Molecules

A molecule is the smallest unit of a chemical compound that can exist independently while preserving the properties of the compound. It consists of two or more atoms held together by chemical bonds, typically covalent bonds formed by the sharing of electron pairs. Ionic compounds, by contrast, are assemblies of ions held by electrostatic forces and are usually not described as discrete molecules.

Molecules can be as simple as diatomic oxygen (O2) or nitrogen (N2), or as complex as organic macromolecules such as proteins, nucleic acids, and polysaccharides. Molecular formulas express the types and numbers of atoms in a molecule, while the molecular structure reveals how the atoms connect and arrange in space. Isomers are molecules with the same formula but different arrangements, leading to distinct properties.

Three-dimensional arrangement—molecular geometry—determines properties such as polarity, reactivity, and phase behavior. Bond lengths and angles, as well as intramolecular forces like hydrogen bonds, influence stability. Intermolecular forces—van der Waals forces and hydrogen bonding—govern interactions between molecules and with their environment.

Techniques such as spectroscopy, X-ray crystallography, and cryo-electron microscopy illuminate molecular structures. Molecules underpin chemistry, biology, materials science, and pharmacology; understanding them enables design of drugs, catalysts, and new materials.