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'''Chemistry''' is the [[science]] of [[matter]] at the [[atom]]ic to [[molecular]] scale, dealing primarily with  the collections of atoms known as [[molecule]]s.  Chemists consider matter as made up of individual atoms or molecules, and use the known properties of these atoms or molecules to predict the outcome of interactions between different substances in bulk. These interactions always involve interactions between [[electrons]].These interactions always consist of making or breaking of semi-fixed bonds [[Covalent Bonds]] versus [[Ionic Bonds]] for molecules respectively crystals or entering a state where electro statical interaction plays a role [[Hydrogen bonding]]. Such interactions are called  [[chemical reactions]].These reactions include the influence of [[Electro Magnetic]] [[Radiation]] where that radiation affects the electrons belonging to atoms or molecules. Some chemical interactions can require energy, the substances must be mixed, and heat added as well, for a transformation to occur [[Endothermic Reactions]]. Other chemical reactions release energy, and these reactions are spontaneous if the substances are brought together in the proper ratios, or under the influence of the proper [[Catalyst]], [[Exothermic Reactions]], depending on their chemical properties. The formation of bonds between molecules   transforms one substance into another, such as the synthesis of water (H<sub>2</sub>O) from two gases, hydrogen (H<sub>2</sub>) and oxygen (O<sub>2</sub>).
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'''Chemistry''' is the [[science]] of materials. Chemists consider that all of the materials in the world are matter, primarily made up of [[atom]]s. The combination of at least two atoms connected by a chemical bond leads to molecules. [[Ion]]s are derived from atoms or molecules by loss or gain of one or more electrons leading to charged particles. [[Salt]]s are composed of [[ion|cations]] (positively charged ions) and [[ion|anions]] (negatively charged ions), so that the substance or material is neutrally charged (without net charge). Chemists use their view of [[matter]] at the [[atom]]ic to [[molecule|molecular]] level to explain how different materials interact, and how they change under varying conditions.  
Chemists can induce practical changes of substances, and make new compounds including [[drug]]s, explosives, [[cosmetic]]s and [[food]]s. Chemical synthesis of materials involves bringing together substances in bulk under conditions that they can interact to give different substances. These interactions are called [[chemical reaction|chemical reactions]], and always involve rearrangement of [[electrons]] around the reacting atoms of each molecule.
 
The ''formation of bonds'' between atoms or molecules means ''sharing electrons'' between the composite atoms, and this can transform one substance into another; such as the synthesis of [[water (molecule)|water]] (H<sub>2</sub>O) from two gases: [[hydrogen]] (H<sub>2</sub>) and [[oxygen]] (O<sub>2</sub>). Some chemical interactions require energy: the substances must be mixed and heated for a [[ chemical reaction]] to occur. These energy requiring reactions are called [[endothermic reaction]]s.  Whereas other chemical reactions release heat, and those energy releasing reactions are called [[exothermic reaction]]s. On a molecular level, reactions can also be initiated by the addition or removal of electrons using [[electromagnetic radiation]] ([[light]]).


== Introduction ==
== Introduction ==


[[Image:Lab bench.jpg|thumb|300px|Laboratory, Institute of Biochemistry, University of Cologne]]
[[Image:Lab bench.jpg|thumb|300px|Laboratory, Institute of Biochemistry, University of Cologne]]
Chemistry is about the electrical or electrostatical interactions of [[matter]]. These interactions might be between two substances, or between matter (electrons) and [[energy]], especially in conjunction with the [[First Law of Thermodynamics]]. Traditionally, chemistry involves interactions between the electrons of [[chemical substance|substances]] in chemical reactions, where one or more substances are changed into other substances. Sometimes these reactions are facilitated and enhanced in efficiency by a [[catalyst]], which might be another chemical substance (such as sulfuric acid catalyzing the [[electrolysis]] of water), or a non-material phenomenon (such as [[electromagnetic radiation]] in [[photochemistry|photochemical]] reactions). Traditional chemistry also deals with the analysis of chemicals both in and apart from a reaction, as in [[spectroscopy]].


Chemistry is often called the "central science" because it connects other sciences, such as [[physics]], [[material science]], [[nanotechnology]], [[biology]] [[pharmacy]], [[medicine]], [[bioinformatics]] and [[geology]].<ref>{{cite web | title = Chemistry - The Central Science | work = The Chemistry Hall of Fame | publisher = York University | url = http://www.chem.yorku.ca/hall_of_fame/whychem.htm | accessdate = 2006-09-12}}</ref> These connections are formed through various sub-disciplines that utilize concepts from multiple scientific disciplines. For example, [[physical chemistry]] involves applying the principles of [[physics]] to materials at the atomic and subatomic level.
Ordinary matter consists of [[atom]]s and the subatomic components that make up atoms; protons, electrons and neutrons. Atoms can be combined to produce more complex forms of matter such as [[ion]]s, [[molecule]]s or [[crystal]]s. The structure of the world we experience, and the properties of the matter we interact with, are determined by the properties of chemical substances and their interactions. [[Steel]] is harder than iron due to the incorporation of [[carbon]] resulting 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]].


