CBSE Class 12, Unit-4: d-Block Elements Notes
d-Block Elements:
The d-block elements are those elements in which the last electron enters the d–subshell of penultimate shell. The general electronic configuration of these elements is (n – 1) d1-10ns1-2 , where n is outermost shell. The d-block consisting of groups 3–12 occupies the large middle section of the periodic table.
Transition Elements:
The elements of d-block are known as transition elements as they possess properties that are transitional between the s and p block elements. A transition element is defined as an element which has incompletely filled d-orbitals in its ground state or any one of its oxidation states. There are four series of transition elements spread between group 3 and 12.
First transition series or 3d-series: Scandium (21Sc) to Zinc (30Zn)
Second transition series or 4d-series: Yttrium (39Y) to Cadmium (48Cd)
Third transition series or 5d-series: Lanthanum (57La) and Hafnium (72Hf) to Mercury (80Hg) (Omitting 58Ce to 71Lu)
Fourth transition series or 6d-series: Begins with Actinium (89Ac) is still incomplete.
Zinc, cadmium and mercury of group 12 have full d10 configuration in their ground state as well as in their common oxidation states and hence, are not regarded as transition metals. However, being the end elements of the three transition series, their chemistry is studied along with the chemistry of the transition 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 increase 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 first 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 transitional metals show multiple oxidation states and have large surface area so, all metals work as a catalyst.
14. Magnetic properties: On the basis of the behaviour of substances in magnetic field, they are of two types:
Diamagnetic and 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, (µ) = √n (n + 2) 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:
A. High melting points
B. Hard
C. Chemically inert
D. Retain metallic conductivity
E. Non-stoichiometric
What are interstitial compounds? Why are such compounds well known for transition metals?
Interstitial compounds are those in which small atoms occupy the interstitial sites in the crystal lattice. Interstitial compounds are well known for transition metals because small-sized atoms of H, B, C, N, etc., can easily occupy positions in the voids present in the crystal lattices of transition metals.
What is the effect of increasing pH on a solution of potassium dichromate?
On increasing the pH, i.e., on making the solution alkaline, dichromate ions (orange coloured) are converted into chromate ions and thus, the solution turns yellow.
Preparation and Properties of Potassium Dichromate (K2Cr2O7)
Preparation of Potassium Dichromate:
It is prepared from chromate ore in the following steps:
1. Chromite ore is fused with sodium carbonate in the presence of air to give sodium chromate.
4FeCr2O4 + 8Na2CO3 +7O2 → 2Fe2O3 + 8Na2CrO4 + 8CO2
2. Na2CrO4 is filtered and acidified with conc. H2SO4 to give Na2Cr2O7.
2Na2CrO4 + 2H+ → Na2Cr2O7 + 2Na+ + H2O.
3. Sodium dichromate solution is treated with KCl to give
K2Cr2O7.
Na2Cr2O7 + 2KCl → K2Cr2O7 + 2NaCl
Properties of Potassium Dichromate:
1. It is an orange, crystalline solid.
2. Reaction with alkali: Cr2O72– + 2OH– → 2CrO42– + H2O
Chromate ion(Yellow)
3. Reaction with acid: 2CrO42– + 2H+ → Cr2O72– + H2O
Dichromate ion (orange red)
In acidic solutions, oxidising action is
Cr2O72– + 14H+ + 6e– → 2Cr3+ + 7H2O
4. It is a powerful oxidising agent
Uses of Potassium Dichromate:
1. In leather industry for chrome tanning.
2. Preparation of azo compounds.
3. As a primary standard in volumetric analysis for the estimation of reducing agent.
Structures of Chromate and Dichromate Ions:
Preparation and Properties of Potassium Permanganate (KMnO4)
Preparation of Potassium Permanganate:
Potassium permanganate is commercially prepared from MnO2 (Pyrolusite). The preparation involves two steps:
1. Conversion of MnO2 to potassium manganate (K2MnO4) by fusing with KOH in presence of air(or an oxidizing agent like KNO3).
2MnO2 + 4KOH + O2 → 2K2MnO4 + 2H2O
2. Potassium manganate is oxidised to potassium permanganate either by electrolysis or by acidification.
3MnO42- + 4H+ → 2MnO4- + MnO2 + 2H2O
Properties of Potassium Permanganate:
Potassium permanganate forms dark purple crystals which are iso-structural with those of potassium perchlorate (KClO4).
The colour of permanganate is due to ligand to metal charge transfer. i.e. an electron is transferred from oxygen atom to the vacant d-orbital
of Mn.
When heated it decomposes and liberate O4.2
2KMnO4 → K2MnO4 + MnO2 + O2
Structure of Potassium permanganate:
The manganate and permanganate ions are tetrahedral. The green manganate is paramagnetic with one unpaired electron but the permanganate is diamagnetic.
Uses of Potassium permanganate:
1. It is used as an oxidising agent in acidic, basic and neutral medium.
2. It is used as a primary standard in volumetric analysis.
3. It is used for the bleaching of wool, cotton, silk and other textile fibres and also for the decolourisation of oils.
