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.

We need to identify element E and its first ionisation enthalpy.

- E forms a binary cation $$(EH_x)^+$$ with hydrogen → this is $$NH_4^+$$ (ammonium ion), so E = Nitrogen.

- $$EH_3$$ with Nessler's reagent ($$K_2HgI_4$$ in alkaline medium) gives a brown precipitate → this confirms $$EH_3 = NH_3$$ (ammonia), and E = Nitrogen.

The first ionisation enthalpies given are for the first element of groups 13, 14, 15, and 16:

- B (Group 13): 801 kJ/mol

- C (Group 14): 1086 kJ/mol

- N (Group 15): 1402 kJ/mol

- O (Group 16): 1312 kJ/mol (note: N has higher IE than O due to half-filled 2p stability)

Nitrogen's first ionisation enthalpy = 1402 kJ/mol.

The correct answer is Option (4): 1402.