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Experimental work in progress in a chemistry laboratory.

Chemistry (from Egyptian kēme (chem), meaning "earth") is the science concerned with the composition, structure, and properties of matter, as well as the changes it undergoes during chemical reactions.[1] Historically, modern chemistry evolved out of alchemy following the chemical revolution (1773). Chemistry is a physical science related to studies of various atoms, molecules, crystals and other aggregates of matter whether in isolation or combination. Chemistry incorporates the concepts of energy and entropy in relation to the spontaneity of chemical processes.

Disciplines within chemistry are traditionally grouped by the type of matter being studied or the kind of study. These include inorganic chemistry, the study of inorganic matter; organic chemistry, the study of organic matter; biochemistry, the study of substances found in biological organisms; physical chemistry, the energy related studies of chemical systems at macro, molecular and submolecular scales; analytical chemistry, the analysis of material samples to gain an understanding of their chemical composition and structure. Many more specialized disciplines have emerged in recent years, e.g. neurochemistry, the study of the chemical dynamics of the brain.

Chemistry works extensively with multiple variations on the basic pattern of beneficial interactions between complementary, oppositely-charged pairs of entities. These include such representative cases as the positively charged protons and negatively charged electrons in an atom, the ions (some positively charged and others negatively charged) bound together forming crystals such as ordinary table salt, and the partially charged (positive) portions of a water molecule forming hydrogen bonds with partially charged (negative) portions of the DNA molecule.


Chemistry is the scientific study of the interaction of chemical substances,[2] which comprise often multiple copies of and/or combinations of atoms composed of protons, electrons and neutrons.[3] Atoms combine to produce ions, molecules or crystals. Chemistry can be called "the central science" because it connects the other natural sciences, such as astronomy, physics, material science, biology, and geology.[4][5]

The genesis of chemistry can be traced to certain practices, known as alchemy that were practiced for several millennia in various parts of the world, particularly the Middle East.[6]

The structure of objects we commonly use and the properties of the matter we commonly interact with, are a consequence of the properties of chemical substances and their interactions. For example, steel is harder than iron because its atoms are bound together in a more rigid crystalline lattice; wood burns or undergoes rapid oxidation because it can react spontaneously with oxygen in a chemical reaction above a certain temperature; sugar and salt dissolve in water because their molecular/ionic properties are such that dissolution is preferred under the ambient conditions.

Chemistry is a large field comprising many sub-disciplines that often overlap with significant portions of other sciences. The defining feature of chemistry is that it involves matter in some way, which may include the interaction of matter with non-material phenomenon, such as energy for example. More central to chemistry is the interaction of matter with other matter such as in the classic chemical reaction where chemical bonds are broken and made, forming new molecules.

Chemistry is in some ways physics on a larger scale and in some ways it is biology or geology on a smaller scale. Chemistry is used to understand and make better materials for engineering. It is used to understand the chemical mechanisms of disease as well as to create pharmaceuticals to treat disease. Chemistry is somehow involved in almost every science, every technology and every "thing."

Most chemists have a broad general knowledge of many areas of chemistry as well as one or two areas of specialization. Chemistry is divided into many areas of study called sub-disciplines in which chemists specialize. The chemistry taught at the high school or early college level is often called "general chemistry" and is intended to be an introduction to a wide variety of fundamental concepts and to give the student the tools to continue on to more advanced subjects. Many concepts presented at this level are often incomplete and technically inaccurate, yet of extraordinary utility. Chemists regularly use these simple, elegant tools and explanations in their work when they suffice because the best solution possible is often so overwhelmingly difficult and the true solution is usually unobtainable.

Presented below are summaries and links to other articles that contain knowledge on a wide variety of sub-disciplines, techniques, theories, and tools used in chemistry. Although a good knowledge of chemistry only comes with many years of study, you may find small bits of knowledge here that may be helpful.

Subdisciplines of chemistry

Chemistry typically is divided into several major sub-disciplines. There are also several main cross-disciplinary and more specialized fields of chemistry.

Analytical chemistry
Analytical chemistry is the analysis of material samples to gain an understanding of their chemical composition and structure.
Biochemistry is the study of the chemicals, chemical reactions, and chemical interactions that take place in living organisms.
Inorganic chemistry
Inorganic chemistry is the study of the properties and reactions of inorganic compounds. The distinction between organic and inorganic disciplines is not absolute and there is much overlap, most importantly in the sub-discipline of organometallic chemistry.
Organic chemistry
Organic chemistry is the study of the structure, properties, composition, mechanisms, and reactions of organic compounds.
Physical chemistry
Physical chemistry is the study of the physical basis of chemical systems and processes. In particular, the energetic description of diverse chemical transformations is of interest to physical chemists. Important areas of study include chemical thermodynamics, chemical kinetics, statistical mechanics, and spectroscopy. Physical chemistry has large overlap with molecular physics.
Theoretical chemistry
Theoretical chemistry is the study of chemistry via theoretical reasoning (usually within mathematics or physics). In particular the application of quantum mechanics to chemistry is called quantum chemistry. Since the end of the Second World War, the development of computers has allowed a systematic development of computational chemistry, which is the art of developing and applying computer programs for solving chemical problems. Theoretical chemistry has large overlap with molecular physics.
Other fields
Astrochemistry, atmospheric chemistry, chemical Engineering, electrochemistry, environmental chemistry, geochemistry, history of chemistry, materials science, medicinal chemistry, molecular biology, molecular genetics, nuclear chemistry, organometallic chemistry, petrochemistry, pharmacology, photochemistry, phytochemistry, polymer chemistry, supramolecular chemistry, surface chemistry, and thermochemistry.

