Saturday, 14 July 2012

F-BLOCK ELEMENTS OF THE MODERN PERIODIC TABLE . What are F-BLOCK ELEMENTS ???? Their POSITION in the Modern Periodic Table and their PROPERTIES.....!!!!!


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 (n-1) p6 (n-1 ) d10, ns 2 .


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 1s 2 2s 2 p 6 3s 2
p 6 d 10 4s 2 p 6 d 10 f 0-14 5s 2 p 6 d 0-1 6s 2 . 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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