The number of valence electrons present in the metal among Cr$\mathrm{Cr}$, Co$\mathrm{Co}$, Fe$\mathrm{Fe}$ and Ni$\mathrm{Ni}$ which has the lowest enthalpy of atomisation is:

Core Logic

In an acidic medium, both K₂Cr₂O₇$\mathrm{K}_2\mathrm{Cr}_2\mathrm{O}_7$ and KMnO₄$\mathrm{KMnO}_4$ act as strong oxidizing agents and carry out the following transformations:

• A: Oxidize iodide to iodine: I^- arrow I₂$\mathrm{I}^- \rightarrow \mathrm{I}_2$
• B: Oxidize sulfide to elemental sulfur: S²⁻ arrow S$\mathrm{S}^{2-} \rightarrow \mathrm{S}$
• C: Oxidize ferrous ions to ferric ions: Fe²⁺ arrow Fe³⁺$\mathrm{Fe}^{2+} \rightarrow \mathrm{Fe}^{3+}$

For reactions D and E:

• Iodide is oxidized to iodate (IO₃^-$\mathrm{IO}_3^-$) by KMnO₄$\mathrm{KMnO}_4$ primarily in a neutral or faintly alkaline medium, not acidic.
• Thiosulfate (S₂O₃²⁻$\mathrm{S}_2\mathrm{O}_3^{2-}$) undergoes a disproportionation/precipitation reaction in acid rather than clean oxidation to sulfate by dichromate.

Thus, statements A, B, and C are valid for both under acidic conditions.

Pattern Recognition

Sees: Shared oxidation products in an acidic environment. Shortcut: Remember that I^- arrow IO₃^-$\mathrm{I}^- \rightarrow \mathrm{IO}_3^-$ is the signature reaction for alkaline permanganate, allowing quick elimination of choices containing D.

Chapter Mix

Class 12 Chemistry: The d-and f-Block Elements

Core Logic

Let us identify the sequential chemical components via the chromyl chloride test pathway:

• Reactant A represents a dichromate salt such as K₂Cr₂O₇$\text{K}_2\text{Cr}_2\text{O}_7$. Heating it with a metal chloride and concentrated sulfuric acid generates deep red chromyl chloride vapors (CrO₂Cl₂$\text{CrO}_2\text{Cl}_2$).
• Passing these vapors into sodium hydroxide dissolves them, producing yellow sodium chromate compound B (Na₂CrO₄$\text{Na}_2\text{CrO}_4$).
• Acidifying the chromate solution dimerizes it into orange sodium dichromate compound C (Na₂Cr₂O₇$\text{Na}_2\text{Cr}_2\text{O}_7$).

Step 1: Structural Analysis of Dichromate

The dichromate ion (Cr₂O₇²⁻$\text{Cr}_2\text{O}_7^{2-}$) consists of two tetrahedral chromium units sharing a single bridging oxygen atom (Cr-O-Cr$\text{Cr}-\text{O}-\text{Cr}$). Each chromium atom retains 3 localized terminal oxygen atoms:

Thus, the total count of terminal oxygen atoms present in compound C$C$ is 6.

Pattern Recognition

Shortcut: The chromyl chloride sequence moves from dichromate arrow$\rightarrow$ chromate arrow$\rightarrow$ dichromate. In the dichromate ion (Cr₂O₇²⁻$\text{Cr}_2\text{O}_7^{2-}$), out of the 7 oxygen atoms, exactly 1 is bridging, leaving 7 - 1 = 6$7 - 1 = 6$ terminal oxygen atoms.

Evaluation Rubric / Model Answer

6

Chapter Mix

Class 12 Chemistry: d- and f-Block Elements

Related Formula

Core Logic

Let's examine the configurations:

• For Mn²⁺$Mn^{2+}$, the electronic configuration is [Ar]3d⁵$[Ar]3d^5$, which features a highly stable, symmetric half-filled d-subshell.
• For Fe²⁺$Fe^{2+}$, the configuration is [Ar]3d⁶$[Ar]3d^6$.

Because of the extra exchange energy and stability of the half-filled 3d⁵$3d^5$ state, it is harder to remove an electron from Mn²⁺$Mn^{2+}$ than from Fe²⁺$Fe^{2+}$. Therefore, the ionisation enthalpy of Mn²⁺$Mn^{2+}$ is greater than that of Fe²⁺$Fe^{2+}$:

Hence, the expression Mn²⁺ < Fe²⁺$Mn^{2+} < Fe^{2+}$ is incorrect.

Pattern Recognition

Whenever you see manganese (Mn$Mn$) in the +2$+2$ oxidation state, remember its exceptionally stable d⁵$d^5$ config. This creates anomalous spikes in successive ionisation energies compared to neighboring iron (Fe$Fe$).

Chapter Mix

Class 12 Chemistry: The d and f Block Elements

Core Logic

Let's map the electronic configurations and count the unpaired d-orbital electrons for each option:

• For pair (1):

Both ions contain exactly 3 unpaired electrons.

• For other ions:

Pattern Recognition

D-orbital counts follow a predictable symmetry: a 3dⁿ$3d^n$ system contains the same number of unpaired electrons as a 3d¹⁰⁻ⁿ$3d^{10-n}$ system under high-spin conditions. This explains why 3d³$3d^3$ (V²⁺$V^{2+}$) and 3d⁷$3d^7$ (Co²⁺$Co^{2+}$) match perfectly with 3 unpaired electrons each.

Chapter Mix

Class 12 Chemistry: The d- and f-Block Elements

Core Logic

Let's resolve both components step by step:

• Finding X: In an acidic medium, the permanganate ion (KMnO₄$KMnO_4$, where Mn$Mn$ is in the +7$+7$ state) is reduced to the divalent manganese cation (Mn²⁺$Mn^{2+}$, state +2$+2$):

X = 7 - 2 = 5$X = 7 - 2 = 5$

• Finding Y: During qualitative salt analysis, the acetate ion reacts with neutral ferric chloride to produce a characteristic blood-red coordination solution. Boiling this solution throws down a brown-red precipitate of basic ferric acetate, [Fe(OH)₂(CH₃COO)]$[Fe(OH)_2(CH_3COO)]$. In this complex, Iron retains its +3$+3$ oxidation state:

Summing the values yields:

Pattern Recognition

This problem elegantly links standard redox transitions with qualitative inorganic salt tests. Remember that throughout the basic ferric acetate precipitation test, Iron remains steadily in its ferric +3$+3$ (d⁵$d^5$) core configuration.

Chapter Mix

Class 12 Chemistry: The d- and f-Block Elements Inorganic Qualitative Analysis