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CBSE Class 12th Chemistry Notes: The d- and f- Block Elements (Part - I)

Oct 7, 2016 18:00 IST

    This article provides you with the revision notes on CBSE Class 12 Chemistry: Chapter - 8: The d- and f-Block Elements. These notes will give you a quick glance of the chapter. These quick notes are prepared strictly according to the latest CBSE syllabus for Class 12th Chemistry.

    •    d –Block elements

    •    Features of d-block elements

    •    Transition metals

    •    General Properties of d –Block elements

    •    Some Important Compounds of Transition Elements

         o    Potassium dichromate, K2Cr2O7

         o    Potassium permanganate, KMnO4

    The key notes of the chapter are as follows:

    d –Block elements and transition metals

    The elements lying in the middle of periodic table belonging to groups 3 to 12 are known as d – block elements.

    Features of d-block elements:

    •     The general electronic configuration of d-block elements is (n −1)d1─10 ns1─2, where (n −1) stands for the inner d orbitals.

    •     In d-block, each horizontal row consists of ten elements as d-subshell can accommodate a maximum of 10 electrons.

    •     The d-block elements having incompletely filled d-subshell are called transition metals.

    •     Zinc, cadmium, mercury having the general electronic configuration as (n –1)d10 ns2, are not regarded as transition metals due to completely filled d – orbital.

    •     There are mainly three series of the transition metals:

    •     3d series starts with Sc (Z = 21) and ends with Zn (Z = 30)

    •     4d series starts with Y (Z = 39) and ends with Cd (Z = 48)

    •     5d series starts with La (Z = 57) and ends with Hg (Z = 80)

    NCERT Solutions for Class 12

    General General Properties of transition elements:

    Metallic character

    Almost all the transition elements display metallic properties such as metallic luster, high tensile strength, ductility, malleability and high thermal and electrical conductivity.

    In any row, the melting point of these metals rises to a maximum at d5 and after that as the electrons start pairing up so the melting point decreases regularly as the atomic number increases with an excepyion of Mn and Tc are exception.

    Atomic and ionic radii

    Due to the addition of new electron to a d-orbital each time the effective nuclear charge increases which causes the atomic radii to decrease in a series of transition elements. However, the atomic size of Fe, Co, Ni is almost the same because the attraction due to increase in nuclear charge is cancelled by the repulsion because of increased in shielding effect. The size of the 4d series elements is almost the same as the size of the 5d series elements. This phenomenon is associated with the intervention of the 4f orbitals which must be filled before the filling starts in 5d subshell.

    Lanthanoid contraction

    The filling of the 4f before the 5d orbital results in a regular decrease in size called lanthanoid contraction. This compensates for the expected increase in the atomic size with increasing atomic number. The net result of the lanthanoid contraction is is that the 4d and 5d series elements exhibit similar radii and show similarity in their physical and chemical properties.

    Melting point

    Due to the strong interatomic bonding which involves both (n−1)d and ns electrons participation, transition metals have high melting points.

    Ionization enthalpy

    In a particular transition series, there is an increase in ionization enthalpy from left to right which is due to the increase in effective nuclear charge along a series. But the trend is not very regular. The exceptions are chromium and copper which have notably larger ionization enthalpy than their neighbours. These exceptions are due to the extra stability associated with the half-filled and fully-filled set of d-orbitals.

    Oxidation States

    Transition metals show variable oxidation states due to tendency of (n-1)d as well as ns electrons to take part in bond formation.

    For example:- Oxidation states of the first row transition metals are:

    Image Source: NCERT Books

    Enthalpy of atomization

    The transition elements have high enthalpy of atomization which is due to the presence of strong metallic bonding. The elements with highest enthalpy of atomization tend to be noble metals. The elements of 4d and 5d series have greater enthalpies of atomization than the elements of 3d series. That’s why the elements of 4d and 5d series have more frequent metal-metal bonding in their compounds.

    Reactivity

    The metals of the 3d series are more reactive than the elements of the 4d or 5d series. All 3d series elements with the exception of Cu are highly reactive and are oxidized by 1 M H+. The tendency to form divalent cation decreases along the 3d series as indicated by their E°(M2+/M) values. The E° value does not follow a regular trend. This is due to irregularity in IE and the heat of atomisation.

    Magnetic properties

    Most of transition metals are paramagnetic in nature due to the presence of unpaired electrons. It increases from Sc to Mn due to the increased number of unpaired electrons and then starts decreasing as the number of unpaired electrons decreases.

