JEE Main · 2026 · 22 Jan 2026 · Shift 1 · p-Block Elements

A 'p'-block element (E) and hydrogen form a binary cation

(EH_{x})^{+}

, while

EH_{3}

on treatment with

K_{2} HgI_{4}

in alkaline medium gives a precipitate of basic mercury(II)amido-iodine. Given below are first ionisation enthalpy values (

kJ mol^{- 1}

) for first element each from group 13, 14, 15 and 16. Identify the correct first ionisation enthalpy value for element E .

(A)1312
(B)1086
(C)1402✓ Correct
(D)801

Answer: (C)

Solution

The hydride that gives a binary cation

E H_{4}^{+}

and whose

E H_{3}

forms the brown basic mercury(II) amido-iodide with alkaline

K_{2} Hg I_{4}

(Nessler's reagent) is ammonia; the cation is the ammonium ion and element E is nitrogen. The listed first ionisation enthalpies of the first members of groups 13 to 16 are boron 801, carbon 1086, oxygen 1314 and nitrogen 1402. Nitrogen has the highest value because its half-filled

2 p^{3}

configuration is especially stable, so an electron is removed least easily. The first ionisation enthalpy of E is

1402 kJ mo l^{- 1}

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JEE Main · 2026 · 22 Jan 2026 · Shift 1 · p-Block Elements

Solution

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