Physical Properties of Sulfur

Sulfur has an atomic weight of 32.066 grams per mole and is part of group 16, the oxygen family. It is a nonmetal and has a specific heat of 0.706 J g^{-1 o}C^-1. The electron affinity if 200 kJ mol^-1 and the electronegativity is 2.58 (unitless). Sulfur is typically found as a light-yellow, opaque, and brittle solid in large amounts of small orthorhombic crystals. Not only does sulfur have twice the density of water, it is also insoluble in water. On the other hand, sulfur is highly soluble in carbon disulfide and slightly soluble in many common solvents. Sulfur can also vary in color and blackens upon boiling due to carbonaceous impurities. Even as little as 0.05% of carbonaceous matter darkens sulfur significantly.

Figure 1.1 : A piece of sulfur burning with a blue flame that can hardly be seen at daylight, but it can be seen in this photograph if you look closely. Image by Johannes Hemmerlein and used with permission.

, S2, S4, S6, and S8

• .
• At room temperature, rhombic sulfur (S_α) is a stable solid comprised of cyclic S8
• molecules.
• At 95.5 °C, rhombic sulfur becomes monoclinic sulfur (S_β). The crystal structure found in monoclinic sulfur differs from that of rhombic sulfur. Monoclinic sulfur is also made up of S8
• molecules. 
• Monoclinic sulfur becomes liquid sulfur (S_λ) at 119 °C. Liquid sulfur is straw-colored liquid made up of S8
• molecules and other cyclic molecules containing a range of six to twenty atoms. 
• At 160 ^oC, this becomes a dark, viscous liquid called Liquid sulfur (S_μ). The molecules are still made up of eight Sulfur atoms but the molecule opens up and transforms from a circle into a long spiral-chain molecule.
• At 180 °C, the chain length and viscosity reach their maximum. Chains break and viscosity decreases at temperatures that exceed 180 °C. 
• Sulfur vapor is produced  when liquid boils at 445 °C. In the vapor that is produced, S8
• molecules dominate but as the vapor continues to heat up, the molecules break up into smaller groups of Sulfur.
• To produce plastic sulfur, Sis poured into cold water. Plastic sulfur is rubberlike and is made up of long, spiral-chain molecules. If plastic sulfur sits for long, it will reconvert to rhombic sulfur.

Figure 1.2 : Two allotropes of sulfur.

While oxygen has fewer allotropes than sulfur, including O , O2, O3, O4, O8, metallic O

(and four other solid phases), many of these actually have a corresponding sulfur variant. However, sulfur has more tendency to catenate (the linkage of atoms of the same element into longer chains). Here are the values of the single and double bond enthalpies:

This means that O=O is stronger than S=S, while O–O is weaker than S–S. So, in sulfur, single bonds are favored and catenation is easier than in oxygen compounds. It seems that the reason for the weaker S=S double bonds has its roots in the size of the atom: it's harder for the two atoms to come at a small enough distance, so that the p orbitals overlap is small and the π bond is weak. This is attested by looking down the periodic table: Se=Se has an even weaker bond enthalpy of 272kJ/mol .

molecules break up. When suddenly cooled, long chain molecules are formed in the plastic sulfur which behave as rubber. Plastic sulfur transform into rhombic sulfur over time.

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