Chemistry is the natural science that studies substances, both elements and compounds. It looks at what they are made of, how they are built and what properties they have. It also studies the reactions that change one substance into another and the energy released or absorbed during those changes. Chemistry is often called "the central science" because it sits between physics and biology. It uses physical laws to explain the behaviour of matter, and it gives the life sciences, earth sciences and engineering a way to talk about matter at the molecular level. Chemistry treats the atom as its basic building block. Most of the behaviour of the atomic nucleus and of subatomic particles belongs to physics.
Fundamental concepts
Chemists describe matter in terms of atoms, molecules and charged particles. The particles are held together by chemical bonds: covalent bonds, ionic bonds and metallic bonds. All of these come from the way atoms share or transfer their outer electrons. An atom's chemical identity depends on how many protons it has. Elements are arranged by this number in the periodic table, which shows repeating patterns in how the elements behave. The table now holds 118 confirmed elements. The International Union of Pure and Applied Chemistry (IUPAC) gave final approval to the names of the four most recent ones (nihonium, moscovium, tennessine and oganesson) in 2016, which completed the table's seventh row.
In a chemical reaction, atoms are rearranged but never created or destroyed. This is the principle of conservation of mass. Stoichiometry uses it to work out how much of each substance takes part in a reaction. Whether a reaction can happen depends on thermodynamics, through quantities such as enthalpy, entropy and free energy. How fast it happens is the subject of chemical kinetics. Catalysts speed up reactions without being used up and are central to both biology and industry. Other key ideas include states of matter, solutions, acid–base reactions, oxidation–reduction (redox) reactions and chemical equilibrium.
History
People used chemical processes long before chemistry was a science. Smelting metals, making pottery and glass, dyeing, fermenting and preparing medicines were all practised in early civilizations, including ancient Egypt. Greek thinkers offered theories of matter. Democritus proposed indivisible atoms, while Aristotle taught a theory of four elements that dominated thinking for centuries.
The tradition of alchemy grew up separately in the Hellenistic world, China, the Islamic world and later Europe. It mixed practical laboratory work with philosophical and mystical goals, such as turning base metals into gold and making elixirs to prolong life. Chinese experiments eventually led to gunpowder. Writings attributed to Jabir ibn Hayyan, from the period of the Abbasid Caliphate, described distillation, crystallization, sublimation and evaporation, and grouped substances by their properties. The word "chemistry" comes from "alchemy", which in turn comes from the Arabic al-kīmiyā.
Chemistry became a modern science in the late 18th century. Antoine Lavoisier showed how important oxygen is in combustion and helped disprove the phlogiston theory. He also stated the law of conservation of mass, helped create a systematic way of naming chemicals and in 1789 published a major textbook. Lavoisier insisted on careful weighing, which turned chemistry into a quantitative science. Early in the 19th century, John Dalton proposed an atomic theory based on the fixed weight ratios in which elements combine. In 1828 Friedrich Wöhler made urea, an organic compound, from inorganic starting materials. This weakened vitalism, the idea that organic substances could only be made by living things. In 1869 Dmitri Mendeleev arranged the roughly sixty elements then known into a periodic system and left gaps for elements that had not yet been discovered.
In the 20th century, the discovery of the electron and the development of quantum mechanics gave chemical bonding a physical explanation. They also explained how atoms and molecules interact with light, which made spectroscopy possible. Synthesis grew far more powerful, and chemists learned to make dyes, drugs, plastics and, eventually, superheavy elements that last only fractions of a second.
Branches
Chemistry is traditionally divided into five main branches:
- Organic chemistry studies carbon compounds, including most fuels, plastics, drugs and the molecules of life.
- Inorganic chemistry covers all other elements and compounds, such as metals, minerals, salts and coordination complexes.
- Physical chemistry uses physics, including thermodynamics, kinetics and quantum theory, to explain how chemical systems behave.
- Analytical chemistry develops methods for finding out what is in a sample and how much of each component it contains.
- Biochemistry studies the chemical processes inside living organisms.
There are also many specialized and cross-disciplinary fields, including theoretical and computational chemistry, polymer chemistry, materials chemistry, medicinal chemistry, environmental chemistry, geochemistry, electrochemistry and nuclear chemistry.
Methods
Chemistry is mainly an experimental science, and it follows the general approach of forming hypotheses and testing them in controlled experiments. Synthetic chemists plan step-by-step routes to build target molecules. Separation methods such as distillation, crystallization and chromatography are used to purify substances. Several techniques reveal structure: nuclear magnetic resonance (NMR) spectroscopy, mass spectrometry, infrared and ultraviolet–visible spectroscopy, and X-ray crystallography. Computational chemistry uses approximate solutions of quantum-mechanical equations and molecular simulations to predict structures, properties and reaction pathways. Machine-learning methods are now used more and more to support this work.
Applications and impact
Chemical knowledge is the basis of a large industrial sector. Chemical engineering turns laboratory chemistry into processes that work at industrial scale. The Haber process fixes nitrogen from the air as ammonia and underpins modern fertilizer production. Synthetic polymers gave rise to plastics, synthetic fibres and rubbers. Pharmaceutical chemistry discovers and manufactures drugs. Materials chemistry provides the semiconductors used in electronics, as well as battery materials and porous solids such as metal–organic frameworks. Chemistry is also important in food processing, water treatment, energy production and conservation.
Chemical industries have also created problems, including pollution, toxic waste, ozone depletion and greenhouse gas emissions. These problems have led to environmental regulation and to the development of green chemistry, which aims to design products and processes that use and produce fewer hazardous substances. Chemistry is also needed to understand and address these problems.
References
- Concept and branches of chemistrybritannica.com
- Chemistry Definition, History & Branches - Lessonstudy.com
- Chemistry The Study Of Matterbyjus.com
- Confirmation of four new elements completes seventh row of periodic tablechemistryworld.com
- Names for elements 113, 115, 117, and 118 finalized by IUPACcen.acs.org
- Naming of chemical elementsen.wikipedia.org
- Chemistryinters.org
- History of chemistry facts for kidskids.kiddle.co
- The Most Important Discoveries in Chemistry Historysciencenewstoday.org