CBSE Class 12 Chemistry The D And F Block Elements Notes Set 04

Revision Notes

d-Block Elements, their Properties and Compounds

• d-block elements: The elements in which last electron enters in the \( d \) - sub-shell i.e., penultimate shell and lies in the middle of the periodic table belonging to groups 3-12.

• Transition elements: The elements of d-block are known as transition elements as they possess properties that are transitional between the s-block and p-block elements. Transition elements are defined as elements which have incompletely filled d-orbitals in their ground states or in its most common oxidation state.

Transition elements have four series:

1. First transition series: These elements have incomplete \( 3d \)-orbitals and they are from Sc (21) to Zn (30).
2. Second transition series: These elements have incomplete \( 4d \)-orbitals and they are from Y (39) to Cd (48).
3. Third transition series: These elements have incomplete \( 5d \)-orbitals and they are La (57) and then from Hf (72) to Hg (80).
4. Fourth transition series: These elements have incomplete \( 6d \)-orbitals. Known elements of this series are actinium (89) and then from Rf (104) Cn (112).

3. General electronic configuration of transition elements: Valence shell electronic configuration is \( (n-1)d^{1-10}, ns^{1-2} \), where n is the outermost shell.

Electronic configuration of d-block elements

General characteristics of Transition Elements:

Physical Properties:

1. All are metals.
2. All are malleable and ductile except mercury (liquid).
3. High thermal and electrical conductivity.
4. Metallic lustre and sonorous.
5. Atomic radii: Smaller than atomic size of s-block elements, larger than atomic size of p-block elements in a period. In a transition series, as the atomic number increases, the atomic radii first decreases till the middle, becomes constant and then increases towards end of the period.
   It usually increases down the group. The size of 4d elements is almost of the same size as of the 5d series elements. The filling of 4d before 5d orbitals results in regular decrease in atomic radii which is called as lanthanoid contraction.
6. Ionic radii: The ionic radii decrease with increase in oxidation state.
7. Density: From left to right in a period, density increases.
8. Ionisation enthalpy: Along the series from left to right, there is an increase in ionisation enthalpy. Irregular trend in the I^st ionisation enthalpy of 3d metals is due to irregularity in electronic configuration of 4s and 3d orbitals. In a group, IE decreases from 3d to 4d-series but increases from 4d to 5d series due to lanthanoid contraction.
9. Metallic bonding: In metallic bonding, regular lattice of positive ions are held together by a cloud of free electrons, which can move freely through the lattice. Transition metal atoms are held together by strong metallic bonds.
10. Enthalpy of atomisation: Enthalpy of atomisation is the heat required to convert 1 mole of crystal lattice into free atoms. Transition elements have high enthalpy of atomisation. It first increases, becomes maximum in the middle of the series and then decreases regularly.
11. Variable oxidation state: Since the energies of ns and (n–1) d electrons are almost equal, therefore the electrons of both these orbitals take part in the reactions, due to which transition elements show variable oxidation states. Transition metal ions show variable oxidation states except the first and last member of the series.
12. Electrode potential: The electrode potential develops on a metal electrode when it is in equilibrium with a solution of its ions, leaving electrons from the electrode. Transition metals have lower value of reduction potential. Variation in E° value is irregular due to the regular variation in ionisation enthalpies (I.E₁ + I.E₂), sublimation and hydration enthalpies.
13. Catalytic properties: Many of the transition metals and their compounds, particularly oxides act as catalysts for a number of chemical reactions. Iron, cobalt, nickel, platinum, chromium, manganese and their compounds are the commonly used catalysts.
    All transition metals show multiple oxidation states and have large surface area, so, all metals act as a catalyst.
14. Magnetic properties: On the basis of the behaviour of substances in magnetic field, they are of two types: (i) Diamagnetic, (ii) Paramagnetic.
    Diamagnetic substances have paired electrons only. e.g., Zn has no (zero) paired electrons.
    In paramagnetic substances, it is necessary to have at least one unpaired electron. Paramagnetism increases with the increase in number of unpaired electrons.
    Paramagnetism may be measured by magnetic moment.
    Magnetic moment, \( (\mu) = \sqrt{n(n + 2)} \text{ B.M.} \)
    where \( n \) = number of unpaired electrons in atom or ion and B.M. = Bohr magneton (unit of magnetic moment). Diamagnetic and paramagnetic substances are repelled and attracted in the magnetic field respectively (Magnetic properties of transition elements).
15. Melting and boiling points: Except zinc, cadmium and mercury, all other transition elements have high melting and boiling points. This is due to strong metallic bonds and presence of partially filled d-orbitals in the shell of the atom of element.
16. Complex formation: They have tendency to form complex ions due to high charge on the transition metal ions and the availability of d-orbitals for accommodating electrons donated by the ligand atoms.
17. Formation of coloured compounds: Transition metals form coloured ions due to the presence of unpaired d-electrons. As a result, light is absorbed in the visible region to cause excitation of unpaired d-electrons (d – d transition) and colour observed corresponds to the complementary colour of the light absorbed. Cu⁺, Zn²⁺ and Cd²⁺ are colourless due to the absence of unpaired d-electron (d¹⁰).
18. Formation of alloys: Alloy formation is due to almost similar size of the metal ions, their high ionic charges and the availability of d-orbitals for bond formation. Therefore, these metals can mutually substitute their position in their crystal lattice to form alloys. e.g., steel, brass.
19. Formation of interstitial compounds: Interstitial compounds are known for transition metals as small-sized atoms of H, B, C, N, etc. can easily occupy positions in the voids present in the crystal lattices of transition metals. Characteristics of interstitial compounds:
    • High melting points
    • Hard
    • Chemically inert
    • Retain metallic conductivity
    • Non-stoichiometric
       

