CBSE Class 12 Chemistry The d and f Block Elements MCQs Set 10

MCQs

Question. Electronic configuration of a transition element X in +3 oxidation state is \( [Ar]3d^5 \). What is its atomic number?
(a) 25
(b) 26
(c) 27
(d) 24
Answer: (b) 26
Explanation: It is formed by the loss of 3 electrons, the configuration of element X is \( [Ar] 3d^6 4s^2 \). Therefore, Atomic number Number of electrons in a neutral atom of an element = 26.

Question. The electronic configuration of Cu(II) is \( 3d^9 \) whereas that of Cu(I) is \( 3d^{10} \). Which of the following is correct?
(a) Cu(II) is more stable.
(b) Cu(II) is less stable.
(c) Cu(I) and Cu(II) are equally stable.
(d) Stability of Cu(I) and Cu(II) depends on nature of copper salts.
Answer: (a) Cu(II) is more stable.
Explanation: Cu (I) should be stable as fullyfilled d-orbital (\( d^{10} \)) than incompletely filled (\( d^9 \)) d-orbital of Cu(II). But Cu(II) has a greater charge density than Cu(I) ion and therefore forms much stronger bonds releasing more energy.

Question. The oxidation state of Ni in \( [Ni(CO)_4] \) is:
(a) 0
(b) 2
(c) 3
(d) 4
Answer: (a) 0
Explanation: Let oxidation state of Ni = \( x \)
\( x + 0 = 0 \) (as CO is neutral ligand.)
\( x = 0 \)

Question. Which of the following is the reason of Zinc for not exhibiting variable oxidation states ?
(a) Inert pair effect
(b) Completely filled 3d subshell
(c) Completely filled 4s subshell
(d) Common ion effect
Answer: (b) Completely filled 3d subshell
Explanation: Zinc does not exhibit variable oxidation states as it has completely filled 3d subshell.
Zn — \( [Ar] 3d^{10} 4s^2 \) (Atomic number 30)
In only exhibit +2 oxidation state.

Question. Which of the following is a diamagnetic ion ? (Atomic numbers of Sc, V, Mn and Cu are 21, 23, 25 and 29 respectively)
(a) \( V^{2+} \)
(b) \( Sc^{3+} \)
(c) \( Cu^{2+} \)
(d) \( Mn^{3+} \)
Answer: (b) \( Sc^{3+} \)
Explanation: The electronic configuration of \( Sc^{3+} \) is \( [Ar]3d^0 \). Since its d subshell is empty, it is diamagnetic as it has attained noble gas configuration.

Question. Which set of ions exhibit specific colours? (Atomic number of Sc = 21, Ti = 22, V=23, Mn = 25, Fe = 26, Ni = 28 Cu = 29 and Zn =30)
(a) \( Sc^{3+}, Ti^{4+}, Mn^{3+} \)
(b) \( Sc^{3+}, Zn^{2+}, Ni^{2+} \)
(c) \( V^{3+}, V^{2+}, Fe^{3+} \)
(d) \( Ti^{3+}, Ti^{4+}, Ni^{2+} \)
Answer: (c) \( V^{3+}, V^{2+}, Fe^{3+} \)
Explanation: \( V^{3+}, V^{2+}, Fe^{3+} \) ions exhibit specific colours.
Atomic number of V = 23,
Electronic configuration of V - \( [Ar]3d^3 4s^2 \)
Electronic configuration of \( V^{2+} \) - \( [Ar]3d^3 \)
Electronic configuration of \( V^{3+} \) - \( [Ar] 3d^2 \)
Atomic number of Fe = 26
Electronic configuration of Fe - \( [Ar]3d^6 4s^2 \)
Electronic configuration of \( Fe^{3+} \) - \( [Ar]3d^5 \)
Since these ions have partially filled d- subshells, they exhibit colour because unpaired electrons of d-orbital of lower energy excites to the higher energy level, and absorbs radiations from visible region and exhibits complementary colours.
As for other ions,
Atomic number of Sc = 21
Electronic configuration of Sc - \( [Ar]3d^1 4s^2 \)
Electronic configuration of \( Sc^{3+} \) - \( [Ar]3d^0 \)
Since d subshell is empty, it shows no colour.
Atomic number of Ti = 22
Electronic configuration of Ti- \( [Ar]3d^2 4s^2 \)
Electronic configuration of \( Ti^{4+} \) - \( [Ar]3d^0 \)
Since d subshell is empty, it shows no colour.
Atomic number of Mn =25
Electronic configuration of Mn- \( [Ar]3d^5 4s^2 \)
Electronic configuration of \( Mn^{2+} \) \( [Ar]3d^5 \)
Since d subshell is partially filled, it shows colour.
Atomic number of Ni = 28
Electronic configuration of Ni- \( [Ar]3d^8 4s^2 \)
Electronic configuration of \( Ni^{2+} \) \( [Ar]3d^8 \)
Since d subshell is partially filled, it shows colour.
Atomic number of Zn =30
Electronic configuration of Zn- \( [Ar]3d^{10} 4s^2 \)
Electronic configuration of \( Zn^{2+} \)- \( [Ar]3d^{10} \)
Since d subshell is fully filled, it shows no colour.

