d and f Block Elements class 12 Chemistry

d- and f-Block Elements

d-Block Elements:

Strictly speaking, the elements which have incompletely filled d-orbitals in this ground state or in any of its oxidation states are called d-block elements.

Electronic Configurations of d-Block Elements:

(i) General electronic configuration of transition metals is (n – 1) d 1 – 10 ns1 – 2. Zinc, cadmium, mercury do not correspond to this electronic configuration. Their configuration is represented by a general formula (n-1)d10 ns2. These are not regarded as transition metals due to completely filled d-orbitals.

(ii) There are mainly four series of the transition metals, 3 d-series (Sc to Zn), 4 d-series (Y to Cd) and 5d-series (La to Hg omitting Ce to Lu).

Electronic Configuration of Transition Metals:

  1. First (3d) Transition Series (Sc-Zn):

First (3d) Transition Series (Sc-Zn)

2. Second (4d) Transition Series (Y-Cd):

Second (4d) Transition Series (Y-Cd)

3. Third (5d) Transition Series (La-Hg):

Third (5d) Transition Series (La-Hg)

4. Fourth (6d) Transition Series:

Fourth (6d) Transition Series

Note: Fourth transition series or 6 d-series begins with actinium and still incomplete.

General Properties of Transition Elements:

Physical Properties:

These elements have metallic properties such as metallic lustre, high tensile strength, ductility, malleability, high thermal and electrical conductivity, low volatility (except Zn, Cd, Hg), hardness, etc. Their melting points are high. The density of transition metals from Sc to Cu increases due to high atomic mass and small volume.

Atomic Radii:

From Sc to Cr, atomic radii decreases because of effective nuclear charge increases. The atomic sizes of Fe, Co and Ni are almost same because the pairing of electrons in d-orbitals causes repulsion and hence effective nuclear charge does not increase appreciably.

Ionisation Enthalpy:

Ionisation enthalpy increases with increase in nuclear charge along each series. However, its value for Cr is lower because of the absence of any change in d-configuration and the value for Zn is higher because it represents an ionisation from the completely filled 4s level.

Oxidation States:

Transition metals show variable oxidation states due to the participation of (n -1) d as well as ns electrons in bond formation. The maximum oxidation states are shown by Mn (in the first series). Sc shows only + 3 oxidation state. Since fluorine and oxygen are strong oxidising agents, high oxidation state is shown in fluorides and oxides.

Cu2+ is more stable than Cu+ due to lower reduction potential which is because of the higher hydration energy.

Trends in M2+/M E° Values:

The general trend towards less negative E° values across the series is related to the general increase in the sum of the first and second ionisation enthalpies.

Trends in M3+ / M2+ E° Values:

Low value of E° shows the stability of ion (either d 5 of d 1° configuration.) The comparatively high value for Mn indicates the stability of Mn2+ (d5), whereas comparatively low value for Fe indicates the extra stability of Fe3+ (d5).

Magnetic Properties:

Most of the transition metals are paramagnetic due to the presence of unpaired electrons (Paramagnetic character α number of unpaired electrons). The species having all paired electrons are diamagnetic in nature. The magnetic moment is determined by the formula, µ= √n(n+ 2) BM, where, n is the number of unpaired electrons and BM is Bohr Magnetons units of magnetic moment.

Coloured Salts Formation:

Transition elements form coloured ions due to the presence of unpaired electrons in d-orbitals as they can undergo d-d transition by absorbing colour from the visible region and radiating complementary colour.

Formation of Complex Compounds:

These elements form complex compounds due to their small size and high charge density on cations and presence of vacant d-orbitals, e.g.,{PtCl4)2- .

Catalytic Properties:

Most of the transition metals and their compounds are used as a catalyst because they show variable oxidation states and have the ability to form a complex.

Formation of Interstitial Compounds:

Transition elements form interstitial compounds. It means the compounds in which H, C or etc are trapped inside the crystal lattices of metals.

Note:

■Transition metal oxides in lowest oxidation state are basic, in intermediate oxidation state is amphoteric and in highest oxidation state are acidic.

■ Transition metal halides in lower oxidation state are ionic and in higher oxidation state are covalent. Mostly fluorides are ionic and chlorides and bromides are covalent.

■ Copper (I) compounds are unstable in aqueous solution and undergo disproportionation.