Oxides of Transition metals: They form oxides of the general composition \( \text{MO}, \text{M}_2\text{O}_3, \text{MO}_2, \text{M}_2\text{O}_5 \) and \( \text{MO}_6 \). Oxides in the lower oxidation states are generally basic while those in the higher oxidation states are amphoteric or acidic. For example,

f-Block Elements : Lanthanoids and Actinoids

• f-block elements: The elements in which filling of electrons takes place in \( (n-2) \) f-subshell which belongs to anti-penultimate (third to the outermost) energy shell. This block consists of two series of elements known as Lanthanoids and Actinoids. These elements are also known as inner transition elements. The general electronic configuration of the f - block elements is
  \( (n-2)f^{1-14} (n-1)d^{0-1} ns^2 \)
  For lanthanoids, \( n \) is 6 while its value is 7 for actinoids. There are many exceptions in the electronic configuration.

• Lanthanoids: The series involves the filling of 4f-orbitals following lanthanum La (Z = 57) is called the lanthanoid series. There are 14 elements in this series starting with Ce (Z = 58) to Lu (Z = 71).
  • Electronic configuration: [Xe] \( 4f^{1-14} 5d^{0-1} 6s^2 \)
  • Physical properties:
    1. Highly dense metals, soft, malleable and ductile.
    2. High melting point.
    3. Forms alloys easily with other metals.
    4. Magnetic properties: Among lanthanoids, La³⁺ and Lu³⁺ which have 4f⁰ or 4f¹⁴ electronic configurations are diamagnetic and all other trivalent lanthanoid ions are paramagnetic due to the presence of unpaired electrons.
    5. Atomic and ionic sizes: With increasing atomic number, the atomic and ionic radii decreases from one element to the other but the decrease is very small.
       A steady decrease in the size of lanthanoids with increase in atomic number is known as lanthanoid contraction.
       Consequences of Lanthanoid contraction:
       (a) It leads to similar physical and chemical properties among lanthanoids.
       (b) Zr and Hf have same properties due to similar atomic radii.
       (c) Chemical separation of lanthanoids become difficult.
  • (vi) Oxidation state: They mainly give +3 oxidation state. Some elements show +2 and +4 oxidation states.