The smallest unit of a compound, which consists of atoms bonded together in a unique arrangement. A diatomic molecule consists of two atoms linked together by chemical bonds. Examples include the oxygen and nitrogen of the air (O₂ and N₂, respectively) and the neurotransmitter nitric oxide (NO). A polyatomic molecule consists of more than two linked atoms. Examples include water (H₂O), carbon dioxide (CO₂), ethanol (C₂H₅OH), and the vast molecule of deoxyribonucleic acid (DNA) with thousands of atoms.