Question. Which of the following oxidation state is common for all lanthanoids?
(a) +2
(b) +3
(c) +4
(d) +5
Answer: (b) +3
Explanation: Most common oxidation state for lanthanoids is +3.

Question. There are 14 elements in lanthanoid series. Which of the following element belong to this series?
(a) Ce
(b) Sm
(c) Tm
(d) All of the options
Answer: (d) All of the options

Question. Gadolinium belongs to 4f series. Its atomic number is 64. Which of the following is the correct electronic configuration of Gadolinium?
(a) \( [Xe] 4f^7 5d^1 6s^2 \)
(b) \( [Xe] 4f^6 5d^2 6s^2 \)
(c) \( [Xe] 4f^8 6d^2 \)
(d) \( [Xe] 4f^9 5s^1 \)
Answer: (a) \( [Xe] 4f^7 5d^1 6s^2 \)
Explanation: \( _{64}Gd \) : \( [Xe] 4f^7 5d^1 6s^2 \)
Rest of the configurations violates the Aufbau principle to fill electrons in the orbitals.

Question. Interstitial compounds are formed when small atoms are trapped inside the crystal lattice of metals. Which of the following is not the characteristic property of interstitial compounds?
(a) They have high melting points in comparison to pure metals.
(b) They are very hard.
(c) They retain metallic conductivity.
(d) They are chemically very reactive.
Answer: (d) They are chemically very reactive.
Explanation: The non-stoichiometric interstitial compounds have voids present in their crystal lattice. That is occupied by small atoms and hence remain chemically inert.

Question. Which of the following statements is not correct?
(a) La is actually transition element.
(b) In Lanthanide series, ionic radii decrease from \( La^{3+} \) to \( Lu^{3+} \).
(c) \( La(OH)_3 \) is less basic than \( Lu(OH)_3 \).
(d) Ionic radii of Zr and Hf are almost similar due to lanthanoid contraction.
Answer: (c) \( La(OH)_3 \) is less basic than \( Lu(OH)_3 \).
Explanation: \( La(OH)_3 \) is more basic than \( Lu(OH)_3 \). Due to lanthanoid contraction the size of lanthanoid ion decreases regularly with increase in atomic size. Thus, ionic character between lanthanoid ion and \( OH^- \) decreases from \( La^{3+} \) to \( Lu^{3+} \). Thus the basic character of hydroxides decreases from \( La(OH)_3 \) to \( Lu(OH)_3 \).

Question. Lanthanoid contraction is caused due to:
(a) Atomic number
(b) Size of 4f orbitals
(c) Effective nuclear charge
(d) Poor shielding effect of 4f electrons
Answer: (d) Poor shielding effect of 4f electrons
Explanation: The lanthanoid contraction is due to poor shielding effect of 4f electrons. Thus, atomic radii decreases as the atomic number increases.

Question. In which of the following elements, 5f orbitals are progressively filled?
(a) Alkaline earth metals
(b) Actinoids
(c) Lanthanoids
(d) Transition elements
Answer: (b) Actinoids
Explanation: Actinoids are elements of 5f block, so in actinoids, 5f orbitals are progressively filled.

