Question is based on following passage:
p-block elements are placed in groups 13 to 18 of the periodic table. Group 15 is nitrogen family. Nitrogen makes various compounds like \(NH_3\), \(HNO_3\), \(N_2O\), \(NO_2\) etc. Phosphorus exists in the form of white and red phosphorus. White phosphorus glows in dark. \(PH_3\), gas is used as smoke screen. Group 16 is called oxygen family. Dry air contains 21% \(O_2\) gas by volume. Combined sulphur exists as sulphates such as gypsum, Epsom salt, baryte etc. The allotropic forms of sulphur are Rhombic and monoclinic sulphur. \(S_2\) is paramagnetic like \(O_2\) and it is stable at high temperature. Group 17 is Halogen family. \(F_2\), is highly reactive, \(Cl_2\), is greenish yellow gas. Both \(SO_2\), and \(Cl_2\), are used for bleaching. Group 18 is Noble gas family but some noble gases also form compounds. Complete hydrolysis of \(XeF_6\) gives \(HF\) and \(XeO_3\), as main products.
At elevated temperature (∼ 1000K) the dominant form of sulphur is:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → \(S_2\)
Several forms of sulphur exist; rhombic and monoclinic sulphur are both made of puckered \( \ce{S8} \) rings. In the vapour, at elevated temperatures (~1000 K), the \( \ce{S8} \) rings break up and S2 is the dominant species. Like \( \ce{O2} \), \( \ce{S2} \) has two unpaired electrons in antibonding π* orbitals and is paramagnetic.
Why the other options are incorrect
Option (1) Rhombic Sulphur — Incorrect. Rhombic (α) sulphur is the stable form only below 369 K.
Option (2) Monoclinic Sulphur — Incorrect. Monoclinic (β) sulphur is stable from 369 K up to its melting point (393 K), far below 1000 K.
Option (3) S8 molecules — Incorrect. \( \ce{S8} \) rings make up the solid allotropes and dominate at lower temperatures; they dissociate at about 1000 K.
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):
Allotropic Forms of Sulphur – …