                                                            2Cu+ —->  Cu2+ + Cu

The stability of Cu2+ (aq) rather than Cu+ (aq) is due to the much more negative ∆ hyd H of Cu2+ (aq) than Cu+ , which is more than that compensates for the second ionisation enthalpy of Cu.

Alloy Formation:

Alloys may be homogeneous solid solutions of two or more metals in which the atoms of one metal are distributed randomly among the atoms of the other. Transition metals have approximately the same size, therefore in the molten form they can fit in each other’s crystalline structure and form a homogeneous mixture and the alloy. Eg: brass (copper-zinc) and bronze (copper-tin)

Potassium Dichromate:

Preparation:

It is prepared from chromite ore (FeO.Cr2O3 ) in three steps

(i) Fusing with Na2CO3 and quicklime and extracting with water to get Na2CrO4

4 FeO.Cr2O3 + 8Na2CO3 + 7O2 –> 8Na2CrO4 + 2Fe203 + 8CO2

(ii) Treating with conc. H2SO4 to convert Na2CrO4 to Na2Cr2O7 and separating out less soluble Na2SO4 by fractional crystallisation.

                                     2Na2CrO4 + H2SO4 —+ Na2Cr2O7 + Na2SO4 + H2O

(iii) Treating with KCl to convert Na2Cr2O7 to K2Cr2O7 and separating out less soluble orange crystals of K2Cr2O7 by fractional crystallisation.

                                Na2Cr2O7 + 2KCl —> K2Cr2O7 + 2NaCl

Properties:

 (i) 2K2Cr2O7 + 2KOH —> 2K2CrO4 + H2O

      2K2CrO4 + H2SO4 —> K2Cr2O7 + K2SO4 + H2O

Thus,               Cr2O72- (Orange) + H2O   BaseAcid  2CrO42- (Yellow) + 2H+

(ii) Oxidising Propdrties

                         K2Cr2O7 + 4H2SO4 (dil)  —–> K2SO4 + Cr2(SO4 )3 + 4H2O + 3(O)

 (v) Chromyl Chloride Test:

   K2Cr2O7 (s) + 4KCl (s) + 6H2SO4 (conc.) —-Heat–>  2CrO2Cl2 (Reddish Brown vapour) + 6KHSO4 + 3H2O

(vi) With H202

    Cr2O72- + 2H+ + 4H202 ——>   2Cr05 + 5H20 (Deep blue)

Note:  Colour of CrO24- andCr2O72-   Cr in both has oxidation state +6 and hence d° configuration. Hence, colour is not due to d-d transition but due to charge transfer (from 0-atom to metal atom thereby changing 02- ion to 0 ion and reducing the oxidation state of Cr from +6 to +5).

Potassium Permanganate:

Potassium permanganate is prepared by pyrolusite (MnO2) ore. When pyrolusite ore is fused with alkali (KOH)  in the presence of air or an oxidising agent like KNO3, potassium permanganate (KMnO4) is formed.

Preparation:

It is prepared from the mineral pyrolusite (MnO2) in two steps

(i) Conversion of MnO2 into K2MnO4 (potassium manganate) by fusing with KOH of K2CO3 in the presence of air or oxidising agent like KNO3, KClO3 etc.

2MnO2 + 4KOH + O2 –> 2K2MnO4 + 2H2O

or

3MnO2 + 6KOH + KClO3 —>3K2MnO4 + KCl + 3H2O

(ii) Oxidation of K2MnO4 to KMnO4 chemically with CO2, Cl2, 03 etc or electrolytically (by electrolysis)

         3K2MnO4 + 2CO2 —> 2KMnO4 + MnO2 + 2K2CO3

or     K2MnO4  ↔ 2K+ + MnO42- ;  H2O ↔  H+ +  OH

     MnO42-  –> MnO4 + e (at anode) and H+ + e —> ½ H2  (at cathode)

Properties:

 (i) 2KMnO4 (s) –513 K —>K2MnO4 + MnO2 + O2

 (ii) Oxidising properties

Neutral solution:-  2KMnO4 + H2O —> 2KOH + 2MnO2 + 3(0),

Alkaline solution:-  2KMnO4 + 2KOH —> 2K2MnO4 + H2O + (0)

In the presence of reducing agent:-  K2MnO4 + H2O —> MnO2 + 2KOH + (0)

Complete reaction              2KMnO4 + H2O —> MnO2 + 2KOH + 3(0),

Alkaline KMnO4 is called Baeyer’s reagent, it oxidises olefinic compound to glycols,

e.g., CH2 =CH2 + H2O + (O) —> CH2(OH) — CH2(OH)

Acidic medium:  2KMnO4 + 3H2SO4 (dil.) —> K2S04 + 2MnSO4 + 3H20+ 5(0)

Note:

■ KMn04 cannot be used in the presence of HCI orHNO3 in titration because oxygen produced from KMnO4 and HCI not only oxidises the reducing agent but also oxidises HCI to CI2. HNO3 itself is an oxidising agent.

