CnHm

CnHm is a general chemical formula used to represent hydrocarbons—compounds composed solely of carbon and hydrogen. The notation denotes a family of possible molecules with n carbon atoms and m hydrogen atoms, but it does not specify how the atoms are connected or whether there are rings, multiple bonds, or functional groups. As a result, many isomers can share the same CnHm formula.

For acyclic, saturated hydrocarbons (alkanes), the hydrogen count follows m = 2n + 2. Each decrease of two hydrogens corresponds to one degree of unsaturation (a double bond or a ring). In common unsaturated subclasses, alkenes have one double bond and general formula m = 2n, while alkynes have a triple bond and m = 2n − 2. The degree of unsaturation DoU = (2n + 2 − m)/2; higher DoU corresponds to more rings or multiple bonds. Aromatic hydrocarbons and cycloalkanes have characteristic patterns that can also fit the CnHm notation.

Examples: CH4 (n=1, m=4) methane; C2H6 ethane; C2H4 ethylene; C2H2 acetylene; C6H6 benzene. The CnHm formula thus encompasses a wide range of structures from simple alkanes to complex polycyclic aromatics.

In practice, CnHm is used in formula-based searches in chemical databases and analytical contexts like mass spectrometry, where formula information helps constrain possible structures but does not uniquely identify a single compound.