A molecule is usually called organic if it is carbon‑based and built around carbon atoms bonded to hydrogen, often with other elements like oxygen, nitrogen, sulfur, or phosphorus. In practice, this means most molecules that make up living things and many synthetic chemicals (like plastics and fuels) are classified as organic.

Core idea: carbon and C–H bonds

Most modern definitions of an organic molecule include these features:

  • Contains carbon atoms that form the main skeleton (often chains or rings).
  • Has at least one carbon–hydrogen (C–H) covalent bond in many definitions.
  • Often also includes other elements such as O, N, S, P, and halogens.

Some textbooks simplify and say “organic = contains carbon,” but they usually exclude things like carbon dioxide, carbon monoxide, carbonates, and simple carbides, which are treated as inorganic even though they have carbon.

Why carbon makes this special

Carbon is uniquely suited to form organic molecules because:

  • Each carbon can form four strong covalent bonds, allowing long chains, branches, and rings.
  • These carbon skeletons can be decorated with different functional groups (like hydroxyl, amino, or carboxyl), giving molecules very diverse properties.
  • This versatility lets carbon‑based molecules store energy, encode information (like DNA), and build structures in living organisms.

Common examples of organic molecules

Typical organic molecules include:

  • Hydrocarbons (only C and H): alkanes, alkenes, alkynes, aromatic rings (e.g., methane, benzene).
  • Oxygen‑containing molecules: alcohols, aldehydes, ketones, carboxylic acids, esters, sugars.
  • Nitrogen‑ or sulfur‑containing molecules: amines, amides, amino acids, many drugs and vitamins.

Large biological macromolecules—carbohydrates, lipids, proteins, and nucleic acids—are all considered organic because they are built on carbon skeletons.

Edge cases and exceptions

Chemists do not all draw the line in exactly the same place, so definitions differ slightly:

  • Some definitions require C–H bonds; others allow carbon compounds without hydrogen (like some organometallics) to be called organic.
  • Carbon dioxide, carbonates, and cyanides are usually listed as inorganic in general chemistry courses, even though they contain carbon.
  • In everyday language (like food labels), “organic” means grown without certain synthetic chemicals, which is unrelated to the chemical definition.

In short, what makes a molecule “organic” in chemistry is that it is a carbon‑based molecule built on a carbon skeleton, typically with C–H bonds and often with other elements that together support the complex structures and reactions associated with life.