Class 12th

59.  (B) and (C) 

Electronic configuration of cerium:

Ce  -  4f¹ 5d¹ 6s²

Ce⁴⁺?  4f⁰ 5d⁰ 6s⁰ 

Lead has the tendency to lose its four outer electrons to attain Noble gas configuration and this removal of electrons corresponds to the f? configuration and cause it to be stable in +4 oxidation state

58.  (B) and (C) 

A compound act as oxidising agent when it's central atom is reduced to its lower oxidation state. This occurs only when the lower oxidation state of metal is more stable than the higher oxidation states. 

In metal WO₃ and CrO₄^2- , both W and Cr are stable in their higher oxidation state and won't get reduced to their lower oxidation state. Therefore, will not act as oxidising agents. 

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.