this questions will help you the understand the topics of the d and f block. students can use this material for their board preparations as well as for their jee preparation
ENGLISH d and f-block element(A.K.SAMAL,PGT(CHEM.)
On what ground can you say that scandium (Z = 21) is a transition element but zinc (Z = 30) is 1
1
not?
ANS: It is because Sc (21) has incompletely filled d-orbital, that is why it is transition element,
whereas Zn(30) does not have incompletely filled d-orbitals, therefore, it is not regarded as
transition element.
2 Why do transition metals show variable oxidation states? 1
ANS: It is because electrons from both ‘s’ and d-orbitals can take part in bond formation.
3 Lanthanoids form primarily +3 ions, while the actinoids usually have higher oxidation states in
1
their compounds, +4 or even +6 being typical. Give reason.
ANS: In Actinoids, 5f, 6d and 7s orbitals have comparable energies and electrons from these
orbitals can take part to show higher oxidation states.
4 Among lanthanoids, Ln(III) compounds are predominant. However, occasionally in solutions or in
1
solid compounds, +2 and +4 ions are also obtained. Give reason.
ANS: Lanthanoids show +3 oxidation state mostly as 2 electrons from outer 6s orbital and one
electron from 5d orbital take part in bond formation. Some show +2 and +4 oxidation states due to
stability of half filled and completely filled 4f orbitals.
5 Out of Cu2Cl2 and CuCl2, which is more stable and why? 1
ANS: CuCl2 is more stable due to more hydration energy.
6 Although Zr belongs to 4d and Hf belongs to 5d transition series but it is quite difficult to separate
1
them. Why?
ANS: It is due to almost same size (Zr = 160 pm, Hf = 159 pm) which is due to lanthanoid
contraction.
7 E° of Cu is +0.34 V while that of Zn is –0.76 V. Explain. 1
ANS: It is because Cu(s) is more stable than Cu 2+ due to high ionisation enthalpy which is not
overcome by its hydration energy.
In the case of Zn, after removal of 2 electrons from 4s orbtital, stable 3d 10 configuration is
acquired.
8 Why do the transition metals have higher enthalpy of atomisation? In 3d series (Sc to Zn), which
2
element has lowest enthalpy of atomisation and why?
ANS: It is due to the involvement of greater number of unpaired electrons from (n – 1)d as well
as ns orbitals in the strong inter-atomic metallic bonding. Zinc has lowest enthalpy of atomisation
due to larger size and in the absence of unpaired electrons, it forms weak metallic bond.
9 For the first row transition metals, the E° values are given below:
2
Explain the
irregularity in the above values.
ANS: It is due to irregular variation of sublimation enthalpies and ionisation enthalpies of
elements of 3d transition series.
10 How would you account for the following?
(i) Cr2+ is reducing in nature while with the same d-orbital configuration (d4), Mn3+ is an oxidising
agent.
2
(ii) In a transition series of metals, the metal which exhibits the greatest number of oxidation
states occurs in the middle of the series. or N ame the element showing maximum number of
oxidation states among the first series of transition metal Sc (21) to Zn (30).
ANS: (i) It is because Cr2+ loses electron to become Cr3+ which is more stable due to half filled
t2g orbitals, whereas Mn3+ will gain electron to become Mn2+ which is more stable due to half filled
1
, d-orbitals.
(ii) Manganese. It is due to large number of unpaired electrons in d-orbitals in middle of the series.
Mn (25) 4s23d5.
11 Explain the following observations giving an appropriate reason for each.
(i) There occurs much more frequent metal-metal bonding in compounds of heavy transition
2
metals (i.e. 3rd series).
(ii) Mn2+ is much more resistant than Fe 2+ towards oxidation.
ANS: (i) Due to lanthanoid contraction, effective nuclear charge remains almost same
therefore, metallic radii are nearly same, therefore, metal-metal bonding is more.
(ii) Mn2+ (3d5) has stable electronic configuration, therefore, it does not get oxidised. Fe 2+ (3d6)
gets oxidised to form Fe3+(3d5) which is more stable.
12 State reasons for the following:
(i) Actinoids exhibit greater range of oxidation states than lanthanoids.
2
(ii) Unlike Cr3+, Mn2+, Fe3+ and the subsequent other M2+ ions of the 3d series of elements, the 4d
and the 5d series metals generally do not form stable cationic species.
ANS: (i) It is due to poor shielding effects of 4f and 5f electrons, more number of electrons take
part in bond formation in actinoids.
