Urenides.
Properties
of Actinides :-
1)
Electronic Configuration :-The general electronic configuration of Actinides is
[Rn] 5f1-14 6d0-1 7s2 . The Actinoids have
partially filled 5f-orbital. The electronic configuration of Actinum which is
followed by 14 Actinides is 2,8,18,32,18,9,2 . In Actinum 5f subshell is vacant
and 6d subshell contains one electron. Thus in Actinides 5f
orbitals
are successively filled,means the Additional electron should occupy vacant 5f
orbital and 6d orbital should remain singly filled up. Thus the electronic
configuration of Actinides should be 2,8,18,32,19-32,9,2 .
But
since it is not sure whether the last electron enters the 5f orbital or the 6d
orbital as both have the same energy. Hence, there are two views regarding the
electronic configuration of actinides.
According
to Seaborg view, the 5f orbital is filled from thorium, while in Dawson view
the 5f orbital is filled from uranium and are called uranides.
2)
Metallic Character :- Actinides
are silvery metals .
3)
Atomic Density :- All
Actinides except Thorium and Americium have high Atomic density.
4)
Melting and Boiling Points :- These
elements posses high M.P. and B.P. than the Lanthanides , but no regularity is
seen with the increase in atomic number.
5)
Oxidation state :- All
Lanthanides show a common stable oxidation state +3 . The most stable oxidation
state for first 3 elements
Thorium,Protactinium
and Uranium is +4 , +5 and +6 .
Plutonium(Pu)shows
all oxidation states from +3 to +7 but most stable is +4.
Americium
shows oxidation state from +2 to +6.
6)
Atomic and Ionic radii :- In
the Actinide series ,there is regular decrease in atomic size as well as atomic
radii as the atomic number increases from Actinium to Lawrencium. The size of
ions decreases gradually along the series because extra charge of the nucleus
is poorly shielded by f-electrons. It is called as Actinide Contraction and is
similar to Lanthanide Contraction.
7)
Electrode Potential :- Due
to Actinide Contraction the value of standard electrode potential increases
regularly from Ac to Lw.
8)
Ionisation Energies :- Actinides
have lower ionisation energies than lanthanides because 5f is more effectively
shielded from nuclear charge than 4f. Due to low ionisation energies ,
Actinides are highly electropositive in nature.
9)
Magnetic Properties :- All
Actinides have unpaired electrons and hence are paramagnetic in nature.
10)
Complex Formation :- Actinides
have higher tendency to form complexes than Lanthanides. It is because of their
higher charge and smaller sizes of cations . Halides of actinides form
complexes with alkali metals . Theses elements form chelates with organic
compounds such as EDTA and oxime.
11)
Chemical Reactivity :- Actinides
are highly reactive metals in fine state and very strong reducing agents.
A
few properties are given below.
a)
They react with boiling water to give oxide and hydride.
b)
They combine with most of non-metals at moderate temperature.
c)
All these metals are attacked by HCl acid but the effect of nitric acid is very
small.
d)
They react with tarnish in air forming oxide coating.
12)
Conductivity :- Actinides
are good conductors of heat and electricity.
13)
Colour :- Actinide
ions are coloured . It depends upon the number of electrons in 5f orbitals.
Ions having no electron in f orbital or 7 electrons in 5f orbital are
colourless. The ions with 2 or 6 electrons in 5f orbitals are coloured in both
the crystalline and in solution state due to f-f transition of electrons.
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