    CBSE Class 10th & 12th Question Papers

    Formation of complexes

    The transition metals form a large number of complexes. This is due to their
          o Comparatively small sizes of the metal ions.
          o High ionic charges.
          o Availability of vacant d atomic orbitals

    Formation of coloured compounds

    Transition metals and their compounds show colour. The colour is due to the excitation of electron from one d atomic orbital to higher energy d atomic orbital in the same subshell. The frequency of the light absorbed generally lies in the visible region. The colour observed is due to the colour of the complementary light. The colour is due to the presence of unpaired electrons. All Zn2+ compounds are white.

    Formation of interstitial compounds

    Transition metals have lattice structure in which the unoccupied space is called void or hole or interstices. Transition metals entrap smaller but highly electronegative elements in these interstices and results in the formation of interstitial compounds. Interstitial com-pounds have high melting points, hardness and retain metallic conductivity. The interstitial compounds are chemically inert. Examples are TiC, steel.

    Formation of alloys

    Alloy is a homogeneous mixture of two or more metals. Due to the comparable size of transition metals, one metal can displace other metal in the crystal lattice and this results in the alloy formation. The alloys so formed are hard and have high melting points. The best known are ferrous alloys; chromium, vanadium, tungsten, manganese are used for the production of variety of steels and stainless steels.

    Catalytic properties

    Most of transition metals are used as catalysts.

    This is due to the

    (i) presence of incomplete or empty d-orbitals,

    (ii) large surface area and

    (iii) variable oxidation state. For example Fe, Ni, V2O3, Pt, Mo, Co, etc., are used as catalyst.

    Formation of oxides

    Transition metals form oxides on reaction with oxygen at elevated temperature. Transition metal form oxide in oxidation state of + 1 (in Ag2O) to + 7 (Mn2O7) to + 8 (in OsO4). As the oxidation number increases in case of same elements,

    (a) The covalent character of oxides increases.

    (b) The acidic strength of the oxides increases.

    (c) The oxidizing power of oxides increases.

    For example, Cr2O3 is amphoteric while CrO is basic and CrO3 is acidic.

    CBSE Class 12 Papers

    Some Important Compounds of Transition Elements

    Potassium dichromate, K2Cr2O7

    Preparation:

    It is prepared by fusion of chromate ore (FeCr2O4) with sodium carbonate in excess of air.

    8Na2CO3 + 4FeCr2O4 + 7O2 → 8Na2CrO4 + 2Fe2O3 + 8CO2

                                                      Yellow

    Na2CrO4 produced in the above reaction is then acidified to get sodium dichromate, Na2Cr2O7

    Na2CrO4 produced in the above reaction is then acidified to get sodium dichromate, Na2Cr2O7

    2Na2CrO4 + H2SO4 → Na2Cr2O7 + Na2SO4 + H2O

                                        Orange

    Solution of sodium dichromate treated with potassium chloride to get the final product, K2Cr2O7.

    Na2Cr2O7   +   2KCl   →   K2Cr2O7       +   2NaCl

                                       Orange Crystals

    Structures of CrO42‒ and Cr2O72‒ ions:

    Uses:

    Potassium dichromate is used as a primary standard in volumetric analysis and as an oxidizing agent. In acidic medium, the oxidation state of Cr changes from + 6 in Cr2O72‒  to + 3 in Cr3+.

    Cr2O72‒ + 14 H+ + 6e → 2Cr3+ + 7H2O

    Potassium permanganate, KMnO4

    Preparation:

    It is prepared by fusion of MnO4 with alkali metal hydroxide (KOH) in presence of O2 or oxidising agent like KNO3. It produces dark green coloured compound , K2MnO4 which undergoes oxidation as well as reduction in neutral or acidic solution to give permanganate.   

    Commercially it is prepared by the alkaline oxidative fusion of MnO2 followed by the electrolytic oxidation of manganate (Vl).

    Uses:

    Potassium permanganate acts as a strong oxidizing agent in acidic, neutral or faintly basic medium.

    In acidic medium:

    MnO4 + 8H+ + 5e → Mn2+ + 4 H2O

                                        Oxidises Fe2+ to Fe3+:

                                         Fe2+ → Fe3+ + e

    In an alkaline medium:

    MnO4 + 2H2O + 3e → MnO2 + 4OH

    Oxidises I to iodate IO3:

    6OH + I → IO3 + 6e + 3H2O

    CBSE Class 12th Chemistry Notes: The d- and f-Block Elements (Part-II)

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