A molecule is a discrete group of two or more atoms held together by chemical interactions, with the atoms forming a distinct entity that has a characteristic composition and structure. Molecules can contain atoms of the same element, such as O2, N2 and H2, or atoms of different elements, such as H2O, CO2 and NH3. The atoms within a molecule are connected through chemical bonding and their arrangement determines many of the molecule's physical and chemical properties.
Molecules are commonly associated with covalent bonding, in which electrons are shared or distributed between atoms, although the broader concept of molecular systems can involve other interactions as well. A molecule is not simply any collection of atoms that happen to be close together. It is a defined structural and electronic entity with a stable or metastable arrangement. Molecules can range from very small diatomic species to extremely complex biological macromolecules containing thousands or millions of atoms.
Molecular structure is determined by the arrangement of nuclei and electrons and by the interactions that minimise the relevant energy under the given conditions. Covalent bonding arises from quantum mechanical interactions between atomic orbitals and molecular orbitals. Electrons can occupy bonding, antibonding or nonbonding molecular orbitals, and the resulting electron distribution influences bond length, bond strength, polarity and reactivity. Molecular geometry is governed by the distribution of electron density and by quantum mechanical principles.
Models such as valence shell electron pair repulsion provide useful qualitative predictions for many simple molecules, while molecular orbital theory provides a more detailed description. Molecules can have linear, bent, trigonal planar, tetrahedral, pyramidal and many other geometries. Three dimensional structure is particularly important because molecules with the same atoms and formula can have different arrangements and consequently different properties.
Molecules have characteristic molecular formulas and can be represented in several ways. A molecular formula states the number of atoms of each element in one molecule, while a structural formula conveys information about how atoms are connected. Lewis structures represent valence electrons and bonding patterns, while line angle structures are widely used in organic chemistry to represent carbon frameworks efficiently. Three dimensional models and computational structures can show molecular geometry and stereochemistry.
Isomers are compounds or molecular species that have the same molecular formula but different arrangements of atoms. Structural isomers differ in connectivity, while stereoisomers have the same connectivity but different spatial arrangements. Enantiomers are non superimposable mirror images, whereas diastereomers are stereoisomers that are not mirror images. Such structural differences can profoundly affect biological activity, physical properties and chemical reactivity, demonstrating that a molecular formula alone does not fully specify molecular behaviour.
Molecules can be polar or nonpolar depending on their bonding and overall geometry. A polar bond arises when electrons are distributed unequally between atoms with different electronegativities, producing partial charges. However, a molecule containing polar bonds is not necessarily polar overall because the individual bond dipoles can cancel through symmetry.
Carbon dioxide has polar carbon oxygen bonds but a linear symmetrical structure that produces no permanent molecular dipole, while water has polar oxygen hydrogen bonds and a bent geometry that produces a net dipole. Molecular polarity affects intermolecular forces, solubility, boiling point, melting point and interactions with biological molecules. Molecules can interact through London dispersion forces, permanent dipole interactions, hydrogen bonding and other noncovalent forces. These interactions are weaker than many covalent bonds but are crucial to molecular aggregation, liquids, molecular recognition, protein folding and materials properties.
Molecules undergo constant motion and can rotate, vibrate and translate depending on their environment and energy state. Molecular vibrations correspond to characteristic patterns in which atoms move relative to one another, and these vibrational frequencies can be measured using infrared and Raman spectroscopy. Rotational transitions can provide detailed information about molecular structure and are particularly important in gas phase spectroscopy.
Electronic transitions occur when electrons move between allowed quantum states and can produce absorption or emission of ultraviolet, visible or other electromagnetic radiation. Spectroscopic fingerprints allow scientists to identify molecules, determine structures, measure concentrations and study dynamic processes. Nuclear magnetic resonance spectroscopy provides information about the chemical environments of certain atomic nuclei and is particularly important for determining molecular structures. Molecular spectroscopy therefore provides an experimental connection between quantum theory and observable chemical behaviour.
Not every substance consists of discrete molecules. Ionic crystals such as sodium chloride are better described as extended lattices of ions rather than collections of independent NaCl molecules under ordinary solid conditions. Metals form extended lattices with delocalised electronic structures, and network covalent solids such as diamond and quartz contain continuous frameworks of bonded atoms. In such materials, writing a chemical formula does not necessarily mean that the formula corresponds to a discrete molecule.
NaCl can be described as a formula unit representing the stoichiometric ratio of sodium and chloride ions in the crystal. Silicon dioxide similarly forms extended network structures in many solid forms rather than discrete SiO2 molecules. This distinction is important because the everyday statement that every substance is made of molecules is scientifically inaccurate. The appropriate microscopic description depends on the bonding and structure of the material.
Molecules are central to biology because biological function depends heavily on molecular structure and interactions. Proteins are polymers of amino acids that fold into specific three dimensional structures, nucleic acids consist of nucleotide chains, carbohydrates contain linked sugar units and lipids include diverse molecular structures involved in membranes and energy storage. DNA and RNA molecules carry and process genetic information, while proteins act as enzymes, receptors, transporters, structural components and signalling molecules.
Small molecules such as oxygen, carbon dioxide, glucose, water, neurotransmitters and hormones participate in metabolism and cellular communication. Molecular recognition depends on shape, charge distribution, hydrogen bonding, hydrophobic interactions and other physical properties. Drug molecules work by interacting with biological targets through molecular complementarity and chemical forces. Molecular chemistry therefore provides a foundation for understanding physiology, pharmacology, genetics and biotechnology.
Molecules can be extremely stable or highly reactive depending on their electronic structures and environmental conditions. Chemical reactions involve breaking, forming or rearranging chemical bonds and therefore transform molecules into different molecular species. Reaction mechanisms describe the sequence of elementary molecular events through which reactants become products. Reaction rates depend on temperature, concentrations, catalysts, molecular orientation and activation barriers. Catalysts change reaction pathways and lower activation barriers without being consumed overall.
Molecules can also absorb or release energy during chemical reactions, with energy changes related to bond energies, enthalpy, entropy and Gibbs free energy. The concept of the molecule therefore connects atomic structure with observable properties such as reactivity, spectroscopy, phase behaviour and biological function. Understanding molecules requires integrating quantum mechanics, thermodynamics, kinetics, structural chemistry and experimental observation.