Chemistry pertains to the electrical or electrostatical interactions of [[matter]]. These interactions may be between two material substances or between matter ie. electrons and [[energy]], especially in conjunction with the [[First Law of Thermodynamics]]. Traditional chemistry involves interactions between [[chemical substance|substances]] in [[chemical reaction]]s, where one or more substances become one or more other substances. Sometimes these reactions are driven by a [[catalyst]], which may be another chemical substance present at the reaction (such as sulfuric acid catalyzing the [[electrolysis]] of water) or a non-material phenomenon (such as [[electromagnetic radiation]] in [[photochemistry|photochemical]] reactions). Traditional chemistry also deals with the analysis of chemicals both in and apart from a reaction, as in [[spectroscopy]].
Substances tend to be classified in terms of their energy or phase as well as their chemical compositions. The three [[phases of matter]] are [[Solid]], [[Liquid]], and [[Gas]]. Solids at room temperature have low kinetic energy and are fixed structures which can resist gravity and other weak forces attempting to rearrange them, due to their tight bonds. Liquids have weaker bonds, with no structure, and they flow with gravity. Gases have no bonds and act as free particles. [[water (molecule)|Water]] (H<sub>2</sub>O) is a [[liquid]] at room temperature because its molecules are bound by intermolecular forces called [[hydrogen bond]]s. [[Hydrogen sulphide]] (H<sub>2</sub>S) on the other hand is a gas at room temperature and pressure, as its molecules are bound by weaker hydrogen bonds [[Intermolecular force#Dipole-dipole interactions|dipole-dipole interactions]]. Due to the higher [[electro negativity]] of oxygen compared to sulphur, the individual atoms in water tend to have higher charges than in hydrogen sulphide, leading to stronger hydrogen bonding in H<sub>2</sub>O than in H<sub>2</sub>S. Hydrogen bonds in water have enough energy to keep the water molecules from separating from each other but not enough to stop them from 'sliding around', making it a liquid at temperatures between [[Celsius|C]] and 100°C at sea level. Lowering the temperature or energy allows tighter organized bonds to form, creating a solid, and releasing energy. In this, water (H<sub>2</sub>O) behaves anomalously, as its largest density as a fluid is at 4 ˚C, not at its freezing point. Increasing the energy [[heat of fusion]] will melt the ice although the temperature will not change until all the ice is melted. Increasing the temperature of the water will cause boiling (see [[heat of vaporization]]) when there is enough energy to break the weak polar bonds at 100 °C, allowing the H<sub>2</sub>O molecules to disperse enough to be a gas. Note that in each case, energy is required to break the hydrogen bonds, as well as to to further separate the molecules from each other.
 
All ordinary matter consists of [[atom]]s or the subatomic components that make up atoms; protons, electrons and neutrons. Atoms may be combined to produce more complex forms of matter such as [[ion]]s, [[molecule]]s or [[crystal]]s. The structure of the world we commonly experience and the properties of the matter we commonly interact with are determined by properties of chemical substances and their interactions. [[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]].
 
Substances tend to be classified in terms of their energy or phase as well as their chemical compositions. The three [[phases of matter]] at low energy are [[Solid]], [[Liquid]] and [[Gas]]. [[Solids]] have fixed structures at room temperature which can resist gravity and other weak forces attempting to rearrange them, due to their tight bonds. [[Liquids]] have limited bonds, with no structure and flow with gravity. [[Gases]] have no bonds and act as free particles. [[Water]] (H<sub>2</sub>O) is a [[liquid]] at room temperature because its molecules are bound by intermolecular forces called [[Hydrogen bonds]]. [[Hydrogen sulfide]] (H<sub>2</sub>S) on the other hand is a gas at room temperature and pressure, as its molecules are bound by weaker [[Intermolecular force#Dipole-dipole interactions|dipole-dipole interactions]]. Due to the higher [[electro negativity]] of Oxygen compared to Sulphur the individual atoms in water tend to be on average more a carrier of charge than in hydrogen sulphide, leading to a more stronger Hydrogen bonding in H<sub>2</sub>O than in H<sub>2</sub>S. The Hydrogen bonds in water have enough energy to keep the water molecules from separating from each other but not from sliding around, making it a liquid at temperatures between 0 °[[Celsius|C]] and 100 °C at sea level. Lowering the temperature or energy further, allows tighter organized bonds to form, creating a solid, and releasing energy. In this water (H<sub>2</sub>O) has anomalous behavior as its largest density as a fluid is at 4 ˚ [[Celcius|C]], not at its freezing point. Increasing the energy [[heat of fusion]] will melt the ice although the temperature will not change until all the ice is melted. Increasing the temperature of the water will eventually cause boiling (see [[heat of vaporization]]) when there is enough energy to break the weak polar bonds at 100 °C, allowing the H<sub>2</sub>O molecules to disperse enough to be a [[gas]]. Note in each case, there is energy required to break the bonds, and more energy to move the molecules away from each other.
 
