Identify the one that does not exist

The correct answer is Option (2) → \(NeF_2\)

The noble gases are the least reactive elements because they have completely filled ns²np⁶ valence shells and very high ionisation enthalpies. Compound formation becomes possible only for the heavier noble gases, whose ionisation enthalpy is lower. Xenon (1170 kJ mol⁻¹) forms several fluorides and oxyfluorides with the most electronegative elements, fluorine and oxygen. Krypton forms very few compounds, and no true compounds of argon, neon or helium are known. Neon is small and its ionisation enthalpy (2080 kJ mol⁻¹) is far too high, so NeF₂ does not exist.

• Option (1) \(XeOF_4\) — Exists. It is formed by partial hydrolysis of \( \ce{XeF6} \):

      \( \ce{XeF6 + H2O -> XeOF4 + 2HF} \)

• Option (2) \(NeF_2\) — Does not exist. Neon is small and its ionisation enthalpy (2080 kJ mol⁻¹) is far too high for it to form a compound, even with fluorine.

• Option (3) \(XeF_2\) — Exists. It is made from xenon (in excess) and fluorine:

      \( \ce{Xe(g) + F2(g)} \xrightarrow{673\ \mathrm{K},\ 1\ \mathrm{bar}} \ce{XeF2(s)} \)

• Option (4) \(XeF_6\) — Exists. It is made with a large excess of fluorine at high pressure:

      \( \ce{Xe(g) + 3F2(g)} \xrightarrow{573\ \mathrm{K},\ 60-70\ \mathrm{bar}} \ce{XeF6(s)} \)

Note: This question is from the pre-rationalised NCERT book (2022-23). The topic is not there in the latest NCERT book (2026-27), and it is also out of the CUET 2026 syllabus.

NCERT (Pre-rationalised edition, 2022-23):
Chemical Properties – …