Question

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

• A. $\mathrm{Sc}<\mathrm{Ti}<\mathrm{Cr}<\mathrm{Mn}$: Number of oxidation states
• B. ${\mathrm{Ni}}^{2+}<{\mathrm{Co}}^{2+}<{\mathrm{Fe}}^{2+}<{\mathrm{Mn}}^{2+}$: Ionic size
• C. ${\mathrm{V}}^{2+}<{\mathrm{Cr}}^{3+}<{\mathrm{Mn}}^{2+}<{\mathrm{Fe}}^{2+}$: Paramagnetic behavior
• D. ${\mathrm{Co}}^{3+}<{\mathrm{Fe}}^{3+}<{\mathrm{Cr}}^{3+}<{\mathrm{S}}^{3+}$: Stability in aqueous solution

Answer 1

The correct option is C

${\mathrm{V}}^{2+}<{\mathrm{Cr}}^{3+}<{\mathrm{Mn}}^{2+}<{\mathrm{Fe}}^{2+}$: Paramagnetic behavior

Explanation for correct option

(c) ${\mathrm{V}}^{2+}<{\mathrm{Cr}}^{3+}<{\mathrm{Mn}}^{2+}<{\mathrm{Fe}}^{2+}$: Paramagnetic behavior

• When there is large number of unpaired electrons, higher is the paramagnetic property.
• $$\begin{gathered} \mathrm{V}=3{\mathrm{d}}^{2}4{\mathrm{s}}^2,{\mathrm{V}}^{2+}=3{\mathrm{d}}^3=3\mathrm{unpaired}\mathrm{electrons} \\ \mathrm{Cr}=3{\mathrm{d}}^{5}4{\mathrm{s}}^1,{\mathrm{Cr}}^{2+}=3{\mathrm{d}}^4=4\mathrm{unpaired}\mathrm{electrons} \\ \mathrm{Mn}=3{\mathrm{d}}^{5}4{\mathrm{s}}^2,{\mathrm{Mn}}^{2+}=3{\mathrm{d}}^5=5\mathrm{unpaired}\mathrm{electrons} \\ \mathrm{Fe}=3{\mathrm{d}}^{6}4{\mathrm{s}}^2,{\mathrm{Fe}}^{2+}=3{\mathrm{d}}^6=4\mathrm{unpaired}\mathrm{electrons} \end{gathered}$$
• Hence, the correct order of parametric behavior is

${\mathrm{V}}^{2+}<{\mathrm{Cr}}^{2+}\simeq {\mathrm{Fe}}^{2+}<{\mathrm{Mn}}^{2+}$

• Hence, option (c) ${\mathrm{V}}^{2+}<{\mathrm{Cr}}^{3+}<{\mathrm{Mn}}^{2+}<{\mathrm{Fe}}^{2+}$: Paramagnetic behavior is incorrect.

Explanation for incorrect options

(a) $\mathrm{Sc}<\mathrm{Ti}<\mathrm{Cr}<\mathrm{Mn}$: Number of oxidation states

• $$\begin{gathered} \mathrm{Sc}-(+2)(+3)\mathrm{has}\mathrm{two}\mathrm{oxidation}\mathrm{states}. \\ \mathrm{Ti}-(+2),(+3),(+4)\mathrm{has}\mathrm{three}\mathrm{oxidation}\mathrm{states}. \\ \mathrm{Cr}-(+2),(+3),(+4),(+5),(+6)\mathrm{has}\mathrm{five}\mathrm{oxidation}\mathrm{states}. \\ \mathrm{Mn}-(+2),(+3),(+4),(+5),(+6),(+7)\mathrm{has}\mathrm{six}\mathrm{oxidation}\mathrm{states}. \end{gathered}$$
• The increasing order for the number of oxidation states is $\mathrm{Sc}<\mathrm{Ti}<\mathrm{Cr}<\mathrm{Mn}.$
• Hence option (a) $\mathrm{Sc}<\mathrm{Ti}<\mathrm{Cr}<\mathrm{Mn}$: Number of oxidation states is correct.

(b) ${\mathrm{Ni}}^{2+}<{\mathrm{Co}}^{2+}<{\mathrm{Fe}}^{2+}<{\mathrm{Mn}}^{2+}$: Ionic size

• Since oxidation state of all the given elements is same, the ionic radii usually decreases.
• This is so because atomic number increases in a particular transition series.
• Therefore, the order will be

${\mathrm{Ni}}^{2+}<{\mathrm{Co}}^{2+}<{\mathrm{Fe}}^{2+}<{\mathrm{Mn}}^{2+}$

• Hence option (b) ${\mathrm{Ni}}^{2+}<{\mathrm{Co}}^{2+}<{\mathrm{Fe}}^{2+}<{\mathrm{Mn}}^{2+}$: Ionic size is correct.

(d) ${\mathrm{Co}}^{3+}<{\mathrm{Fe}}^{3+}<{\mathrm{Cr}}^{3+}<{\mathrm{S}}^{3+}$: Stability in aqueous solution

• The stability of the compound depends upon the electrode potentials.
• Standard electrode potential of the given transition metals are as follows

$$\begin{gathered} {\mathrm{Co}}^{3+}/\mathrm{Co}=+1.97 \\ {\mathrm{Fe}}^{3+}=+0.77 \\ {\mathrm{Cr}}^{3+}/{\mathrm{Cr}}^{2+}=-0.41 \\ {\mathrm{Sc}}^{3+}\mathrm{is}\mathrm{highly}\mathrm{stable}\mathrm{as}\mathrm{it}\mathrm{does}\mathrm{not}\mathrm{show}+2\mathrm{O}.\mathrm{S}. \end{gathered}$$

• When the standard electrode potential is more positive, the ion in the aqueous solution is less stable.
• Therefore, the increasing order of stability in aqueous solution is ${\mathrm{Co}}^{3+}<{\mathrm{Fe}}^{3+}<{\mathrm{Cr}}^{3+}<{\mathrm{S}}^{3+}$.
• Hence option (d) ${\mathrm{Co}}^{3+}<{\mathrm{Fe}}^{3+}<{\mathrm{Cr}}^{3+}<{\mathrm{S}}^{3+}$: Stability in aqueous solution is correct.

Hence the correct option which does not represent the correct order of property is (c) ${\mathrm{V}}^{2+}<{\mathrm{Cr}}^{3+}<{\mathrm{Mn}}^{2+}<{\mathrm{Fe}}^{2+}$: Paramagnetic behavior.