[[Scientists]] who study chemistry are known as [[chemists]]. Most chemists specialize in one or more sub-disciplines. 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 they are of extraordinary utility. Chemists regularly use these simple, elegant tools and explanations in their work because the best solution possible is often so overwhelmingly difficult and the true solution is usually unobtainable.
 
The science of chemistry is historically a recent development but has its roots in [[alchemy]] which has been practiced for [[millennia]] throughout the world. The word chemistry is directly derived from the word alchemy; however, the etymology of alchemy is unclear (see [[alchemy]]).


== History of chemistry ==
== History of chemistry ==
{{main|History of chemistry}}
{{main|History of chemistry}}


The roots of chemistry can be traced to the phenomenon of [[combustion|burning]]. [[Fire]] was a mystical force that transformed one substance into another and thus was of primary interest to mankind. It was fire that led to the discovery of [[iron]] and [[glass]]. After [[gold]] was discovered and became a precious metal, many people were interested to find a method that could convert other substances into gold. This led to the [[protoscience]] called [[Alchemy]]. Alchemists discovered many chemical [[processes]] that led to the development of modern chemistry. Chemistry as we know it today was invented by [[Antoine Lavoisier]] with his law of [[Conservation of mass]] in 1783. The [[discoveries of the chemical elements]] has a long history culminating in the creation of the [[periodic table]] of the chemical elements by [[Dmitri Mendeleev]]. The [[Nobel Prize in Chemistry]] created in 1901 gives an excellent overview of chemical discovery in the past 100 years.
The roots of chemistry can be traced to the phenomenon of [[combustion|burning]]. [[Fire]] was a mystical force that transformed one substance into another and was of primary interest to mankind. It was fire that led to the discovery of [[iron]] and [[glass]]. After [[gold]] was discovered, and became used as a precious metal, many people were interested to find a method that could convert other substances into gold. This led to the [[protoscience]] called [[Alchemy]]. Alchemists discovered many chemical [[processes]] that led to the development of modern chemistry. Chemistry as we know it today was invented by [[Antoine Lavoisier]] with his law of [[Conservation of mass]] in 1783. The [[discoveries of the chemical elements]] has a long history culminating in the creation of the [[periodic table of elements]] by [[Dmitri Mendeleev]]. The [[Nobel Prize in Chemistry]], created in 1901 gives an excellent overview of chemical discovery in the past 100 years.


The [[chemical industry]] represents an important economic activity. The global top 50 chemical producers in 2004 had sales of 587 billion [[dollars]] with a profit margin of 8.1% and [[research and development]] spending of 2.1% of total chemical sales.<ref>{{cite journal | title = Top 50 Chemical Producers | journal = [[Chemical & Engineering News]] | date = [[July 18]], [[2005]] | volume = 83 | issue = 29 | pages = 20–23 | url = http://pubs.acs.org/cen/coverstory/83/8329globaltop50.html}}</ref>
The [[chemical industry]] represents an important economic activity. The global top 50 chemical producers in 2004 had sales of 587 billion [[dollars]] with a profit margin of 8.1% and [[research and development]] spending of 2.1% of total chemical sales.<ref>{{cite journal | title = Top 50 Chemical Producers | journal = [[Chemical & Engineering News]] | date = [[July 18]], [[2005]] | volume = 83 | issue = 29 | pages = 20–23 | url = http://pubs.acs.org/cen/coverstory/83/8329globaltop50.html}}</ref>


== Subdisciplines of chemistry ==
== Subdisciplines of chemistry ==
[[Image:Pipetten.JPG|thumb|Lab [[pipette]]s]]
[[Image:Pipetten.JPG|thumb|Lab [[pipette]]s]]


Chemistry typically is divided into several major sub-disciplines. There are also several main cross-disciplinary and more specialized fields 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]] is the analysis of material samples to gain an understanding of their [[chemical composition]] and [[structure]]. Analytical chemistry incorporates standardized experimental methods in chemistry. These methods may be used in all subdiciplines of chemistry, excluding purely theoretical chemistry.
* [[Analytical chemistry]] is the analysis of material samples to gain an understanding of their [[chemical composition]] and structure. Analytical chemistry incorporates standardized experimental methods in chemistry. These methods may be used in all subdiciplines of chemistry, excluding purely theoretical chemistry.
 
