Question

Carbon disulphide is a liquid but silicon disulphide is a polymeric solid. This is because (1) \(\mathrm{CS}_{2}\) is nonpolar but \(\mathrm{SiS}_{2}\) is polar (2) carbon can form strong \(\mathrm{p} \pi\) p \(\pi\) bonds with sulphur but silicon cannot (3) of strong molecular attractive forces between larger \(\mathrm{SiS}_{2}\) molecules (4) \(\mathrm{CS}_{2}\) is covalent whilc \(\mathrm{SiS}_{2}\) is ionic

Short answer

Option (2): carbon can form strong p\( \pi \) p\( \pi \) bonds with sulfur but silicon cannot.

Step-by-step solution

Title - Understanding the Compounds' Properties

Analyze the properties of \(\text{CS}_2\) and \(\text{SiS}_2\). Carbon disulfide (\(\text{CS}_2\)) is a covalent compound which is typically nonpolar and exists as a liquid. Silicon disulfide (\(\text{SiS}_2\)), on the other hand, is a polymeric solid, indicating differences in bonding and structure.

- Evaluate Bonding Nature

Consider the bonding strengths and types. Carbon can form strong p\( \pi \) p\( \pi \) bonds with sulfur due to the compatibility in their atomic sizes and electron configurations, leading to small, discrete \(\text{CS}_2\) molecules. Silicon, due to its larger atomic radius and different electron configuration, cannot form strong p\( \pi \) p\( \pi \) bonds with sulfur.

- Analyze the Structure of the Compounds

Examine the resulting structures and intermolecular forces. The inability of silicon to form strong p\( \pi \) p\( \pi \) bonds means \(\text{SiS}_2\) forms a polymeric structure with extensive covalent networks connecting molecules, leading to a solid state. Whereas, \(\text{CS}_2\)'s strong p\( \pi \) p\( \pi \) bonds result in discreet, simple molecular interactions and a liquid state.

- Determine Correct Answer

Based on the analysis, the primary reason that \(\text{CS}_2\) is a liquid and \(\text{SiS}_2\) is a polymeric solid is because carbon forms strong p\( \pi \) p\( \pi \) bonds with sulfur but silicon cannot.

- Conclusion

Thus, the correct answer to the exercise is option (2): \[ \text{carbon can form strong } p \pi \text{ p } \pi \text{ bonds with sulfur but silicon cannot} \]

Key concepts

Bonding Nature

The bonding nature of a compound greatly influences its physical state. Carbon disulfide (CS₂) and silicon disulfide (SiS₂) offer a perfect example of this distinct difference. In CS₂, covalent bonds dominate with carbon and sulfur atoms sharing electrons to form strong p_π p_π bonds, resulting in discrete molecules that are nonpolar. Due to these characteristics, CS₂ remains a liquid at room temperature.

Conversely, SiS₂ does not form strong p_π p_π bonds because silicon’s larger atomic radius and different electron configuration make such bonding unfavorable. Instead, silicon forms extended covalent networks. These extensive networks lead to a more rigid structure, turning SiS₂ into a solid. Thus, the intrinsic bonding nature in each compound dictates their resulting states - liquid for CS₂ and polymeric solid for SiS₂.

Polymeric Structures

In chemistry, the formation of polymeric structures is a decisive factor for the physical state of a compound. Silicon disulphide (SiS₂) is a fantastic illustration of how polymeric structures influence the material's properties. Unlike carbon disulfide (CS₂), which forms simple, discrete molecules due to its capability to establish strong p_π p_π bonds, SiS₂ forms long, interconnected covalent structures.

Silicon atoms, due to their larger size and different electron configurations, prefer forming bonds that extend in three dimensions. These extensive chains and networks of bonds impart a solid and often rigid nature to the compound. This phenomenon is observed in many silicon-based compounds that are typically solids at room temperature, contrasting with carbon analogs that often appear in liquid form.

p π p π Bonds

Understanding p_π p_π bonds is crucial in explaining the differences between carbon disulfide (CS₂) and silicon disulfide (SiS₂). Carbon and sulfur atoms in CS₂ can form strong double bonds, referred to as p_π p_π bonds. This p_π-p_π bonding happens when the p-orbitals of carbon and sulfur overlap effectively due to their similar atomic sizes.

As a result, CS₂ consists of discrete, simple molecules with minimal intermolecular forces, maintaining a liquid state. On the other hand, silicon atoms, with larger atomic radii, have less effective p-orbital overlap with sulfur. This results in a lack of strong p_π p_π bonds, leading silicon to form extended networks or polymeric structures instead of discrete molecules.
Hence, SiS₂ becomes a solid due to extended covalent bonding throughout the material.

Atomic Radius Effects

The effect of atomic radius is a significant factor in the dissimilar states of carbon disulfide (CS₂) and silicon disulfide (SiS₂). Atomic radius refers to the size of an atom. Carbon, with a smaller atomic radius, forms strong p_π p_π bonds with sulfur. These bonds are highly effective due to the close proximity of the atomic orbitals.

Silicon, being larger in size, has an increased atomic radius, which means its atomic orbitals are further apart compared to carbon. This difference inhibits the formation of strong p_π p_π bonds with sulfur. Instead, silicon tends to form bonds that create a polymeric structure, leading to a solid state for SiS₂.
The increased atomic radius of silicon compared to carbon is a key factor why SiS₂ forms a solid polymeric network, while CS₂ remains a liquid with discrete molecular structure.