allotropes and catenation of sulfur

Sulfur is the yellow element of brimstone and matchsticks, and it has an unusual talent: its atoms love to bond to one another, forming rings and chains in a way oxygen never does. This ability of an element to link to itself in long bonded sequences is called catenation, and sulfur is, after carbon, one of its great practitioners. The result is a surprising variety of solid forms — allotropes — for what looks like a simple yellow powder.

Sulfur's stable everyday form is rhombic sulfur, built from S8 molecules: eight sulfur atoms joined by single bonds into a puckered, crown-shaped ring. Warm it gently and it shifts to monoclinic sulfur, a different crystal packing of the same S8 rings. The real drama comes on stronger heating: molten sulfur first thins, then near 160 degrees Celsius the S8 rings crack open and the chains they form tangle into longer and longer polymers, so the liquid grows dark and thick — almost too viscous to pour. Quench that hot tangle into cold water and you get plastic sulfur, a rubbery, stretchy form made of long disordered chains, which slowly reverts to ordinary yellow S8 over days. Why sulfur catenates so well while oxygen does not comes down to bonding: the S-S single bond is reasonably strong, whereas oxygen prefers a strong O=O double bond and forms only short O-O chains.

This catenation theme runs right across the p-block and explains a deep contrast. The first-row elements (nitrogen, oxygen) form strong multiple bonds and so prefer small molecules with double or triple bonds (N2, O2, CO2). Their heavier relatives below (phosphorus, sulfur) make weak multiple bonds but strong single bonds, so they prefer to catenate into rings, chains, and cages (P4, S8, polyphosphates). Practically, the polymeric chains of sulfur are why molten sulfur behaves so strangely, and catenated sulfur chains turn up in vulcanised rubber, where sulfur bridges cross-link polymer chains to make a tyre tough.