Allotropes of Carbon

Allotropes of Carbon

Carbon's ability to show allotropy comes directly from the two properties introduced in §11.5.6: its strong tendency to catenate (link into chains/rings via very strong C–C bonds) and its unique ability to form $\pi$–$\pi$ multiple bonds with itself and with small, electronegative atoms ($C=C$, $C \equiv C$, $C=O$, $C=S$, $C \equiv N$). Heavier Group 14 elements cannot form these $\pi$–$\pi$ bonds because their orbitals are too diffuse to overlap effectively, and their catenating ability falls off sharply down the group ($C \gg Si > Ge \approx Sn$, negligible for Pb; bond enthalpies C–C 348, Si–Si 297, Ge–Ge 260, Sn–Sn 240 kJ/mol).

As a result, carbon exists in several allotropic forms, crystalline and amorphous:

• Diamond and graphite — the two long-known crystalline allotropes. …