The most widely represented class of hydrocarbons in essential oils is the terpenes. The basic unit of every variety of terpene is a five-carbon molecule called an isoprene; however, isoprenes do not occur in nature as free molecules. Instead, multiple isoprene units are joined together by chemical bonding to form molecules composed of 10, 15, 20, etc. carbons. This class of molecule can be identified from its scientific name, which typically uses the suffix -ene.
When a chemical bond forms between two isoprene units, a 10-carbon molecule called a monoterpene results. Monoterpenes can assume many shapes including cyclic, straight, and branched varieties. Common physical characteristics of monoterpenes are a low molecular weight (meaning they are small and light compared to other types of molecules), clear in color, free flowing (not viscous), high volatility, and strong aroma. You will see below that monoterpenes are present in varying amounts in nearly every essential oil! Common monoterpenes found in essential oils are limonene, pinene, terpinene, and cymene.