JEE Main 2026 — Practical Chemistry Question with Solution

The borax bead test is given by transition metal ions that have unpaired electrons and form coloured compounds.

$Z{n}^{2+}$ has a completely filled $3d^{10}$ configuration, making it colourless, so it does not give a positive borax bead test.

The remaining ions and their electronic configurations are:
$F{e}^{2+}:[Ar]3d^6$
$C{r}^{2+}:[Ar]3d^4$
$F{e}^{3+}:[Ar]3d^5$

The ionisation enthalpy of an ion refers to the energy required to remove an electron from it. $F{e}^{3+}$ has a highly stable half-filled $d$-subshell configuration ($3d^5$). Removing an electron from this stable configuration requires significantly more energy compared to removing an electron from $F{e}^{2+}$ ($3d^6$) or $C{r}^{2+}$ ($3d^4$).

Therefore, $F{e}^{3+}$ shows a positive borax bead test and has the highest ionisation enthalpy among the given ions.

Answer: $F{e}^{3+}$