(ii) It is because energy required to remove electron is more due to greater effective nuclear
charge which is due to lanthanoid contraction.
13 Assign reasons for each of the following:
(i) T ransition metals generally form coloured compounds.
2
(ii) Manganese exhibits the highest oxidation state of + 7 among the 3d series of transition
elements.
ANS: (i) It is because transtion metals have unpaired electron in d-orbitals and undergo d-d-
transitions by absorbing light from visible region and rediate complementary colour.
(ii) Mn has electronic configuration (Ar)4s2 3d5 and all the electrons in ‘s’ as well as ‘d’ orbitals can
take part in bond formation, therefore, it shows +7 (highest) oxidation state.
14 Explain the following observations:
(i) Generally there is an increase in density of elements from titanium (Z = 22) to copper (Z = 29)
in the first series of transition elements. 2
(ii) T ransition elements and their compounds are generally found to be good catalysts in chemical
reactions.
ANS: (i) It is because atomic mass increases more than atomic volume, therefore, density
increases from titanium (Z = 22) to copper (Z = 29).
(ii) It is because they show variable oxidation states and have vacant d-orbitals forming unstable
intermediates which readily change into products.
15 Explain the following observations:
(i) Transition elements generally form coloured compounds. 2
(ii) Zinc is not regarded as a transition element.
ANS: (i) It is due to presence of unpaired electrons in d-orbitals, therefore, they undergo d-d
transitions by absorbing light from visible region and radiate complementary colour.
(ii) It is because neither Zn nor Zn2+ ion has incompletely filled d-orbital.
16
2
Complete the following equations:
ANS:
17
2
Complete the following equations:
2
, ANS:
18
2
Complete the following chemical equations:
ANS:
19 Name the oxometal anions of the first series of the transition metals in which the metal exhibits
2
the oxidation state equal to its group number.
ANS: In MnO4–, oxidation state of Mn is +7 which is equal to its group number.
In CrO42–, oxidation state of Cr is +6 which is equal to its group number.
20 Write complete chemical equations for:
(i) Oxidation of Fe2+ by Cr2O72– in acid medium. 2
(ii) Oxidation of S2O32– by MnO4– in neutral aqueous medium.
ANS:
21 Write one similarity and one difference between the chemistry of lanthanoids and that of actinoids. 2
ANS: Similarity
Lanthanoids show lanthanoid contraction like actinoids contraction.
Dissimilarity
Lanthanoids show mostly +3 oxidation state. Few show +2 and +4, whereas Actinoids show +3,
+4, +5, +6 and +7 oxidation states.
22 Give reasons for the following observations:
(i) Mn(II) ion shows maximum paramagnetic character amongst the bivalent ions of first transition
2
series.
(ii) Scandium (At. no. 21) salts are white.
ANS: (i) It is due to presence of five unpaired electrons.
(ii) Sc3+ does not have unpaired electrons, therefore, cannot undergo d-d transition by absorbing
light from visible region. Therefore, its salts are white.
23 State reasons for the following observations:
(i) The enthalpies of atomisation of transition elements are quite high.
2
(ii) There is a greater horizontal similarity in the properties of the transition elements than of the
main group elements.
ANS: (i) It is due to smaller size of transition metals and strong metallic bonds due to the
presence of large number of unpaired electrons.
(ii) It is due to similarity in atomic and ionic size, there is more horizontal similarity. Secondly, in
transition elements incoming electron goes to inner shell (d-orbitals), whereas in main group
elements, the incoming electron goes to outermost shell.
24 Assign suitable reasons for the following:
(a) The Mn2+ compounds are more stable than Fe 2+ towards oxidation to their +3 state.
3
(b) In the 3d series from Sc (Z = 21) to Zn (Z = 30), the enthalpy of atomization of Zn is the lowest.
(c) Sc3+ is colourless in aqueous solution, whereas Ti 3+ is coloured.
ANS: (a) Mn2+ has 3d5 (stable electronic configuration), therefore, it does not get oxidised to
Mn3+, whereas Fe2+ has 3d6 which readily changes to Fe 3+ (3d5) which has stable electronic
configuration.
(b) Zinc does not have unpaired electrons and larger in size, therefore, has weak metallic bonds.
That is why it has least enthalpy of atomisation.
(c) Sc3+ is colourless as it does not have unpaired electron and cannot undergo d-d transition,
3
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