Fundamental concepts


Nomenclature refers to the system for naming chemical compounds. There are well-defined systems in place for naming chemical species. Organic compounds are named according to the organic nomenclature system. Inorganic compounds are named according to the inorganic nomenclature system.

See also: IUPAC nomenclature


Main article: Atom.

Atoms are the fundamental units of chemistry as each of the chemical elements comprises one distinctive type of atom. An atom consists of a positively charged core (the nucleus) composed of protons and neutrons surrounded at a relatively great distance by a number of electrons to balance the positive charge in the nucleus.


Main article: Chemical element.

An element is a class of atoms having the same number of protons in the nucleus. This number is known as the atomic number of the element. For example, all atoms with 6 protons in their nuclei are atoms of the chemical element carbon, and all atoms with 92 protons in their nuclei are atoms of the element uranium.

The most convenient presentation of the elements is in the periodic table, which groups elements with similar chemical properties together. Lists of the elements by name, by symbol, and by atomic number are also available.

Because the number of protons in the nucleus dictates the maximum number of electrons (at most three more than the number of protons) surrounding the nucleus and their properties, and because the electrons are the outermost component of atoms (the component which presents a surface to the rest of the universe), the identity of an element dictates the interactions, or chemical transformations, in which it can participate. There may, however, be subtle changes in chemical properties brought about by the number of neutrons in the nucleus of otherwise "same" elements.

See also: isotope


Main article: Chemical compound

A compound is a substance with a fixed ratio of elements which determines the composition, and a particular organization which determines chemical properties. For example, water is a compound comprising hydrogen and oxygen in the ratio of two to one. Compounds are formed and interconverted by chemical reactions.


Main article: Molecule.

A molecule is the smallest indivisible portion of a pure compound that retains the unique chemical properties of that compound. A molecule comprises two or more atoms bonded together.


Main article: Ion.

An ion is a charged species of an atom or a molecule that has lost or gained an electron. Positively charged cations (e.g., sodium cation Na+) and negatively charged anions (e.g., chloride Cl-) build neutral salts (e.g., sodium chloride NaCl). Examples of polyatomic ions that do not split up during acid-base reactions are hydroxide (OH-), or phosphate (PO43-).


Main article: Chemical bond.

A chemical bond is the force that holds together atoms in molecules or crystals. In many simple compounds, valence bond theory and the concept of oxidation number can be used to predict molecular structure and composition. Similarly, theories from classical physics can be used to predict many ionic structures. With more complicated compounds, such as metal complexes, valence bond theory fails and alternative approaches based on quantum chemistry, such as molecular orbital theory, are necessary.

States of matter

Main article: Phase (matter).

A phase is a set of states of a chemical system that have similar bulk structural properties, over a range of conditions, such as pressure or temperature. Physical properties, such as density and refractive index tend to fall within values characteristic of the phase. The phases of matter are usually differentiated by the phase transition zones marking the boundaries between states of matter. At a phase transition zone, energy put into or taken out of the matter goes into rearranging the structure of the matter, instead of changing its bulk conditions.

Sometimes the distinction between phases can be continuous instead of having a discrete boundary. In this case the matter is considered to be in a supercritical state. When three phases meet based on these conditions, it is known as a triple point and since this is invariant, it is a convenient way to define a set of conditions.

The most familiar examples of phases are solids, liquids, and gases. Less familiar phases include plasmas, Bose-Einstein condensates and fermionic condensates, and the paramagnetic and ferromagnetic phases of magnetic materials. Even the familiar ice has many different phases depending on the pressure and temperature of the system. While most familiar phases deal with three-dimensional systems, it is also possible to define analogs in two-dimensional systems, which is getting a lot of attention because of its relevance to biology. In terms of total mass and volume occupied, plasma is by far the most common phase in the universe, as all stars and the interstellar and intergalactic media are plasma. In the cosmic sense the phases of matter familiar on earth are an anomaly.

Chemical Reactions

Main article: Chemical reaction.

Chemical reactions are transformations in the fine structure of molecules. Such reactions can result in molecules attaching to each other to form larger molecules, molecules breaking apart to form two or more smaller molecules, or rearrangement of atoms within or across molecules. Chemical reactions usually involve the making or breaking of chemical bonds.

