At room temperature and pressure, industrially ground sulfur powder takes the S₈ crown-shaped cyclic molecule as its fundamental structural unit, and its crystalline form is dominated by thermodynamically stable orthorhombic sulfur (α-sulfur). The grinding process only changes the physical particle size and does not alter the molecular structure or chemical nature of sulfur.
1. Molecular Level: The S₈ Crown Ring
The basic building block of solid sulfur powder is the octasulfur molecule S₈:
- Ring structure: Eight sulfur atoms are connected end-to-end by single covalent bonds to form a closed eight-membered ring.
- Spatial conformation: The ring adopts a puckered crown conformation (chair-like folded structure) rather than a flat ring, with atoms alternating above and below the average plane of the ring.
- Bonding details: Each sulfur atom undergoes sp³ hybridization, forming two σ bonds with adjacent S atoms and retaining two lone pairs of electrons. The S–S bond length is approximately 206 pm, the bond angle is about 108°, and the dihedral angle of the ring is roughly 90°.
- This cyclic structure has moderate bond energy and is the most thermodynamically stable molecular form of sulfur under ambient conditions.
2. Crystal Level: Orthorhombic Sulfur (α-Sulfur)
Conventional industrial sulfur powder exists mainly as orthorhombic (rhombic) crystalline sulfur:
- S₈ molecules are arranged in an ordered three-dimensional lattice held together by weak van der Waals intermolecular forces, with a density of about 2.07 g/cm³.
- At temperatures above 95.5 °C, orthorhombic sulfur undergoes a reversible phase transition to monoclinic sulfur (β-sulfur). The molecular unit remains S₈; only the packing arrangement of molecules in the crystal changes. Local frictional heating during grinding may produce trace amounts of monoclinic sulfur, but it reverts to the orthorhombic form upon cooling to room temperature.
3. Other Sulfur Allotropes (Not Present in Conventional Ground Sulfur Powder)
- Small ring molecules such as S₂, S₄ and S₆ are only stable at high temperatures or in the gas phase, and do not exist in sulfur powder produced by mechanical grinding at room temperature.
- Polymeric sulfur (elastic sulfur) consists of long linear sulfur chains (Sₙ), produced by quenching molten sulfur. It is an amorphous allotrope that cannot be obtained by ordinary mechanical grinding and is not a component of standard industrial sulfur powder.
- Amorphous precipitated sulfur is made via chemical precipitation; standard ground sulfur powder is predominantly crystalline orthorhombic sulfur.
4. Effect of Grinding on Sulfur Structure
- Grinding is a purely physical size-reduction process. It breaks large sulfur crystals into fine particles but does not break the covalent S–S bonds within S₈ molecules, so the molecular structure remains intact.
- Extremely high-energy ultrafine grinding may cause partial amorphization of crystal surfaces (lattice disorder), but the fundamental molecular unit remains S₈, and the chemical properties of sulfur are unchanged.
