29.1.1—Compounds in the table on page 47 contain

A functional group is the local arrangement of atoms that gives an organic family its characteristic reactions and many of its physical properties.

The carbon skeleton still affects boiling point, solubility and steric access, but the functional group supplies the main reaction pattern. Identify both before predicting behaviour.

Ethanol and ethanoic acid both contain two carbon atoms, yet –OH versus –CO₂H gives very different acidity and reactions. Propene and propane differ by C=C and therefore by addition chemistry.

A functional group is not the whole molecule and one atom alone is not enough to classify it; inspect the complete local connectivity.