Colour of MnO4  Mn isMnO4is in +7 oxidation state with d0 configuration. Hence, colour is not due to d-d transition but due to charge transfer (from O to Mn) changing Mn from +7 to +6.

Inner-Transition Elements:

Elements in which the last electron enters the f-orbital are called f-block elements. They are also known as inner-transition elements. The f-block elements have two series i.e., lanthanoids and actinoids.

Lanthanoids:

The 14 elements after lanthanum are called lanthanoids in which 4f-orbitals are progressively filled. General configuration of lanthanoids is 4f 1 – 14 5d 0-1 6s2

Oxidation States of Lanthanoids:

The most common oxidation state of lanthanoids is +3. However, Ce shows +4, Eu and Yb show +2, oxidation state. However, they tend to change to +3. That’s why Sm2+ , Eu2+ and Yb2- are reducing agents, while Ce4+ and Tb4+ are oxidising agents. Due to large energy gap between 4f and 5d, they show a limited number of oxidation states.

Atomic and Ionic Radii of Lanthanoids: Lanthanoid Contraction

The regular decrease in atomic and ionic radii of lanthanoids, from left to right, is called lanthanoid contraction. The small net decrease is due to increase in nuclear charge which outweighs the imperfect shielding by f-electrons. Due to this contraction, atomic radii of elements of the 3rd transition series (after La) are nearly same as those of 2nd series in the same group.

Note: The radii of members of 5d-series are similar to those of corresponding members of the 4d-series due to lanthanoid contraction, e.g., Zr and Hf have almost some radii and due to identical radii, they have similar physical and chemical properties.

Other Characteristics :

(i) They are silvery-white metals.

(ii) Most of the trivalent metal ions are coloured due to f-f transition. Those with xf -electrons have same colour as with (14 -x)f-electrons.

(iii) Most of the lanthanoids are paramagnetic in nature due to the presence of unpaired electrons.

(iv) Lanthanoids are highly electropositive and reactive metals.

(v) They form basic hydroxides. As size decreases from La3+ to Lu3+ , the covalent character of hydroxides increases and hence, basic strength decreases. Thus, La(OH)3 is most basic while Li(OH)3 is least basic.

Chemical Properties:

The earlier members of the series are quite reactive similar to calcium but, with increasing atomic number, they behave more Iike aluminium.

Uses of Lanthanoids:

They are used only as alloys. The most common being misch metal (Ln = 95%, Fe = 5%, S, C, Ca, Al = traces), It is used in making Mg based alloy. It is a pyrophoric alloy (emits spark when struck), used in making bullets, shells, lighter flints, etc.

Actinoids:

The 14 elements after actinium are called actinoids in which 5f-orbitals are progressively filled. General configuration of actinoids is 5f 1-14 6d 0 – 2 7s2.

Oxidation States of Actinoids:

Actinoids show in general + 3 oxidation state. These resemble the lanthanoids in having more compounds in + 3 state than in the + 4 state. However, + 3 and + 4 ions tend to hydrolyse. There is a greater range of oxidation states in actinoid due to the comparable energies of 5f, 6d and 7s levels.

Atomic and Ionic Radii:  Actinoids Contraction:

The steady decrease in atomic and ionic size along actinoid series is called actinoid contraction and it is due to the poor shielding effect of 5f-electrons which results in the increase in effective nuclear charge.

Other Characteristics:

(i) They are silvery-white metals.

(ii) Their cations are coloured due to f-f transition. Except Ac3+ (5f°), Cm3+ (5f7 ) and Th3+ (5f °) are colourless.

(iii) They are strongly paramagnetic.

Uses of Actinoids:

(i) Thorium is used in atomic reactor and treatment of cancer.

(ii) U and Pu are used as a fuel in a nuclear reactor.

Leave a Comment

Your email address will not be published. Required fields are marked *

Scroll to Top
Share via
Copy link
Powered by Social Snap