molecule

A molecule is the smallest unit of a chemical substance that retains its characteristic properties. It is typically composed of two or more atoms held together by chemical bonds. Molecules can be neutral; they may consist of atoms of the same element, as in O2 or N2, or of different elements, as in H2O or CO2. Larger assemblies are sometimes called compounds or macromolecules depending on context, but the term molecule is commonly used for discrete, covalently bonded units.

Most molecules are held together by covalent bonds in which atoms share electron pairs. Some molecules involve ionic character or weaker interactions, but the essential concept is that the atoms form a defined, discrete unit with a fixed chemical formula. Certain molecules exist as isomers, sharing the same formula but differing in arrangement in space, such as structural isomers or stereoisomers.

Molecular structure determines properties. The angular arrangement of atoms—geometry—gives shapes such as linear, bent, trigonal planar, tetrahedral, and more complex forms. Bond lengths and bond energies influence stability and reactivity. Molecules can be polar or nonpolar, and their polarity affects solubility, boiling points, and intermolecular interactions, including hydrogen bonding and van der Waals forces.

In science and biology, molecules underpin all substances and processes. Important classes include biomolecules such as nucleic acids, proteins, carbohydrates, and lipids. Researchers study molecules using techniques like spectroscopy, crystallography, and mass spectrometry, and describe them with molecular formulas, molecular weights, and, for larger molecules, three-dimensional structures.