Assertion and Reason Based MCQs

Question. Assertion (A): \( Cu^{2+} \) iodide is not known.
Reason (R): \( Cu^{2+} \) oxidises \( I^- \) to iodine.
(a) Both (A) and (R) are true, and (R) is the correct explanation of (A).
(b) Both (A) and (R) are true, but (R) is not the correct explanation of (A).
(c) (A) is true, but (R) is false.
(d) (A) is false, but (R) is true.
Answer: (a) Both (A) and (R) are true, and (R) is the correct explanation of (A).

Question. Assertion (A): The highest oxidation state of osmium is +8.
Reason (R): Osmium is a 5d-block element.
(a) Both (A) and (R) are true, and (R) is the correct explanation of (A).
(b) Both (A) and (R) are true, but (R) is not the correct explanation of (A).
(c) (A) is true, but (R) is false.
(d) (A) is false, but (R) is true.
Answer: (b) Both (A) and (R) are true, but (R) is not the correct explanation of (A).

Question. Assertion (A): Separation of Zr and Hf is difficult.
Reason (R): Zr and Hf lie in the same group of the periodic table.
(a) Both (A) and (R) are true, and (R) is the correct explanation of (A).
(b) Both (A) and (R) are true, but (R) is not the correct explanation of (A).
(c) (A) is true, but (R) is false.
(d) (A) is false, but (R) is true.
Answer: (b) Both (A) and (R) are true, but (R) is not the correct explanation of (A).

Question. Assertion (A): Cu cannot liberate hydrogen from acids.
Reason (R): Cu has positive electrode potential.
(a) Both (A) and (R) are true, and (R) is the correct explanation of (A).
(b) Both (A) and (R) are true, but (R) is not the correct explanation of (A).
(c) (A) is true, but (R) is false.
(d) (A) is false, but (R) is true.
Answer: (a) Both (A) and (R) are true, and (R) is the correct explanation of (A).

Question. Assertion (A): Transition metals have low melting points.
Reason (R): The involvement of greater number of \( (n - 1)d \) and \( ns \) electrons in the interatomic metallic bonding.
(a) Both (A) and (R) are true, and (R) is the correct explanation of (A).
(b) Both (A) and (R) are true, but (R) is not the correct explanation of (A).
(c) (A) is true, but (R) is false.
(d) (A) is false, but (R) is true.
Answer: (d) (A) is false, but (R) is true.

Question. Assertion (A): Chromium is an actinoid.
Reason (R): In chromium, 3d orbitals are filled.
(a) Both (A) and (R) are true, and (R) is the correct explanation of (A).
(b) Both (A) and (R) are true, but (R) is not the correct explanation of (A).
(c) (A) is true, but (R) is false.
(d) (A) is false, but (R) is true.
Answer: (d) (A) is false, but (R) is true.

Question. Assertion (A): Cerium (Ce) exhibits +4 oxidation state.
Reason (R): \( Ce^{4+} \) has \( 4f^4 \) electronic configuration which is less stable.
(a) Both (A) and (R) are true, and (R) is the correct explanation of (A).
(b) Both (A) and (R) are true, but (R) is not the correct explanation of (A).
(c) (A) is true, but (R) is false.
(d) (A) is false, but (R) is true.
Answer: (c) (A) is true, but (R) is false.

Question. Assertion (A): Lanthanoids have poor tendency to form complexes.
Reason (R): Lanthanoids has low density.
(a) Both (A) and (R) are true, and (R) is the correct explanation of (A).
(b) Both (A) and (R) are true, but (R) is not the correct explanation of (A).
(c) (A) is true, but (R) is false.
(d) (A) is false, but (R) is true.
Answer: (a) Both (A) and (R) are true, and (R) is the correct explanation of (A).
Explanation: Lanthanoids have low density due to large size and high charges. Thus, lanthanoids have poor tendency for making complexes.

