CBSE Class 12 Chemistry Transition Elements Notes

 Remedial action plan

D and f block

1. Definitions and examples types of transition series in transition elements

2. Properties (physical and chemical) in transition element

3. Preparation and properties in the transition elements K₂Cr₂O₇, KMnO₄

a. Magnetic nature

b. Melting points

c. Alloys formation

d. Interstitial compounds

e. Oxidation states

f. Coloured complexes 

g.Reduction potential 

4. lanthanoids and actinoids with their properties 

5.lanthanoid contraction reason and concequences 

The d-block consisting of Groups 3-12 occupies the large middle section of the periodic table. In these elements the inner d orbitals are progressively filled. The f-block is placed outside at the bottom of the periodic table and in the elements of this block, 4f and5f orbitals are progressively filled. 

--All the transition elements exhibit typical metallic properties such as –high tensile strength, ductility, malleability, thermal and electrical conductivity and metallic character. 

--Their melting and boiling points are high which are attributed to the involvement of (n –1) d electrons resulting into strong inter atomic bonding 

--Successive ionisation enthalpies do not increase as steeply as in the main group elements with increasing atomic number 

--Electronic Configurations of the d-Block Elements 

In general the electronic configuration of these elements is(n-1)d ^1–10 n s ^1–2.

variation in Atomic and Ionic Sizes of Transition Metals 

In general, ions of the same charge in a given series show progressive decrease in radius with increasing atomic number. This is because the new electron enters a d orbital each time the nuclear charge increases by unity 

Ionisation Enthalpies Due to an increase in nuclear charge which accompanies the filling of the inner d orbitals, there is an increase in ionisation enthalpy along each series of the transition elements from left to right 

. There is the generally expected increasing trend in the values as the effective nuclear charge increases . However, the value of Cr is lower because of the absence of any change in the d configuration and the value for Zn higher because it represents an ionization from the 4s level. The lowest common oxidation state of these metals is +2 

Oxidation States One of the notable features of a transition element is the great variety of oxidation states The elements which give the greatest number of oxidation states occur in or near the middle of the series. Manganese, for example ,exhibits all the oxidation states from +2 to +7. 

Magnetic Properties Many of the transition metal ions are paramagnetic .Paramagnetism arises from the presence of unpaired electrons, each such electron having a magnetic moment associated with its spin angular momentum and orbital angular momentum. For the compounds of the first series of transition metals, the contribution of the orbital angular momentum is effectively quenched and hence is of no significance. For these, the magnetic moment is determined by the number of unpaired electrons and is calculated by using the ‘spin only’ formula ,i.e.,

where n is the number of unpaired electrons and μ is the magnetic moment in units of Bohr magneton (BM)

.Formation of Coloured Ions When an electron from a lower energy d orbital is excited to a higher energy d orbital, the energy of excitation corresponds to the frequency of light absorbed This frequency generally lies in the visible region. The colour observed corresponds to the complementary colour of the light absorbed

. Formation of Complex Compounds The transition metals form a large number of complex compounds .This is due to the comparatively smaller sizes of the metal ions, their high ionic charges and the availability of d orbitals for bond formation. A few examples are: [Fe(CN)₆]^3–, [Fe(CN)₆]^4–,[Cu(NH₃)4]²⁺ and [PtCl₄]^2–. 

