MoleculeEdit
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A molecule is the smallest unit of a chemical substance that can exist while retaining the properties of that substance. It is formed when two or more atoms bond together, typically through shared electrons in covalent bonds or through other bonding interactions that hold the atoms in a defined arrangement. Molecules come in a vast range of sizes and complexities—from simple diatomic species like O2 and N2 to expansive biological macromolecules such as proteins and deoxyribonucleic acid. The study of molecules lies at the heart of chemistry and intersects with physics, biology, and materials science to explain how matter behaves, reacts, and assembles into higher-order structures.
Definition and scope
A molecule represents the discrete unit of a chemical substance that can participate in chemical reactions and determine many of the substance’s intrinsic properties. In contrast to extended crystalline lattices found in many solids, a molecule is a finite, countable assembly of atoms with a specific arrangement and geometry. The concept of a molecule is central to disciplines ranging from organic chemistry and inorganic chemistry to biochemistry and physical chemistry. For certain substances, the term “molecule” refers to neutral, covalently bonded assemblies, while other species—such as polyatomic ions—can also be described as molecular entities in particular contexts. See also molecular formula for shorthand notation that encodes the number and types of atoms in a molecule.
Structure and bonding
The architecture of a molecule arises from the way atoms share or transfer electrons to achieve stability. The most common type of bonding is the covalent bond, where atoms share electron pairs to fill their outer electron shells. Covalent bonds give rise to a wide variety of molecular geometries, described in part by the principles of VSEPR theory (valence shell electron pair repulsion) and hybridization concepts that predict bond angles and shapes. For a broader view of bonding, see chemical bond.
Other attractive forces contribute to the behavior of molecules without forming discrete covalent bonds. Ionic bonds result from electron transfer between atoms in many salts, while noncovalent interactions such as hydrogen bonds and van der Waals forces influence molecular recognition, folding, and assembly—essential in biomolecules and supramolecular chemistry.
Molecular geometry—the three-dimensional arrangement of atoms within a molecule—controls physical properties like polarity, reactivity, and spectroscopic behavior. Stereochemistry explores isomerism arising from different spatial arrangements, including enantiomers and diastereomers, which can have markedly different biological activities. See molecular geometry, isomer, and stereochemistry for related concepts.
Types of molecules
- Small and simple molecules: These include diatomic species such as O2 and N2 and small organics like methane (CH4). See also structure of water for a classic example of a polar molecule.
- Organic molecules: Compounds built primarily from carbon and hydrogen, often with heteroatoms such as oxygen, nitrogen, or sulfur. Key classes include carbohydrate, lipid, protein constituents, and many industrial materials.
- Inorganic molecules: A broad category that includes coordination compounds, oxides, sulfides, and other assemblies not primarily based on carbon-hydrogen frameworks.
- Macromolecules and polymers: Large, chain-like molecules such as proteins and nucleic acid polymers that perform essential functions in biology and technology. See also polymer for extended chain structures.
- Biomolecules: Molecules of life, including amino acids, nucleotides, and metabolites, whose interactions underlie metabolism, signaling, and genetic information transfer. See amino acid, nucleotide, and enzyme.
Detection, analysis, and measurement
A wide array of techniques characterizes molecules and their properties: - Spectroscopy reveals information about bonds and structure. Key methods include NMR spectroscopy, infrared spectroscopy, and UV–visible spectroscopy. - Mass spectrometry provides molecular weights and structural clues. - X-ray crystallography and, when possible, cryo-electron microscopy determine precise three-dimensional arrangements at atomic resolution. - Chromatographic and electrochemical techniques separate, identify, and quantify molecular species. See also spectroscopy and analytical chemistry for broader context.
Synthesis and reactivity
Molecules form and transform through chemical reactions governed by kinetics and thermodynamics. Reactions proceed via mechanisms that involve bond formation and cleavage, often facilitated by catalysts, heat, light, or pressure. The study of reaction pathways, transition states, and energy profiles is central to physical chemistry and organic chemistry. See also chemical reaction and catalyst for foundational topics.
Role in science and technology
Molecules underpin all of chemistry and inform disciplines from medicine to materials science. In medicine, the design of therapeutic drugs depends on understanding how molecules interact with biological targets. In materials science, molecular design drives the development of polymers, coatings, and nanoscale devices. In energy research, molecular processes govern catalysis, energy storage, and conversion. See pharmacology, materials science, and energy for related areas.
History
The concept of molecules emerged with early ideas about atoms and chemical bonds. Foundational figures include John Dalton for atomic theory and molecular composition, Amedeo Avogadro for the mole concept, and later work by Gilbert N. Lewis on Lewis structures and valence. The development of experimental techniques such as X-ray crystallography and advances in spectroscopy dramatically expanded knowledge about molecular structures and bonding.