* [[Biochemistry]] is the study of the [[chemical compound|chemicals]], chemical reactions and [[interaction]]s in living or semi-living (viruses) [[organism]]s. Biochemistry and organic chemistry are closely related, as in [[medicinal chemistry]], [[radiochemistry]] or [[neurochemistry]]. Biochemistry is also associated with [[molecular biology]] and [[genetics]].


* [[Biochemistry]] is the study of the [[chemical compound|chemicals]], [[chemical reaction]]s and chemical [[interaction]]s that take place in living or semi-living (virii) [[organism]]s. Biochemistry and organic chemistry are closely related, as in [[medicinal chemistry]], [[radiochemistry]] or [[neurochemistry]]. Biochemistry is also associated with [[molecular biology]] and [[genetics]].
* [[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]].


* [[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]], in most laborotories [[Analytical Chemistry]] is paired tp [[Inorganic Chemistry]] because of the great overlap they have.
* [[Macromolecular chemistry]] is the study of the physical, biological and chemical structure, properties, composition, mechanisms, and reactions of [[macromolecules]]. Often there is a link with [[Pharmacochemistry]] to engineer macromolecules with a certain function to help research in [[Health Sciences]] as well as [[microbiology]] and biochemistry.


* [[Organic chemistry]] is the study of the structure, properties, composition, mechanisms, and [[chemical reaction|reactions]] of [[organic compound]]s. An organic compound is formally defined as any compound bearing one or more covalent bonds between two or more carbon atoms.
* [[Organic chemistry]] is the study of the structure, properties, composition, mechanisms, and reactions of [[organic compound]]s. An organic compound is formally defined as any compound bearing one or more covalent bonds between two or more carbon atoms.


* [[Physical chemistry]] is the study of the physical and fundamental basis of chemical systems and processes. In particular, the energetics and dynamics of such systems and processes are of interest to physical chemists. Important areas of study include [[chemical thermodynamics]], [[chemical kinetics]], [[electrochemistry]], [[statistical mechanics]], and [[spectroscopy]], [[Macromolecular Chemistry]] and [[Magnetic Resonance]]. Physical chemistry has large overlap with [[molecular physics]]. Physical chemistry involves the use of [[calculus]] and [[Mathematics]] in deriving equations. It is usually associated with [[Quantum Mechanics|Chemistry]], [[Theoretical Chemistry]] and [[Computational Chemistry]].
* [[Physical chemistry]] is the study of the physical and fundamental basis of chemical systems and processes. In particular, the energetics and dynamics of such systems and processes are of interest to physical chemists. Important areas include [[chemical thermodynamics]], [[chemical kinetics]], [[electrochemistry]], [[statistical thermodynamics]], [[statistical mechanics]], and [[spectroscopy]], [[macromolecular chemistry]] and [[magnetic resonance]]. Physical chemistry largely overlaps with [[molecular physics]], and involves the use of [[calculus]] and [[Mathematics]] in deriving equations. It is usually associated with [[quantum chemistry]], [[theoretical chemistry]] and [[computational chemistry]].


* [[Theoretical chemistry]] is the study of chemistry via fundamental 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 program]]s for solving chemical problems. Theoretical chemistry has large overlap with (theoretical and experimental) [[condensed matter physics]] and [[molecular physics]]. Essentially from [[reductionism]] theoretical chemistry is just physical chemistry with a huge emphasize on physics, just like fundamental biology is just chemistry and physics.
* [[Theoretical chemistry]] is the study of chemistry by fundamental 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 program]]s for solving chemical problems. Theoretical chemistry overlaps with (theoretical and experimental) [[condensed matter physics]] and molecular physics.  


* [[Nuclear chemistry]] is the study of how subatomic particles come together and make nuclei. Modern [[Transmutation]] is a large component of nuclear chemistry, and the [[table of nuclides]] is an important result and tool for this field.
* [[Nuclear chemistry]] is the study of the chemistry of radioactive materials, the chemical effects of radiation and all chemistry associated with nuclear equipment and processes. Modern [[transmutation]] is a large component of nuclear chemistry, and the [[table of nuclides]] is an important result and tool for this field.


