What are the allotropic forms of sulphur?
Answers
Sulfur occurs in three allotropic forms:
orthorhombic
The orthorhombic form is the most common crystalline form because it is the most stable form below 96⁰C. It is a yellow, brittle solid which occurs in S8 rings. It can be found in Sicily, Mexico, and some southern states of the USA.
monoclinic
Monoclinic sulfur is another crystalline form of sulfur. It has the same type of S8 rings, packed in different ways to form their crystals. Monoclinic sulfur is stable only between 96oC and 119oC. It forms naturally as molten sulfur and gradually cools through that temperature range. Below 90oC, monoclinic sulfur quickly returns to the orthorhombic form.
amorphous
When heated to above 120⁰C, sulfur becomes a liquid. When heated above 200⁰C it becomes dark-coloured and viscous. This change is due to the breaking open of the S8 rings to form chains and the atoms at the ends of the chains have electrons that are easily excited by absorption of light and cause the dark colour. The S8 chains join up to form long chains which coil around one another giving the increase in viscosity.
When the dark viscous liquid is quenched in cold water, the coiled molecules do not have time to re-form the S8 molecules and to arrange themselves in orthorhombic crystals. The product is a soft, elastic material and is an example of a super-cooled liquid. It is difficult to maintain in its amorphous form because it gradually reverts to the more stable orthorhombic form.
Allotropic Forms of Sulphur
One is the yellow rhombic sulphur (α-sulphur), and the other is monoclinic (β-sulphur). The most interesting feature is that the thermal stability, the allotropes of the sulphur compound are interconvertible. It means rhombic sulphur, when heated more than 369K produces monoclinic sulphur.
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