Chemistry is the branch of science that studies matter, its composition, structure, properties, transformations and the energy changes associated with those transformations. Matter includes atoms, molecules, ions, compounds, mixtures, solids, liquids, gases and plasmas, while chemistry examines how these forms of matter are organised and how they interact. At its most fundamental level, chemistry is concerned with electrons, atomic nuclei and the interactions that determine how atoms bond and how substances behave.
It connects microscopic structure with macroscopic properties such as colour, melting point, boiling point, acidity, electrical conductivity, reactivity and mechanical strength. Chemistry overlaps strongly with physics, biology, materials science, Earth science, medicine and engineering, but it has its own emphasis on chemical composition, bonding, molecular structure, reactions and transformations. The word chemistry has a complex historical origin related to alchemy, with the modern scientific discipline developing gradually through experimental methods, quantitative measurement and atomic theory. Modern chemistry is therefore both a fundamental science concerned with the structure of matter and an applied science used to design, analyse and transform substances.
The basic units studied in chemistry include atoms, molecules and ions. An atom consists of a nucleus containing protons and neutrons surrounded by electrons described by quantum mechanical states. The number of protons defines the element, while different numbers of neutrons produce isotopes of that element. Molecules form when atoms are connected through chemical bonds, while ions are electrically charged species produced by gaining or losing electrons or, in some cases, by other charge separation processes.
Compounds contain two or more different chemical elements combined in definite proportions, whereas mixtures contain substances that are physically combined without necessarily forming new chemical species. The periodic table organises elements according to atomic number and recurring chemical properties. Periodicity arises largely from the arrangement of electrons in atomic orbitals and the resulting patterns of valence electrons. Concepts such as electronegativity, atomic radius, ionisation energy and electron affinity help explain why elements react differently. Chemistry uses these principles to predict and explain the behaviour of substances rather than treating chemical reactions as arbitrary changes.
Chemical bonding is central to understanding chemistry. Ionic bonding involves electrostatic attraction between oppositely charged ions and is particularly important in many salts. Covalent bonding involves sharing electron density between atoms and forms the basis of most molecular compounds and extensive network solids. Metallic bonding involves delocalised electrons associated with arrays of metal atoms and helps explain the electrical and thermal conductivity of metals.
Real chemical bonding is more nuanced than three completely separate categories, because many compounds exhibit combinations of ionic and covalent character. Molecular geometry depends on quantum mechanical electron distributions and can be described using concepts such as hybridisation, valence shell electron pair repulsion and molecular orbital theory, although each model has different strengths and limitations. Intermolecular forces, including hydrogen bonding, dipole interactions and London dispersion forces, influence boiling points, solubility, viscosity, crystal structures and biological interactions. The distinction between intramolecular bonds and intermolecular forces is important because breaking or forming chemical bonds usually involves different energy scales from rearranging interactions between molecules.
Chemical reactions occur when atoms and molecules are rearranged into different chemical species. A reaction can involve bond breaking, bond formation, electron transfer, proton transfer, substitution, addition, elimination, rearrangement or other mechanisms. Chemical equations represent these transformations symbolically, with coefficients used to conserve atoms and, for ionic reactions, charge. The law of conservation of mass is fundamental to ordinary chemical reactions, while nuclear reactions can change atomic nuclei and therefore belong primarily to nuclear physics rather than conventional chemistry.
Reaction mechanisms describe the sequence of elementary molecular events through which reactants become products. A reaction's rate depends on factors such as temperature, concentration, pressure for gases, surface area, solvent, catalysts and molecular structure. Catalysts accelerate reactions by providing alternative pathways with lower activation barriers while not being consumed overall in the net reaction. Reaction kinetics studies rates and mechanisms, whereas thermodynamics studies energy, entropy and equilibrium. These fields are complementary because a reaction can be thermodynamically favourable but kinetically extremely slow.
