Question

Describe the structure of the sulfur molecules in: (a) Rhombic sulfur (b) Monoclinic sulfur (c) Plastic sulfur (d) Liquid sulfur above \(160^{\circ} \mathrm{C}\)

Short answer

Rhombic sulfur is rhombic crystal S₈; monoclinic sulfur is needle-like S₈; plastic sulfur is amorphous with chain-like structures; liquid sulfur above 160°C forms long chains.

Step-by-step solution

Understanding Rhombic Sulfur

Rhombic sulfur exists as an orthorhombic crystal structure which is the most stable form of sulfur at room temperature. It consists of S₈ molecules arranged in a puckered ring configuration. These rings are held together in a specific periodic pattern.

Analyzing Monoclinic Sulfur

Monoclinic sulfur is a crystalline allotrope of sulfur that forms needle-like crystals. It also consists of S₈ molecules, like rhombic sulfur, but with a different crystal lattice. The arrangement within the lattice is such that it becomes stable at higher temperatures (between 96°C and 119°C) before reverting to rhombic sulfur.

Exploring Plastic Sulfur

Plastic sulfur is an amorphous form of sulfur produced when molten sulfur is suddenly cooled, such as by pouring it into cold water. This form doesn't have a regular, crystalline structure but instead consists of long polymeric chains of sulfur atoms.

Examining Liquid Sulfur Above 160°C

Liquid sulfur above 160°C contains molecules that have broken from the S₈ ring structure to form long chains. The longer chains can entangle, which increases the viscosity of the liquid. At this temperature, the sulfur exists as a liquid form with properties differing significantly from its crystalline forms.

Key concepts

Rhombic Sulfur Structure

Rhombic sulfur, also known as α-sulfur, is the most stable form of sulfur at room temperature. This allotrope has an orthorhombic crystal structure, which means it has three axes of differing lengths that intersect at right angles. The sulfur atoms in rhombic sulfur are organized into puckered rings made up of eight sulfur atoms each, known as S₈ rings.
These S₈ molecules are layered in a highly ordered, periodic pattern, providing the characteristic crystalline state. Rhombic sulfur appears as bright yellow crystals, often found in natural deposits. Its stability at room temperature makes it a common form of sulfur found on Earth.

Monoclinic Sulfur Structure

Monoclinic sulfur, or β-sulfur, is another crystalline allotrope of sulfur that forms needle-like crystals. It is less stable than rhombic sulfur at standard conditions, but it becomes the stable form at temperatures between 96°C and 119°C. This transformation occurs because the lattice structure changes to accommodate higher thermal energy.
Similar to rhombic sulfur, monoclinic sulfur consists of S₈ molecules. However, the arrangement of these molecules in the crystal lattice is different, leading to a distinct set of physical properties. Once cooled below 96°C, monoclinic sulfur slowly reverts back to the rhombic form, reestablishing the more stable orthorhombic pattern.

Amorphous Sulfur

Amorphous sulfur, commonly referred to as plastic sulfur, lacks a defined crystalline structure. This non-crystalline form is created when molten sulfur is rapidly cooled, such as by pouring it into cold water. The abrupt cooling prevents the sulfur atoms from organizing into a regular lattice, resulting in long, tangled polymeric chains rather than ordered rings.
This form of sulfur is flexible and often rubbery in texture, hence its name "plastic sulfur." Since it does not have a structured arrangement, amorphous sulfur can eventually crystallize into more stable forms, like rhombic sulfur, if left undisturbed over time.

Liquid Sulfur Properties

When sulfur is heated above 160°C, it transitions into a liquid form with unique properties. At this temperature, the S₈ rings begin to break apart, forming long, chain-like molecules. These chains can become quite entangled, leading to a significant increase in the viscosity of the liquid.
The viscosity reaches its peak near 190°C as the sulfur molecules stretch into longer chains, but as the temperature rises further, the viscosity begins to decrease as the chains break down again. This behavior is a distinctive characteristic of liquid sulfur and contrasts sharply with its solid crystalline states. These changes in molecular structure and properties are crucial for understanding sulfur's role in industrial applications where its liquid form is utilized.