Case-based MCQs

I. Read the passage given below and answer the following questions:
Within the 3d series, manganese exhibits oxidation states in aqueous solution from +2 to +7, ranging from \( Mn^{2+}(aq) \) to \( MnO_4^-(aq) \). Likewise, iron forms both \( Fe^{2+}(aq) \) and \( Fe^{3+}(aq) \) as well as the \( FeO_4^{2-} \) ion. Cr and Mn form oxoanions \( CrO_4^{2-} \), \( MnO_4^- \), owing to their willingness to form multiple bonds. The pattern with the early transition metals—in the 3d series up to Mn, and for the 4d, 5d metals upto Ru and Os—is that the maximum oxidation state corresponds to the number of ‘‘outer shell’’ electrons. The highest oxidation states of the 3d metals may depend upon complex formation (e.g., the stabilisation of \( Co^{3+} \) by ammonia) or upon the pH (thus \( MnO_4^{2-}(aq) \) is prone to disproportionation in acidic solution). Within the 3d series, there is considerable variation in relative stability of oxidation states, sometimes on moving from one metal to a neighbour; thus, for iron, \( Fe^{3+} \) is more stable than \( Fe^{2+} \), especially in alkaline conditions, while the reverse is true for cobalt. The ability of transition metals to exhibit a wide range of oxidation states is marked with metals such as vanadium, where the standard potentials can be rather small, making a switch between states relatively easy.
In the following questions, a statement of assertion followed by a statement of reason is given. Choose the correct answer out of the following choices on the basis of the above passage.

Question. Assertion (A): Highest oxidation state is exhibited by transition metals lying in the middle of the series.
Reason (R): The highest oxidation state exhibited corresponds to number of \( (n−1)d \) electrons.
(a) Assertion and reason both are correct statements and reason is correct explanation for assertion.
(b) Assertion and reason both are correct statements but reason is not correct explanation for assertion.
(c) Assertion is correct statement but reason is wrong statement.
(d) Assertion is wrong statement but reason is correct statement.
Answer: (c) Assertion is correct statement but reason is wrong statement.
Explanation: Higher oxidation states are usually exhibited by the transition metals lying in the middle of the series because of higher number of unpaired electrons in \( (n - 1)d \) and \( ns \) orbitals at the middle of the series.

Question. Assertion (A): \( Fe^{3+} \) is more stable than \( Fe^{2+} \).
Reason (R): \( Fe^{3+} \) has \( 3d^5 \) configuration while \( Fe^{2+} \) has \( 3d^6 \) configuration.
(a) Assertion and reason both are correct statements and reason is correct explanation for assertion.
(b) Assertion and reason both are correct statements but reason is not correct explanation for assertion.
(c) Assertion is correct statement but reason is wrong statement.
(d) Assertion is wrong statement but reason is correct statement.
Answer: (a) Assertion and reason both are correct statements and reason is correct explanation for assertion.
Explanation: \( Fe^{3+} \) is more stable than \( Fe^{2+} \) because of the presence of half-filled d-orbital (\( 3d^5 \)) in \( Fe^{3+} \) ion.

Question. Assertion (A): Vanadium had the ability to exhibit a wide range of oxidation states.
Reason (R): The standard potentials of vanadium are rather small, making a switch between oxidation states relatively easy.
(a) Assertion and reason both are correct statements and reason is correct explanation for assertion.
(b) Assertion and reason both are correct statements but reason is not correct explanation for assertion.
(c) Assertion is correct statement but reason is wrong statement.
(d) Assertion is wrong statement but reason is correct statement.
Answer: (a) Assertion and reason both are correct statements and reason is correct explanation for assertion.

Question. Assertion (A): Transition metals like Fe, Cr and Mn form oxoanions.
Reason (R): Oxygen is highly electronegative and has a tendency to form multiple bonds.
(a) Assertion and reason both are correct statements and reason is correct explanation for assertion.
(b) Assertion and reason both are correct statements but reason is not correct explanation for assertion.
(c) Assertion is correct statement but reason is wrong statement.
(d) Assertion is wrong statement but reason is correct statement.
Answer: (b) Assertion and reason both are correct statements but reason is not correct explanation for assertion.
Explanation: Transition metals like Fe, Cr and Mn form oxoions because oxygen is strong oxidising agent due to its high electronegativity and small size Due to these properties oxygen is able to oxidise the metal to its highest oxidation state.