Other fields include [[Astrochemistry]], [[Atmospheric chemistry]], [[Chemical Engineering]], [[Chemo-informatics]], [[Electrochemistry]], [[Environmental chemistry]], [[Flow chemistry]] or Rheology, [[Geochemistry]], [[Green chemistry]], [[History of chemistry]], [[Materials science]], [[Medicinal chemistry]], [[Molecular Biology]], [[Molecular genetics]], [[Nanotechnology]], [[Organometallic chemistry]], [[Petrochemistry]], [[Pharmacology]], [[Photochemistry]], [[Phytochemistry]], [[Polymer chemistry]], [[Solid-state chemistry]], [[Sonochemistry]], [[Supramolecular chemistry]], [[Surface chemistry]], and [[Thermochemistry]].
Other fields include [[Astrochemistry]], [[Atmospheric chemistry]], [[Chemical Engineering]], [[Chemo-informatics]], [[Electrochemistry]], [[Environmental chemistry]], [[Flow chemistry]] or [[Rheology]], [[Geochemistry]], [[Green chemistry]], [[History of chemistry]], [[Materials science]], [[Medicinal chemistry]], [[Molecular Biology]], [[Molecular genetics]], [[Nanotechnology]], [[Organometallic chemistry]], [[Petrochemistry]], [[Pharmacology]], [[Photochemistry]], [[Phytochemistry]], [[Polymer chemistry]], [[Solid-state chemistry]], [[Sonochemistry]], [[Supramolecular chemistry]], [[Surface chemistry]], and [[Thermochemistry]].


== Fundamental concepts ==
== Fundamental concepts ==
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{{main|IUPAC nomenclature}}
{{main|IUPAC nomenclature}}


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


=== Atoms ===
=== Atoms ===
{{main|Atom}}
{{main|Atoms and Molecules}}
 
An ''atom'' is a collection of matter consisting of a positively charged core (the [[atomic nucleus]]) which contains [[protons]] and [[neutrons]], and which maintains a number of electrons to balance the positive charge in the nucleus.


An ''atom'' is a collection of matter consisting of a positively charged core (the [[atomic nucleus]]) which contains [[protons]] and [[neutrons]], and which maintains a number of [[electron]]s to balance the positive charge in the nucleus.
{|border="0" align="center"
|-
|valign=bottom |{{Image|Atom structure.gif|right|270px|A common concept of an atom's structure}}
|valign=bottom |{{Image|Atom-quantum.jpg|right|360px|Modern concept of the quantum atom}}
|}


=== Elements ===
=== Elements ===
{{main|Chemical element}}
{{main|Elements|Periodic table of elements}}


An ''element'' is a class of atoms which have the same number of [[proton]]s in the [[atomic nucleus|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]].
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 the element [[carbon]], and all atoms with 92 protons in their nuclei are atoms of [[uranium]]. The most convenient presentation of the chemical elements is in the [[periodic table of elements]], which groups elements by atomic number. Due to its ingenious arrangement, [[Periodic table group|groups]], or columns, and [[periods]], or rows, of elements in the table either share several chemical properties, or follow a certain trend in characteristics such as [[atomic radius]], [[electronegativity]], [[electron affinity]], and etc. Lists of the elements [[List of elements by name|by name]], [[List of elements by symbol|by symbol]], and by [[List of elements by number|atomic number]] are also available. In addition, several [[isotope]]s of an element may exist.
 
The most convenient presentation of the chemical elements is in the [[periodic table]] of the chemical elements, which groups elements by atomic number. Due to its ingenious arrangement, [[Periodic table group|groups]], or columns, and [[periods]], or rows, of elements in the table either share several chemical properties, or follow a certain trend in characteristics such as [[atomic radius]], [[electronegativity]], [[electron affinity]], and etc. Lists of the elements [[List of elements by name|by name]], [[List of elements by symbol|by symbol]], and by [[List of elements by number|atomic number]] are also available. In addition, several [[isotope]]s of an element may exist.


=== Ions ===
=== Ions ===
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{{main|Chemical compound}}
{{main|Chemical compound}}


A ''compound'' is a substance with a ''fixed ratio'' of [[chemical element]]s which determines the composition, and a particular organization which determines chemical properties. For example, [[water (molecule)|water]] is a compound containing [[hydrogen]] and [[oxygen]] in the ratio of two to one, with the oxygen between the hydrogens, and an angle of 104.5° between them. Compounds are formed and interconverted by [[chemical reaction]]s.
A ''compound'' is a substance with a ''fixed ratio'' of [[chemical element]]s which determines 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, with the oxygen between the hydrogens, and an angle of 104.5° between them. Compounds are formed and interconverted by chemical reactions.


=== Molecules ===
=== Molecules ===
{{main|Molecule}}
{{main|Molecule|Atoms and Molecules}}


A ''molecule'' is the smallest indivisible portion of a pure [[Chemical compound|compound]] or [[chemical element|element]] that retains a set of unique chemical properties.
 
