Chapter 8 The d and f Block Elements Solutions
Question - 11 : - Actinoid contraction is greater from element to element than lanthanoidcontraction. Why?
Answer - 11 : -
The 5f electrons are more effectively shieldedfrom nuclear charge. In other words the 5f electrons themselves provide poorshielding from element to element in the seriest.
Question - 12 : - Write down the electronic configuration of
Answer - 12 : -
1. Cr_3+ 2. Pm_3+ 3. Cu_+ 4. Ce_4+ 5. CO_2+ 6. Lu_2+
7. Mn_2+ 8. Th_4+
Answer
Question - 13 : - Why are Mn2+compoundsmore stable than Fe2+ towards oxidation to their +3 state?
Answer - 13 : -
Electronic configuration of Mn2+ is[Ar]18 3d5.
Electronic configuration of Fe2+ is[Ar]18 3d6.
It is known that half-filledand fully-filled orbitals are more stable. Therefore, Mn in (+2) state has astable d5 configuration. This is the reason Mn2+ showsresistance to oxidation to Mn3+. Also, Fe2+ has 3d6 configurationand by losing one electron, its configuration changes to a more stable 3d5 configuration.Therefore, Fe2+ easily gets oxidized to Fe+3 oxidationstate.
Question - 14 : - Explain briefly how +2 statebecomes more and more stable in the first half of the first row transitionelements with increasing atomic number?
Answer - 14 : -
With increasing atomic number the effective nuclearcharge increases after losing two electrons from s-orbital. The ionic sizedecreases which results in more stability. The slability is less in thebeginning due to too few electrons to lose or share.
Question - 15 : - To what extent do the electronicconfigurations decide the stability of oxidation states in the firstseries of the transition elements? Illustrate your answer with examples.
Answer - 15 : - The elements in the first-half of the transitionseries exhibit many oxidation states with Mn exhibiting maximum number ofoxidation states (+2 to +7). The stability of +2 oxidation state increases withthe increase in atomic number. This happens as more electrons are gettingfilled in the d-orbital. However, Sc does not show +2 oxidationstate. Its electronic configuration is 4s2 3d1.It loses all the three electrons to form Sc3+. +3 oxidation state ofSc is very stable as by losing all three electrons, it attains stable noble gasconfiguration, [Ar]. Ti (+ 4) and V(+5) are very stable for the same reason.For Mn, +2 oxidation state is very stable as after losing two electrons,its d-orbital is exactly half-filled, [Ar] 3d5.
Question - 16 : - What may be the stable oxidation state of the transition element withthe following d-electron configurations in the ground state of their atoms: 3d3,3d5, 3d8 and 3d4 ?
Answer - 16 : -
1. 3d3 :Stable oxidation state will be +5 due to outer electronic configuration 3d34s2 (+2,+3, +4, +5).
2. 3d5: Stableoxidation state will be +2 and +7 due to outer electronic configuration 3d5 4s2 (+2,+3, +4, +6, +7).
3. 3d8: Stableoxidation state will be +2 due to outer electronic configuration 3d8 4s2 (+2,+3, +4).
4. 3d4 :Stable oxidation state will be +3 and +6 due to outer electronic configuration3d4 4s1. There is no d4 configurationin ground state, as it becomes 3d5 4s1
Question - 17 : - Name the oxometal anions of the first series of the transition metals inwhich the metal exhibits the oxidation state equal to its group number.
Answer - 17 : -
Manganese shows oxidation state of +7 in itsoxometal anion MnO4– which is equivalent to itsgroup number 7. Cr in Cr2O72- and CrO42- showoxidation state +6 which is equivalent to its group number 6.
Question - 18 : - What is lanthanoid contraction? What are the consequences of lanthanoidcontraction?
Answer - 18 : -
The overall decrease in atomic and ionic radiifrom lanthanum to lutetium is a unique feature in the chemistry of thelanthanoids. It has far reaching consequences in the chemistry of the thirdtransition series of the elements.
This contraction is attributed to theimperfect shielding of one electron by another in the same sub-shell. However,the shielding of one 4f electron by another is less than one d electron byanother with the increase in nuclear charge along the series. There is fairlyregular decrease in the sizes with increasing atomic number.
The cumulative effect of the contraction ofthe lanthanoid series, known as lanthanoid contraction, causes the radii of themembers of the third transition series to be very similar to those of thecorresponding members of the second series. The almost identical radii of Zr(160 pm) and Hf (159 pm), a consequence of the lanthanoid contraction, accountfor their occurrence together in nature and for the difficulty faced in theirseparation.
Question - 19 : - What are the characteristics ofthe transition elements and why are they called transition elements? Whichof the d-block elements may not be regarded as the transitionelements?
Answer - 19 : -
Transition elements are thoseelements in which the atoms or ions (in stable oxidation state) containpartially filled d-orbital. These elements lie in the d-blockand show a transition of properties between s-block and p-block.Therefore, these are called transition elements.
Elements such as Zn, Cd, and Hgcannot be classified as transition elements because these have completelyfilled d-subshell.
Question - 20 : - In what way is the electronicconfiguration of the transition elements different from that of the non-transitionelements?
Answer - 20 : -
Transition metals have apartially filled d−orbital. Therefore, the electronic configurationof transition elements is (n − 1)d1-10 ns0-2.
The non-transition elementseither do not have a d−orbital or have a fully filled d−orbital.Therefore, the electronic configuration of non-transition elements is ns1-2 orns2 np1-6.