Question. Assertion (A): The highest oxidation states of the 3d metals depends only on electronic configuration of the metal.
Reason (R): The number of electrons in the (n-1) d and ns subshells determine the oxidation states exhibited by the metal.
(a) Assertion and reason both are correct statements and reason is correct explanation for assertion.
(b) Assertion and reason both are correct statements but reason is not correct explanation for assertion.
(c) Assertion is correct statement but reason is wrong statement.
(d) Assertion is wrong statement but reason is correct statement.
Answer: (d) Assertion is wrong statement but reason is correct statement.

II. Read the passage given below and answer the following questions:
The transition metals when exposed to oxygen at low and intermediate temperatures form thin, protective oxide films of upto some thousands of Angstroms in thickness. Transition metal oxides lie between the extremes of ionic and covalent binary compounds formed by elements from the left or right side of the periodic table. They range from metallic to semiconducting and deviate by both large and small degrees from stoichiometry. Since d-electron bonding levels are involved, the cations-exist in various valence states and hence give rise to a large number of oxides. The crystal structures are often classified by considering a cubic or hexagonal close-packed lattice of one set of ions with the other set of ions filling the octahedral or tetrahedral interstices. The actual oxide structures, however, generally show departures from such regular arrays due in part to distortions caused by packing of ions of different size and to ligand field effects. These distortions depend not only on the number of d-electrons but also on the valence and the position of the transition metal in a period or group.
In the following questions, a statement of assertion followed by a statement of reason is given. Choose the correct answer out of the following choices on the basis of the above passage.

Question. Assertion (A): Cations of transition elements occur in various valence states.
Reason (R): Large number of oxides of transition elements are possible.
(a) Assertion and reason both are correct statements and reason is correct explanation for assertion.
(b) Assertion and reason both are correct statements but reason is not correct explanation for assertion.
(c) Assertion is correct statement but reason is wrong statement.
(d) Assertion is wrong statement but reason is correct statement.
Answer: (b) Assertion and reason both are correct statements but reason is not correct explanation for assertion.
Explanation: Transition element can occur in variable oxidation states due to maximum number of electrons are present in \( (n – 1)d \) and \( ns \) sub-shell that determines oxidation state. Oxides of transition metals stabilises highest oxidation states, from \( Se_2O_3 \) to \( Mn_2O_7 \) and no above \( Fe_2O_3 \), higher oxides are known.

Question. Assertion (A): Crystal structure of oxides of transition metals often show defects.
Reason (R): Ligand field effect cause distortions in crystal structures.
(a) Assertion and reason both are correct statements and reason is correct explanation for assertion.
(b) Assertion and reason both are correct statements but reason is not correct explanation for assertion.
(c) Assertion is correct statement but reason is wrong statement.
(d) Assertion is wrong statement but reason is correct statement.
Answer: (a) Assertion and reason both are correct statements and reason is correct explanation for assertion.
Explanation: The crystal field theory explains the model for interaction between transition metals and ligands. It causes attraction between charges of metal ion with lands by degeneracy of orbital states, producing electric field by charge distribution. It affects different properties like colour, distorted structures, magnetic properties etc.

Question. Assertion (A): Transition metals form protective oxide films.
Reason (R): Oxides of transition metals are always stoichiometric.
(a) Assertion and reason both are correct statements and reason is correct explanation for assertion.
(b) Assertion and reason both are correct statements but reason is not correct explanation for assertion.
(c) Assertion is correct statement but reason is wrong statement.
(d) Assertion is wrong statement but reason is correct statement.
Answer: (c) Assertion is correct statement but reason is wrong statement.
Explanation: Transition metals like Zn, Cr are protected by formation of oxide film to present from corrosion. These oxides of transition metals are likely to form non-stoichiometry due to variable oxidation states and their defects in the lattices.