A '''molecule''' is a combination of two or more [[atom]]s in a definite arrangement held together by [[chemical bond]]s. It is the smallest indivisible portion of a pure compound or element that retains a set of unique chemical properties.


=== Substance ===
=== Substance ===
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=== Bonding ===
=== Bonding ===
[[Image:Electron orbitals.png|right|thumb|350px|[[Electron]] atomic and [[molecular orbital|molecular]] orbitals]]
[[Image:Electron orbitals.png|right|thumb|350px|[[Electron]] atomic and [[molecular orbital|molecular]] orbitals]]


{{main|Chemical bond}}
{{main|Chemical bond}}


A ''chemical bond'' is the [[multipole]] balance between the positive charges in the nuclei and the negative charges oscillating about them. More than simple attraction and repulsion, the energies and distributions characterize the availability of an electron to bond to another atom. These potentials create the [[interaction]]s which holds together [[atom]]s in [[molecule]]s or [[crystal]]s. In many simple compounds, [[Valence Bond Theory]], the Valence Shell Electron Pair Repulsion model ([[VSEPR]]), 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 [[complex (chemistry)|metal complexes]], valence bond theory fails and alternative approaches, primarily based on principles of [[quantum chemistry]] such as the [[molecular orbital]] theory, are necessary.
A ''chemical bond'' is the [[multipole]] balance between the positive charges in the nuclei and the negative charges oscillating about them. More than simple attraction and repulsion, the energies and distributions characterize the availability of an electron to bond to another atom. These potentials create the [[interaction]]s which holds together atoms in molecules or crystals. In many simple compounds, [[Valence Bond Theory]], the Valence Shell Electron Pair Repulsion model ([[VSEPR]]), 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 [[complex (chemistry)|metal complexes]], valence bond theory fails and alternative approaches, primarily based on principles of quantum chemistry such as the [[molecular orbital]] theory, are necessary.
See diagram on electronic orbitals.
See diagram on electronic orbitals.


=== States of matter ===
=== States of matter ===
{{main|Phase (matter)}}
{{main|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 phase of matter is defined by the ''[[phase transition]]'', 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.


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 phase of matter is defined by the ''[[phase transition]]'', 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 is considered to be in a [[Supercritical fluid|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 of 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 fluid|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 of conditions.


The most familiar examples of phases are [[solid]]s, [[liquid]]s, and [[gas]]es. Less familiar phases include [[Plasma physics|plasmas]], [[Bose-Einstein condensate]]s and [[fermionic condensate]]s and the [[paramagnetism|paramagnetic]] and [[ferromagnetism|ferromagnetic]] phases of [[magnet]]ic 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 has received attention for its relevance to systems in [[biology]].
The most familiar examples of phases are [[solid]]s, [[liquid]]s, and [[gas]]es. Less familiar phases include [[Plasma physics|plasmas]], [[Bose-Einstein condensate]]s and [[fermionic condensate]]s and the [[paramagnetism|paramagnetic]] and [[ferromagnetism|ferromagnetic]] phases of [[magnet]]ic 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 has received attention for its relevance to systems in [[biology]].


=== Chemical reactions ===
=== Chemical reactions ===
{{main|Chemical reaction}}
{{main|Chemical reaction}}
 
A ''Chemical reaction'' is a process that results in the interconversion of [[chemical substance]]s.  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 [[atom]]s within or across molecules. Chemical reactions usually involve the making or breaking of [[chemical bond]]s. For example, substances that react with oxygen to produce other substances are said to undergo [[oxidation]]; similarly a group of substances called [[acid]]s or [[alkali]]s can react with one another to neutralize each other's  effect, a phenomenon known as [[neutralization]]. Substances can also be [[chemical decomposition|dissociated]] or synthesized from other substances by various different chemical [[processes]].
A ''Chemical reaction'' is a process that results in the interconversion of [[chemical substance]]s.  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 [[atom]]s within or across molecules. Chemical reactions usually involve the making or breaking of [[chemical bond]]s. For example, substances that react with oxygen to produce other substances are said to undergo [[oxidation]]; similarly a group of substances called [[acid]]s or [[alkali]]s can react with one another to neutralize each other's  effect, a phenomenon known as [[neutralization]]. [[chemical substance|Substances]] can also be [[chemical decomposition|dissociated]] or synthesized from other substances by various different chemical [[processes]].


=== Quantum chemistry ===
=== Quantum chemistry ===
{{main|Quantum chemistry}}
{{main|Quantum chemistry}}
''Quantum chemistry'' mathematically describes the fundamental behavior of [[matter]] at the [[molecule|molecular]] scale. It is, in principle, possible to describe all chemical systems using this theory. In practice, only the most simple chemical systems may qualitatively be investigated in purely [[quantum mechanics|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 primarily necessary for most chemistry, as the important implications of the theory (principally the [[atomic orbital|orbital approximation]]) can be understood and applied in simpler terms.


