The d and f Block Elements

57.  (A) and (B) 

Cr and Co  form tri halide compounds.

55.  (B) and (C) 

Ce (Z = 57)  =  [Xe] 4f⁵ 5d⁰ 6s² , O.S.  =   + 3, + 4

Eu (Z = 63)  =  [Xe] 4f⁷ 5d⁰ 6s², O.S.  =   + 2, + 3

Yb (Z = 70)  =  [Xe] 4f¹⁴ 5d⁰ 6s², O.S.  =   + 2, + 3

Ho (Z = 67)  =  [Xe] 4f¹¹ 5d⁰ 6s², O.S.  =   + 3 

54.  (A) and (B)

Electronic configuration:

92U -  5f³ 6d¹ 7s²

93Np -  5f⁴ 6d¹ 7s² 

53.  (B) & (D) 

Actinides show a variety of oxidation states like +3, +5, +6 and +7. 

From the given option, plutonium and neptunium show all oxidation states up to +7

52.  (B) and (C) 

For the heavier metals of d-block, higher oxidation state is more favourably stable than the lower oxidation state. For example: Mo (VI) and W (VI) are more stable than Cr (VI). For lighter d-block elements like chromium, +3 oxidation state is more stable. So, Cr (VI) in dichromate act as a strong oxidising agent by reducing itself from +6 to +3 oxidation states While Mo (VI) and W (VI) does not. 

51.  (A) & (D) 

The magnetic moment of an electron/dipole moment is caused by its intrinsic properties of spin and electric charge. It depends upon the number of unpaired electrons in its valence shell. The more the number of unpaired electrons, the greater will be the value of the magnetic moment.

Co²⁺  and Cr³⁺ have same unpaired electrons i.e. 3 unpaired electrons. Hence they have the same spin only magnetic moment. 

50.  (A) & (B)

Due to charge transfer phenomenon, which can be understood, for an instant, an electron is transferred from oxygen to the central metal atom, where oxidation state of metal is reduced by one and oxygen gets converted from O^- to O^2- . 

This transfer imparts Colour to the compound. 

49.  (B) KMnO₄ oxidises HCl into Cl₂  which is also an oxidizing agent.

HCl is not used in titrations involving KMnO₄  as it produces unsatisfactory results. When HCl  is added to KMnO₄, it itself acts as reducing agent and coverts HCl to Cl₂. 

48.  (C)  both have similar atomic radius

As an effect of lanthanide contraction, zirconium and hafnium have similar radii of 160 pm and 159 pm respectively. Due to this similarity in their size, they show similar physical and chemical properties. 

Lanthanide contraction: Because the elements in Row 3 have 4f electrons. These electrons do not shield well, causing a greater nuclear charge. This greater nuclear charge has a greater pull on the electrons and results in the decrease in their size and atomic radii.