Thermochemistry and thermodynamics provide the energy framework of chemistry. Chemical systems can absorb or release heat when reactions occur, with exothermic reactions releasing heat to the surroundings and endothermic reactions absorbing heat. Enthalpy changes describe heat transfer under constant pressure under specified conditions, while Gibbs free energy is commonly used to assess spontaneity at constant temperature and pressure.
A negative Gibbs free energy change indicates that a process is thermodynamically favourable under the specified conditions, but it does not determine how quickly the process occurs. Entropy measures the dispersal of energy and the number of accessible microscopic configurations in statistical descriptions. Chemical equilibrium occurs when forward and reverse processes proceed at equal rates, producing stable macroscopic concentrations even though molecular reactions continue microscopically.
Le Chatelier's principle provides a useful qualitative framework for predicting how an equilibrium responds to changes in concentration, pressure or temperature, although quantitative analysis uses equilibrium constants and thermodynamic relationships. These principles explain why some reactions release energy, why others require energy input and why many reactions do not proceed completely to products.
Chemistry is divided into several overlapping fields. Organic chemistry focuses primarily on compounds containing carbon and includes hydrocarbons, polymers, pharmaceuticals, biomolecules and many industrial materials. Inorganic chemistry studies substances outside the traditional scope of organic chemistry, including metals, minerals, coordination compounds and many ionic materials. Physical chemistry applies physical principles to chemical systems, including quantum chemistry, thermodynamics, kinetics and spectroscopy.
Analytical chemistry develops methods for identifying and quantifying substances, using techniques such as chromatography, mass spectrometry, nuclear magnetic resonance and optical spectroscopy. Biochemistry studies chemical processes in living organisms, including proteins, nucleic acids, carbohydrates, lipids and metabolic reactions. Materials chemistry investigates the synthesis and properties of substances designed for particular electrical, optical, mechanical or chemical functions. Environmental chemistry examines pollutants, atmospheric reactions, water chemistry and chemical cycling in ecosystems. These divisions are organisational rather than absolute, because modern research frequently crosses several fields simultaneously.
Chemistry is fundamental to biology and medicine because living systems operate through enormous networks of chemical reactions. Proteins fold according to chemical interactions among amino acid side chains and surrounding molecules. Enzymes accelerate biochemical reactions by binding substrates and stabilising appropriate transition states. DNA and RNA depend on hydrogen bonding, base pairing, covalent backbones and molecular recognition. Cellular energy metabolism involves oxidation reduction reactions, proton gradients and the synthesis and hydrolysis of energy carrying molecules such as ATP.
Pharmaceuticals work through chemical interactions with biological targets, and drug development relies on understanding molecular structure, solubility, reactivity, metabolism and binding. Chemistry also underlies nutrition, toxicology, genetics, immunology and microbiology. The boundary between chemistry and biology is therefore highly interconnected, with biochemistry providing a major bridge between molecular chemistry and living systems. Understanding chemical structure can explain biological function, while biological systems provide complex environments in which chemical principles operate.
Chemistry is also essential to industry and everyday technology. The manufacture of fertilisers, medicines, polymers, detergents, fuels, pigments, batteries, semiconductors and construction materials depends on chemical knowledge. Petroleum refining separates and transforms hydrocarbons, while polymer chemistry creates materials with controlled flexibility, strength, thermal stability or biodegradability. Electrochemistry underlies batteries, fuel cells, electroplating, corrosion and many industrial processes. Semiconductor chemistry contributes to electronic materials and manufacturing, while surface chemistry is important for catalysts, sensors and coatings.
Green chemistry seeks to design chemical processes that reduce hazardous substances, waste, energy consumption and environmental impact. Modern chemical engineering applies chemical principles at industrial scale while considering heat transfer, fluid flow, reaction kinetics, separation processes, safety and economics. Chemistry therefore has both explanatory and practical functions, allowing scientists to understand existing matter and deliberately create substances with desired properties.