''Quantum chemistry'' mathematically describes the fundamental behavior of [[matter]] at the [[molecule|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 mechanics|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 [[atomic orbital|orbital approximation]]) can be understood and applied in simpler terms.
In quantum mechanics (several applications in computational chemistry and quantum chemistry), the [[Hamiltonian (quantum mechanics)|Hamiltonian]], or the physical state, of a particle can be expressed as the sum of two operators, one corresponding to [[kinetic energy]] and the other to [[potential energy]]. The Hamiltonian of a particle with no [[electric charge]] and no [[spin (physics)|spin]] is described by the [[Schrödinger wave equation]] of a particle without forces being exerted on it.
 
In quantum mechanics (several applications in computational chemistry and quantum chemistry), the [[Hamiltonian (quantum mechanics)|Hamiltonian]], or the physical state, of a particle can be expressed as the sum of two operators, one corresponding to [[kinetic energy]] and the other to [[potential energy]], [[vibrational energy]] or [[rotational energy]]. The Hamiltonian of a particle with no [[electric charge]] and no [[spin (physics)|spin]] is described by the [[Schrödinger wave equation]].


Solutions of the Schrödinger equation for the hydrogen atom gives the form of the wave function for [[atomic orbital]]s, and the relative energy of say the 1s,2s,2p and 3s orbitals. The orbital approximation can be used to understand the other atoms e.g. [[helium]], [[lithium]] and [[carbon]].
Solutions of the Schrödinger equation for the hydrogen atom gives the form of the wave function for [[atomic orbital]]s, and the relative energy of say the 1s,2s,2p and 3s orbitals. The orbital approximation can be extended to understand the other atoms e.g. [[helium]], [[lithium]] and [[carbon]].


=== Chemical Laws ===
=== Chemical Laws ===
{{main|Chemical law}}
{{main|Chemical law}}


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 equivalent or [[Mass-energy equivalence|related]]; a concept which becomes important in [[nuclear chemistry]]. [[Conservation of energy]] leads to the important concepts of [[Chemical equilibrium|equilibrium]], [[thermodynamics]], and [[chemical kinetics|kinetics]].
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 equivalent or [[Mass-energy equivalence|related]]; a concept which becomes important in [[nuclear chemistry]]. [[Conservation of energy]] leads to the important concepts of [[Chemical equilibrium|equilibrium]], [[thermodynamics]], and [[chemical kinetics|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.
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.
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: ''Main article: [[Alchemy#Etymology|Etymology of alchemy]]''
: ''Main article: [[Alchemy#Etymology|Etymology of alchemy]]''


The word ''chemistry'' comes from the earlier study of [[alchemy]], from the old French ''alkemie;'' and the Arabic ''al-kimia:'' "the art of transformation."  An alchemist was called a 'chemist' in popular speech, and later the suffix "-ry" was added to this to describe the art of the chemist as "chemistry".
The word ''chemistry'' comes from the earlier study of alchemy, from the old French ''alkemie;'' and the Arabic ''al-kimia:'' "the art of transformation."  An alchemist was called a 'chemist' in popular speech, and later the suffix "-ry" was added to this to describe the art of the chemist as "chemistry".


