How are rhombic and monoclinic sulfur different?

Rhombic and monoclinic sulfur are two distinct crystalline allotropes of sulfur, both composed of eight-membered ring molecules ($S_8$). While often referred to as "rhombic," the former is technically the orthorhombic ($\alpha$-sulfur) form, whereas the latter is the monoclinic ($\beta$-sulfur) form.1

Their primary differences include:

• Stability and Temperature: The orthorhombic form is stable at temperatures below 96 °C (204.8 °F). Above this transition temperature, it converts into the monoclinic form, which remains stable until it reaches its melting point.1
• Crystal Structure: In the orthorhombic form, the molecules are arranged in an orthorhombic crystal lattice. In the monoclinic form, two crystal axes are perpendicular, while the third forms an oblique angle with the first two.1
• Physical Properties: The two forms differ in density and melting points:

| Property | Orthorhombic ($\alpha$) | Monoclinic ($\beta$) |
| :--- | :--- | :--- |
| Density (at 20 °C) | 2.07 g/cm³23| 1.96 g/cm³3|
| Melting Point | 112.8 °C (235 °F)*13| 118.9 °C (246 °F)13|

*Note: The orthorhombic form melts at 112.8 °C if heated rapidly before it can transform into the monoclinic form.1