Question

Which of the following arrangements does not represent the correct order of the property stated against it ?

(a) V²⁺ < Cr²⁺ < Mn²⁺ < Fe²⁺ : paramagnetic behaviour

(b) Ni²⁺ < Co²⁺ < Fe²⁺ < Mn²⁺ : ionic size

(c) Co³⁺ < Fe³⁺ < Cr³⁺ < Sc³⁺ : stability in aqueous solution 

(d) Sc < Ti < Cr < Mn : number of oxidation states

Answer 1

Correct option: (a) V²⁺ < Cr²⁺ < Mn²⁺ < Fe²⁺ : paramagnetic behaviour

Explanation:

 V = 3d³ 4s ² ; V²⁺ = 3d ³ = 3 unpaired electrons

Cr = 3d⁵ 4s¹ ; Cr²⁺ = 3d ⁴ = 4 unpaired electrons

Mn = 3d⁵ 4s² ; Mn²⁺ = 3d ⁵ = 5 unpaired electrons

Fe = 3d⁶ 4s ² ; Fe²⁺ = 3d ⁶ = 4 unpaired electrons

Hence the correct order of paramagnetic behaviour V²⁺ < Cr2+ = Fe²⁺ < Mn²⁺

b) For the same oxidation state, the ionic radii generally decreases as the atomic number increases in a particular transition series. Hence the order is Mn²⁺ > Fe²⁺ > Co²⁺ > Ni²⁺

(c) In solution, the stability of the compound depends upon electrode potentials, SEP of the transitions metal ions are given as Co³⁺ / Co = + 1.97, Fe³⁺ / Fe = + 0.77 ; Cr³⁺ / Cr²⁺ = – 0.41, Sc ³⁺ is highly stable as it does not show + 2 O. S.