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Chemistry is the science of matter and its interactions with energy (see physics , biology ). Because of the diversity of matter (which ismostly atomic), Chemists are often engaged in the pursuit of studying how atoms interactto form molecules , and how molecules interact with each other.


Branches of chemistry

Chemistry typically is divided into several main branches. There are also several main cross-disciplinary and more specializedbranches of chemistry.

Organic chemistry  
Organic chemistry is the science of the structure, properties,composition, and reactions of organic compounds .
Inorganic chemistry  
Inorganic chemistry is the branch of chemistry concerned with 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 thesub-discipline of organometallic chemistry .
Physical chemistry  
Physical chemistry is the study of the physical basis of chemical systems and processes. In particular, theenergetic description of diverse chemical transformations are of interest to physical chemists. Important areas of study include chemical thermodynamics , chemical kinetics , quantumchemistry , statistical mechanics , and spectroscopy .
Biochemistry is the study of the chemicals , chemical reactions and chemical interactions that take place in living organisms .
Analytical chemistry  
Analytical chemistry is the analysis of material samples to gain anunderstanding of their chemical composition and structure .
Other Branches 
Materials science , Polymer chemistry , Environmentalchemistry , Pharmacology , Thermochemistry , Nuclear chemistry , Electrochemistry , Computational Chemistry , Geochemistry , Supramolecular chemistry , Medicinal chemistry .

Fundamental concepts


Main article: Chemical nomenclature .

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


Main article: Atom .

An atom is a collection of matter consisting of a positively charged core (the nucleus ) which usually contains protons and neutrons , and which maintains a number of electrons to balance the positive charge in the nucleus.


Main article: Chemical element .

An element is a class of atoms which have 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 thechemical element carbon , and all atoms with 92 protons in their nuclei are atoms of theelement uranium .

The most convenient presentation of the elements is in the periodictable , 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 number of electrons surrounding the nucleus and their properties,and because the electrons are the outermost component of atoms (the component which 'sees' the rest of the universe), theidentity 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.


Main article: Chemical compound

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


Main article: Molecule .

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


Main article: Chemical bond .

A chemical bond is the force which holds together atoms in molecules or crystals . In many simplecompounds, valence bond theory and the concept of oxidation number can be used to predict molecular structure andcomposition. Similarly, theories from classical physics can beused to predict many ionic structures. With more complicated compounds, such as metal complexes , valence bond theory fails and a more thorough understanding based on quantum mechanics is necessary.

States of matter

Main article: Phase (matter) .

A phase is a set of states of a chemical system that have similar bulk structuralproperties, over a range of conditions, such as pressure or temperature . Physical properties, such as density and refractive index tend to fall withinvalues characteristic of the phase. The phase of matter is defined by the phasetransition, which is when energy put into or taken out of the system goes into rearranging the structure of the system,instead of changing the bulk conditions.

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

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 manydifferent phases, depending on the pressure and temperature of the system. While most familiar phases deal with three-dimensionalsystems, it is also possible to define analogs in two-dimensional systems, which is getting a lot of attention because of itsrelevance to biology .


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 formlarger molecules, molecules breaking apart to form two or more smaller molecules, or rearrangements of atoms within or across molecules. Chemical reactions usually involve the making or breaking of chemical bonds .

Quantum theory

Main article: Quantum theory .

Quantum theory describes the behavior of matter at short length scales. Itis, in principle, possible to describe all chemical systems using this theory, but it is mathematically complex and profoundly non-intuitive . In practice, only the simplest chemical systems may realisticallybe investigated in purely quantum mechanical terms, and approximations must be made for most practical purposes (e.g., Hartree-Fock or Density functional theory ). Hence a detailed understanding of quantum mechanics is not necessaryfor 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 that the Law of conservation of mass states that there is no detectable change in the quantity of matterduring an ordinary chemical reaction . Modern physics shows thatit 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. Proust 's lawof definite composition says that pure chemicals are composed of elements in a definite formulation; we now know thatthe structural arrangement of these elements are also important.

Dalton 's law of small whole numbers says that these chemicals will presentthemselves in proportions that are small whole numbers (i.e. 1:2 O:H in water); although for biomacromolecules and mineralchemistry 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, andare in a ratio defined by the intrinsic energy of the molecules -- the lower the intrinsic energy, the more abundant themolecule.
  • Transforming one structure to another requires the input of energy to cross an energy barrier; this can come from theintrinsic energy of the molecules themselves, or from an external source which will generally accelerate transformations. Thehigher 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 theenergy barrier. The HammondPostulate states that this structure looks most similar to the product or starting material which has intrinsic energyclosest to that of the energy barrier. Stabilizing this hypothetical intermediate through chemical interaction is one way toachieve catalysis .
  • All chemical processes are reversible (law of microscopic reversibility ) although some processes have such an energy bias, they areessentially irreversible.

History of chemistry


Old French: alkemie; Arab al-kimia: the art of transformation.

See also

External links and resources


Further reading

Chang, Raymond. Chemistry. 6th ed. Boston: James M. Smith, 1998. ISBN 0071152210 .


Analytical chemistry | Organic chemistry | Inorganic chemistry | Physicalchemistry | Polymer chemistry | Biochemistry | Materials science | Environmental chemistry | Pharmacy | Thermochemistry | Electrochemistry | Nuclear chemistry | Computationalchemistry
Periodic table | List of compounds

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