Which of the following best defines a hydrocarbon?

• A. A molecule containing only carbon and hydrogen
• B. A molecule containing carbon, hydrogen, and oxygen
• C. A molecule containing carbon, hydrogen, and nitrogen
• D. A molecule containing carbon only

Answer: (A)

Hydrocarbons are organic molecules built only from carbon and hydrogen. The defining idea is that no other elements are part of the structure. That’s why the description of a molecule containing only carbon and hydrogen is the best match. If oxygen appears, as in ethanol, or nitrogen appears, as in ammonia or other amines, the compound is no longer a hydrocarbon. Likewise, a substance that has carbon but no hydrogen isn’t a hydrocarbon either. Examples like methane (CH4) or benzene (C6H6) illustrate hydrocarbons, since they consist solely of C and H.