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, epsome salt, baryte etc. The allotropic forms of sulphur are Rhombic and monoclinic sulphur. \(S_2\) is paramagnetic like \(O_2\) and it 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:

\(S_2\)

The correct answer is option 4. \(S_2\).

At room temperature and under normal conditions, elemental sulphur (\(S_8\)) exists as molecules composed of eight sulphur atoms arranged in a ring-like structure. However, at elevated temperatures, such as around 1000K, the thermal energy becomes sufficiently high to break the bonds holding the sulphur atoms together in the \(S_8\) rings.

As a result of this increased thermal energy, the sulphur molecules begin to dissociate into smaller units. One of the predominant forms of sulphur at these temperatures is \(S_2\), which consists of pairs of sulphur atoms bonded together. These \(S_2\) molecules are more stable under the conditions of elevated temperature.

So, at around 1000K, the dominant form of sulphur shifts from \(S_8\) molecules to \(S_2\) molecules due to the breaking of the larger \(S_8\) rings into smaller \(S_2\) units. This phenomenon is a result of the increased thermal energy causing the sulphur molecules to undergo dissociation.