Among $\mathrm{Fe}^{2+}, \mathrm{Fe}^{3+}, \mathrm{Cr}^{2+}$ and $\mathrm{Zn}^{2+}$, the ion that shows positive borax bead test and with highest ionisation enthalpy is:

JEE Main · 2026 · 02 Apr 2026 · Shift 2 · d and f Block Elements

Among

Fe^{2 +}, Fe^{3 +}, Cr^{2 +}

and

Zn^{2 +}

, the ion that shows positive borax bead test and with highest ionisation enthalpy is:

(A) $Fe^{2 +}$
(B) $Zn^{2 +}$
(C) $Cr^{2 +}$
(D) $Fe^{3 +}$ ✓ Correct

Answer: (D)

Solution

The borax bead test gives a characteristic colour only with ions having partly filled

d

orbitals able to form coloured metaborate complexes;

Zn X^{2 +}

(

3 d^{10}

) is colourless and fails the test, while

Fe X^{2 +}

(

3 d^{6}

Fe X^{3 +}

(

3 d^{5}

) and

Cr X^{2 +}

(

3 d^{4}

) all respond. Among the three that respond, the next electron removed is the fourth for iron(III) against the third for the other two, and prising an electron from the stable half-filled

3 d^{5}

core of

Fe X^{3 +}

demands the most energy. The ion that both answers the borax bead test and has the highest ionisation enthalpy is

Fe X^{3 +}

JEE Main · 2026 · 02 Apr 2026 · Shift 2 · Single correct option

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JEE Main · 2026 · 02 Apr 2026 · Shift 2 · d and f Block Elements

Solution

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