Quantum chemistry

Main article: Quantum chemistry.

Quantum chemistry describes the behavior of matter at the molecular scale. It is, in principle, possible to describe all chemical systems using this theory. In practice, only the simplest chemical systems may realistically be investigated in purely quantum mechanical terms, and approximations must be made for most practical purposes (e.g., Hartree-Fock, post Hartree-Fock, or Density functional theory, see computational chemistry for more details). Hence a detailed understanding of quantum mechanics is not necessary for most chemistry, as the important implications of the theory (principally the orbital approximation) can be understood and applied in simpler terms.


The most fundamental concept in chemistry is the law of conservation of mass, which states that there is no detectable change in the quantity of matter during an ordinary chemical reaction. Modern physics shows that it is actually energy that is conserved, and that energy and mass are related; a concept which becomes important in nuclear chemistry. Conservation of energy leads to the important concepts of equilibrium, thermodynamics, and kinetics.

Further laws of chemistry elaborate on the law of conservation of mass. Joseph Proust's law of definite composition says that pure chemicals are composed of elements in a definite formulation; we now know that the structural arrangement of these elements is also important.

Dalton's law of multiple proportions says that these chemicals will present themselves in proportions that are small whole numbers (i.e., 1:2 O:H in water); although for biomacromolecules and mineral chemistry the ratios tend to require large numbers.

More modern laws of chemistry define the relationship between energy and transformations.

  • In equilibrium, molecules exist in mixture defined by the transformations possible on the timescale of the equilibrium, and are in a ratio defined by the intrinsic energy of the molecules—the lower the intrinsic energy, the more abundant the molecule.
  • Transforming one structure to another requires the input of energy to cross an energy barrier; this can come from the intrinsic energy of the molecules themselves, or from an external source that will generally accelerate transformations. The higher the energy barrier, the slower the transformation occurs.
  • There is a hypothetical intermediate, or transition structure, that corresponds to the structure at the top of the energy barrier. The Hammond-Leffler Postulate states that this structure looks most similar to the product or starting material that has intrinsic energy closest to that of the energy barrier. Stabilizing this hypothetical intermediate through chemical interaction is one way to achieve catalysis.
  • All chemical processes are in principle reversible (law of microscopic reversibility), although some processes have such an energy bias that they are essentially irreversible.

See also


  1. Chemistry. Merriam-Webster's Medical Dictionary. Retrieved March 27, 2017.
  2. John B. Russell, What is Chemistry? General Chemistry, McGraw-Hill International Book Company, 1980. Retrieved March 27, 2017.
  3. Anthony Carpi, Matter: Atoms from Democritus to Dalton Atomic Theory and Structure. Retrieved March 27, 2017.
  4. Theodore L. Brown, H. Eugene Lemay, Bruce Edward Bursten, H. Lemay, Chemistry: The Central Science (Prentice Hall, 1999, ISBN 0130103101), 3-4.
  5. It is sometimes called the central science because it is seen as occupying an intermediate position in a hierarchy of the sciences by "reductive level", between physics and biology. See Carsten Reinhardt, Chemical Sciences in the 20th Century: Bridging Boundaries (Wiley-VCH, 2001, ISBN 3527302719), 1-2.
  6. Michelle Feder, From Alchemy to Chemistry Kahn Academy. Retrieved March 27, 2017.

ISBN links support NWE through referral fees

  • Brown Jr., Theodore L., H. Eugene LeMay, Bruce Edward Bursten, and Julia R. Burdge. Chemistry: The Central Science. 9th ed. Upper Saddle River, NJ: Prentice Hall, 2002. ISBN 0130669970.
  • Chang, Raymond. Chemistry. 9th ed. New York, NY: McGraw-Hill Science/Engineering/Math, 2006. ISBN 0073221031.
  • Cotton, F. Albert, and Geoffrey Wilkinson. Advanced Inorganic Chemistry. 4th ed. New York, NY: Wiley, 1980. ISBN 0471027758.
  • Greenwood, N.N., and A. Earnshaw. Chemistry of the Elements. 2nd ed. Burlington, MA: Butterworth-Heinemann, Elsevier Science, 1998. ISBN 0750633654. Online version available here. Retrieved March 27, 2017.
  • McMurry, John. Organic Chemistry, 6th ed. Belmont, CA: Brooks/Cole, 2004. ISBN 0534420052.
  • Morrison, Robert T., and Robert N. Boyd. Organic Chemistry, 6th ed. Englewood Cliffs, NJ: Prentice Hall, 1992. ISBN 0136436692.
  • Reinhardt, Carsten. Chemical Sciences in the 20th Century: Bridging Boundaries. Wiley-VCH, 2001. ISBN 3527302719.
  • Solomons, T.W. Graham, and Craig B. Fryhle. Organic Chemistry, 8th ed. Hoboken, NJ: John Wiley, 2004. ISBN 0471417998.

External links

All links retrieved December 5, 2023.

General subfields within the Natural sciences
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