Question

(i) With reference to structural variability, chemical reactivity, write the differences between lanthanoids and actinoids. 

(ii) Name a member of the lanthanoid series which is well known to exhibit + 4 oxidation state. 

(iii) Complete the following equation: 

MnO₄^- + 8H⁺ + 5e^- → 

(iv) Out of Mn³⁺ and Cr³⁺, which is more paramag- netic and why? (Atomic number of Mn = 25, Cr = 24) 

Answer 1

(i) Differences between lanthanoids and actinoids 

Structural variability

Lanthanoids	Actinoids
There is small regular decrease in atomic and ionic radii of lanthanoids, known as lanthanoid contraction.	There is also regular decrease in atomic and ionic radii of actinoids, known as actinoid contraction, but this contraction vary from element to element due to poor shielding effect of 5f electrons. They show great structural variability.

Chemical reactivity

Lanthanoids	Actinoids
They have less tendency towards complex formation. Except promethium, they are nonradioactive. They do not form oxocations. Their oxides and hydroxides are less basic.	They have stronger tendency towards complex formation. They are radioactive. 'They form oxocations Their oxides and hydroxides are more basic.

(ii) Cerium (Ce) is well known to exhibit + 4 oxidation state. 

(iii) MnO₄^- + 8H⁺ + 5e^-  → Mn²⁺ + 4H₂O 

(iv) Outer shell electronic configuration of 

Mn = 3d4s² so, Mn³⁺ = 3d⁴