Question. Assertion (A): CrO crystallises in a hexagonal close-packed array of oxide ions with two out of every three octahedral holes occupied by chromium ions.
Reason (R): Transition metal oxide may be hexagonal close-packed lattice of oxide ions with metal ions filling the octahedral voids.
(a) Assertion and reason both are correct statements and reason is correct explanation for assertion.
(b) Assertion and reason both are correct statements but reason is not correct explanation for assertion.
(c) Assertion is correct statement but reason is wrong statement.
(d) Assertion is wrong statement but reason is correct statement.
Answer: (d) Assertion is wrong statement but reason is correct statement.

III. Read the passage given below and answer the following questions:
The d block elements are the 40 elements contained in the four rows of ten columns (3-12) in the periodic table. As all the d block elements are metallic, the term d-block metals is synonymous. This set of d-block elements is also often identified as the transition metals, but sometimes the group 12 elements (zinc, cadmium, mercury) are excluded from the transition metals as the transition elements are defined as those with partly filled d or f shells in their compounds. Inclusion of the elements zinc, cadmium and mercury is necessary as some properties of the group 12 elements are appropriate logically to include with a discussion of transition metal chemistry. The term transition element or transition metal appeared to derive from early studies of periodicity such as the Mendeleev periodic table of the elements. His horizontal table of the elements was an attempt to group the elements together so that the chemistry of elements might be explained and predicted. In this table there are eight groups labeled I-VIII with each subdivided into A and B subgroups. Mendeleev recognized that certain properties of elements in Group VIII are related to those of some of the elements in Group VII and those at the start of the next row Group I. In that sense, these elements might be described as possessing properties transitional from one row of the table to the next.
In the following questions, a statement of assertion followed by a statement of reason is given. Choose the correct answer out of the following choices on the basis of the above passage.

Question. Assertion (A): Group 12 elements are not considered as transition metals.
Reason (R): Transition metals are those which have incompletely filled d sub-shell in their compounds.
(a) Assertion and reason both are correct statements and reason is correct explanation for assertion.
(b) Assertion and reason both are correct statements but reason is not correct explanation for assertion.
(c) Assertion is correct statement but reason is wrong statement.
(d) Assertion is wrong statement but reason is correct statement.
Answer: (a) Assertion and reason both are correct statements and reason is correct explanation for assertion.
Explanation: Elements of Group 12 (Zn, Cd, Hg) are not considered an transition metals because of their fullyfilled d-orbitals [\( (n –1)^{10}ns^2 \)], whereas transition metals are those metals whose d-orbitals are partially filled.

Question. Assertion (A): All d-block elements are metallic in nature.
Reason (R): The d-block elements belong to Group 3 -12 of the periodic table.
(a) Assertion and reason both are correct statements and reason is correct explanation for assertion.
(b) Assertion and reason both are correct statements but reason is not correct explanation for assertion.
(c) Assertion is correct statement but reason is wrong statement.
(d) Assertion is wrong statement but reason is correct statement.
Answer: (b) Assertion and reason both are correct statements but reason is not correct explanation for assertion.
Explanation: Group 3 to Group 12 are the d-block elements, out of which all are considered as transition metals except Group 12 elements. Transition metals show metallic properties due to low ionization energies and their vacant d-orbitals in the valenced shell.

Question. Assertion (A): Group VII elements of Mendeleev periodic table are transition elements.
Reason (R): Group I –VIII in Mendeleev periodic table is divided into two subgroups, A and B.
(a) Assertion and reason both are correct statements and reason is correct explanation for assertion.
(b) Assertion and reason both are correct statements but reason is not correct explanation for assertion.
(c) Assertion is correct statement but reason is wrong statement.
(d) Assertion is wrong statement but reason is correct statement.
Answer: (d) Assertion is wrong statement but reason is correct statement.
Explanation: Mendeleev periodic table is divided into two subgroups, A & B that possess, same physical property and chemical properties. In subgroup B of Group III to VII defines for transition elements.

