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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