Class 12th

61. Although Zr and Hf lie in same group but As an effect of lanthanide contraction, zirconium and hafnium have similar radius of 160pm and 159 pm respectively. Due to this similarity in their size, they show similar physical and chemical properties. As a result, Zr and Hf are difficult to separate. Both the statements assertion and reason are correct but reason is not the correct explanation of assertion. 

60. All halogens combine with copper to form copper halides except iodine. The reason behind this is that Cu²⁺  oxidises iodide (-1)  to iodine (0). Therefore, Cu²⁺ iodide does not exist .Both the statements assertion and reason are correct and reason is the correct explanation of assertion. 

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.