Question. Assertion (A): Nickel is a transition element that belongs to group 10 and period 4 of the modern periodic table.
Reason (R): Electronic configuration of Nickel is \( [Ar]3d^8 4s^2 \).
(a) Assertion and reason both are correct statements and reason is correct explanation for assertion.
(b) Assertion and reason both are correct statements but reason is not correct explanation for assertion.
(c) Assertion is correct statement but reason is wrong statement.
(d) Assertion is wrong statement but reason is correct statement.
Answer: (a) Assertion and reason both are correct statements and reason is correct explanation for assertion.

IV. Read the passage given below and answer the following questions:
In transition elements, generally, 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. But the radii of the third (5d) series are virtually the same as those of the corresponding members of the second series. This phenomenon is associated with the intervention of the 4f orbitals which must be filled before the 5d series of elements begin. The filling of 4f before 5d orbital results in a regular decrease in atomic radii called Lanthanoid contraction.
In these questions, a statement of assertion followed by a statement of reason is given. Choose the correct answer out of the following choices.
Following are the transition metal ions of 3d series:
\( Ti^{4+}, V^{2+}, Mn^{3+}, Cr^{3+} \)
(Atomic number: Ti = 22, V = 23, Mn = 25, Cr = 24)

Question. Assertion (A): Among the given ions, \( Cr^{3+} \) is the most stable in an aqueous environment.
Reason (R): \( Cr^{3+} \) has half filled \( t_{2g}^3 \).
(a) Assertion and reason both are correct statements and reason is correct explanation for assertion.
(b) Assertion and reason both are correct statements but reason is not correct explanation for assertion.
(c) Assertion is correct statement but reason is wrong statement.
(d) Assertion is wrong statement but reason is correct statement.
Answer: (a) Assertion and reason both are correct statements and reason is correct explanation for assertion.
Explanation: Among the given ions, \( Cr^{3+} \) is the most stable in an aqueous environment because of its outermost electronic configuration as \( 3d^3 4s^0 t_{2g}^3 e_g^0 \) i.e., half-filled \( t_{2g} \) orbital.

Question. Assertion (A): Among the given ions, \( Mn^{3+} \) is the most strong oxidising agent.
Reason (R): \( Mn^{3+} \) has an unstable configuration.
(a) Assertion and reason both are correct statements and reason is correct explanation for assertion.
(b) Assertion and reason both are correct statements but reason is not correct explanation for assertion.
(c) Assertion is correct statement but reason is wrong statement.
(d) Assertion is wrong statement but reason is correct statement.
Answer: (c) Assertion is correct statement but reason is wrong statement.
Explanation: \( Mn^{3+} \) is the strong oxidising agent because it has 4 electrons in its valence shell and when it gains one electron then it forms \( Mn^{2+} \), it results in the half-filled (\( d^5 \)) configuration that provides extra stability.

Question. Assertion (A): \( Ti^{4+} \) ion is colourless.
Reason (R): All valence electrons are unpaired in \( Ti^{4+} \) ion.
(a) Assertion and reason both are correct statements and reason is correct explanation for assertion.
(b) Assertion and reason both are correct statements but reason is not correct explanation for assertion.
(c) Assertion is correct statement but reason is wrong statement.
(d) Assertion is wrong statement but reason is correct statement.
Answer: (c) Assertion is correct statement but reason is wrong statement.
Explanation: Electronic configuration of \( Ti^{4+} \) is \( [Ar]3d^0 4s^0 \) shows no unpaired electron in d-orbitals. Thus, no electron will excite to higher energy level to exhibit complementary colours on reaching to ground energy level.

Question. Assertion (A): There is irregular trend in ionization energy in the group of lanthanoid series.
Reason (R): Ionization energy in lanthanoid series vary due to lanthanoid contraction.
(a) Assertion and reason both are correct statements and reason is correct explanation for assertion.
(b) Assertion and reason both are correct statements but reason is not correct explanation for assertion.
(c) Assertion is correct statement but reason is wrong statement.
(d) Assertion is wrong statement but reason is correct statement.
Answer: (a) Assertion and reason both are correct statements and reason is correct explanation for assertion.
Explanation: Due to steady decrease in the size of lanthanide ions with increase in atomic number is called lanthanide contraction. It is caused by poor shielding effect of 4f electrons. Thus, due to decrease in radii, ionization energy varies with stability of half filled d-subhell.