Catalytic Properties The transition metals and their compounds are known for their catalytic activity. This activity is ascribed to their ability to adopt multiple oxidation states and to form complexes. Formation of Interstitial Compounds Interstitial compounds are those which are formed when small atoms like H,C or N are trapped inside the crystal lattices of
metals. They are usually non stoichiometric and are neither typically ionic nor covalent, for example, TiC, Mn₄N, Fe₃H, VH0.56 and TiH1.7, etc

(i) They have high melting points, higher than those of pure metals.
(ii) They are very hard, some borides approach diamond in hardness.
(iii) They retain metallic conductivity.
(iv) They are chemically inert

Alloy Formation---alloys are formed by atoms with metallic radii that are within about 15percent of each other. Because of similar radii and other characteristics of transition metals, Alloys of transition metals with non transition metals such as brass (copper-zinc) and bronze (copper-tin), are also of considerable industrial importance Some Important Compounds of Transition Elements Potassium dichromate K₂Cr₂O₇
4 FeCr₂O₄ + 8 Na₂CO₃ + 7 O₂ →8 Na2CrO₄ + 2 Fe₂O₃ + 8 CO₂
2Na₂CrO₄ + 2 H+ →Na₂Cr₂O₇ + 2 Na+ + H₂O
Na2Cr2O7 + 2 KCl →K2Cr2O7 + 2 NaCl

The chromates and dichromates are interconvertible in aqueous solution depending upon pH of the solution. The oxidation state of chromium in chromate and dichromate is the same.

2 CrO4₂– + 2H+ →Cr₂O7₂– + H₂O
Cr₂O7₂– + 2 OH- →2 CrO₄₂– + H₂O

potassium dichromates as oxidising agent;----

Cr₂O₇₂^– + 14H+ + 6e– →2Cr₃⁺ + 7H₂O (EV = 1.33V)
Cr2O₇₂– + 14 H⁺ + 6 Fe₂⁺ →2 Cr₃⁺ + 6 Fe₃⁺ + 7 H₂O

Potassium permanganate KMnO4=
2MnO2 + 4KOH + O2 →2K₂MnO₄ + 2H₂O
3MnO₄₂^– + 4H+ →2MnO₄^– + MnO₂ + 2H₂O

In acid solutions:

(a) Iodine is liberated from potassium iodide :
10I– + 2MnO₄^– + 16H+ ——> 2Mn₂⁺ + 8H₂O + 5I₂

(b) Fe₂+ ion (green) is converted to Fe₃+ (yellow):
5Fe2+ + MnO₄^– + 8H+ ——> Mn2+ + 4H2O + 5Fe3+

Oxalate ion or oxalic acid is oxidised at 333 K:
5C₂O4₂^– + 2MnO₄^– + 16H+ ——> 2Mn₂+ + 8H₂O + 10CO₂

THE INNER TRANSITION ELEMENTS ( f-BLOCK)

The f-block consists of the two series, lanthanoids (the fourteen elements
following lanthanum) and actinoids (the fourteen elements followingactinium).
lanthanoid contraction The overall decrease in atomic and ionic radii from lanthanum to
lutetium is a unique features & called lanthanoid contraction
Consequence of lanthanoid contraction
-Similar sizes of lanthanoid ions (M³⁺)
-Decrease in basic character from La(OH)₃ to Lu (OH)₃
-Similar size & properties of 4d & 5d series members e.g Zr-Hf, Nb-Ta, Mo-W
The Actinoids- The actinoids include the fourteen elements from Th to Lr
The actinoids are radioactive elements and the earlier members haverelatively long half-lives, the latter ones have half-life values ranging froma day to 3 minutes for lawrencium (Z =103)
ElectronicConfigurations All the actinoids are believed to have the electronic configuration of 7s2and variable occupancy of the 5f and 6d sub shells

Ionic Sizes- There is a gradual decrease in the size of atoms or M3+ ions across the series. This may be referred to as the actinoid contraction (like lanthanoidcontraction). The contraction is,however, greater from element to elementin this series resulting from poor shielding by 5f electrons
OxidationStates--- There is a greater range of oxidation states, which is in part attributed tothe fact that the 5f, 6d and 7s levels are of comparable energies The actinoids show in general +3 oxidation state. The elements, in the first half of the series frequently exhibit higher oxidation states. For example ,the maximum oxidation state increases from +4 in Th to +5, +6 and +7respectively in Pa, U and Np but decrease in succeeding elements

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