''Alchemy'' in turn is thought to possibly derive either from [[Greek language|Greek]] word ''chemeia'' (χημεία) meaning "cast together", "pour together", "weld", "alloy", etc. (from ''khumatos'', "that which is poured out, an ingot"), or from the Coptic name for Egypt ''kēme'', or alternately, from [[Persian language|Persian]] ''Kimia'' meaning "gold".
''Alchemy'' in turn is thought to possibly derive either from [[Greek language|Greek]] word ''chemeia'' (χημεία) meaning "cast together", "pour together", "weld", "alloy", etc. (from ''khumatos'', "that which is poured out, an ingot"), or from the Coptic name for Egypt ''kēme'', or alternately, from [[Persian language|Persian]] ''Kimia'' meaning "gold".
== See also ==
* [[Common chemicals]] - Where to find common chemical components
* [[List of chemistry topics]]
* [[List of chemists]]
* [[List of compounds]]
* [[List of publications in chemistry|List of important publications in chemistry]]
* [[Perfection#Perfection in physics and chemistry|Perfection]] (''"Perfection in physics and chemistry"'')
* [[Royal Society of Chemistry]]
* [[Unsolved problems in chemistry]]
* [[Periodic Table of the Elements]]
== External links ==
{{sisterlinks|Chemistry}}
{{wikibookspar|Wikiversity|School of Chemistry}}
{{portal}}
* [http://chemie-wereld.uwstart.nl Chemie-Wereld: Startpage (chemistry-world)]
* [http://ocw.mit.edu/OcwWeb/Chemistry/index.htm MIT OpenCourseWare | Chemistry]
* [http://www.chemportal.sk/ Chemportal - The navigator for the slovak and czech chemical industry (slovak)]
* [http://wikidchem.org/ Wikidchem, The Free Chemistry Archive Wiki]
* [http://www.allchemicals.info/ Chemical Glossary]
* http://www.eurochem.eu/ [[EuroChem]] (European Portal for Chemistry - Database of Hazard Compounds, European Legislation)
* [http://www.webqc.org/ Chemical Portal]
* [http://chem.sis.nlm.nih.gov/chemidplus/ Chemistry Information Database]
* [http://www.chemicalforums.com/ Chemistry Forum]
* [http://www.chemistryguide.org/ Chemistry Guide]
* [http://www.chemistry-conferences.com/ Chemistry Conferences]
* [http://www.sciencedaily.com/news/matter_energy/chemistry/ Chemistry Research]
* [http://www.iupac.org/dhtml_home.html International Union of Pure and Applied Chemistry]
* [http://www.chem.qmw.ac.uk/iupac/ IUPAC Nomenclature Home Page], see especially the "Gold Book" containing definitions of standard chemical terms
* [http://www.cci.ethz.ch/index.html Experiments] videos and photos of the techniques and results
* [http://81.207.88.128/science/ More experiments] - lots of information about the elements too.
* [http://physchem.ox.ac.uk/MSDS/ Material safety data sheets for a variety of chemicals]
* [http://www.flinnsci.com/search_MSDS.asp Material Safety Data Sheets]
* [http://www.chemicalblogs.com Chemical Blogs]
For a full list of external links and suppliers see [[Wikipedia:Chemical sources]]


== References ==
== References ==
<references/>
<references/>
== Further reading ==
* Chang, Raymond. ''Chemistry'' 6th ed. Boston: James M. Smith, 1998. ISBN 0-07-115221-0.
* Pauling, L. ''The Nature of the chemical bond'' (Cornell University Press) ISBN 0-8014-0333-2
* Pauling, L., and Wilson, E. B. ''Introduction to Quantum Mechanics with Applications to Chemistry'' (Dover Publications) ISBN 0-486-64871-0
* Pauling, L. ''General Chemistry'' (Dover Publications) ISBN 0-486-65622-5
* Atkins, P.W. ''Galileo's Finger'' (Oxford University Press)
== Reading list for university students ==
* Atkins,P.W. ''Physical Chemistry'' (Oxford University Press) ISBN 0-19-879285-9
* Atkins,P.W. et al. ''Molecular Quantum Mechanics'' (Oxford University Press)
* McWeeny, R. ''Coulson's Valence'' (Oxford Science Publications) ISBN 0-19-855144-4
* Stephenson, G. ''Mathematical Methods for Science Students'' (Longman)ISBN 0-582-44416-0
* Smart and Moore ''Solid State Chemistry: An Introduction'' (Chapman and Hall) ISBN 0-412-40040-5
* Atkins,P.W., Overton,T., Rourke,J., Weller,M. and Armstrong,F. ''Shriver and Atkins inorganic chemistry''(4th edition) 2006(Oxford University Press) ISBN 0-19-926463-5
* Clayden,J., Greeves,N., Warren,S., Wothers,P. ''Organic Chemistry'' 2000 (Oxford University Press) ISBN 0-19-850346-6
* Voet and Voet ''Biochemistry'' (Wiley) ISBN 0-471-58651-X
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Chemistry is the science of materials. Chemists consider that all of the materials in the world are matter, primarily made up of atoms. The combination of at least two atoms connected by a chemical bond leads to molecules. Ions are derived from atoms or molecules by loss or gain of one or more electrons leading to charged particles. Salts are composed of cations (positively charged ions) and anions (negatively charged ions), so that the substance or material is neutrally charged (without net charge). Chemists use their view of matter at the atomic to molecular level to explain how different materials interact, and how they change under varying conditions. Chemists can induce practical changes of substances, and make new compounds including drugs, explosives, cosmetics and foods. Chemical synthesis of materials involves bringing together substances in bulk under conditions that they can interact to give different substances. These interactions are called chemical reactions, and always involve rearrangement of electrons around the reacting atoms of each molecule.

The formation of bonds between atoms or molecules means sharing electrons between the composite atoms, and this can transform one substance into another; such as the synthesis of water (H2O) from two gases: hydrogen (H2) and oxygen (O2). Some chemical interactions require energy: the substances must be mixed and heated for a chemical reaction to occur. These energy requiring reactions are called endothermic reactions. Whereas other chemical reactions release heat, and those energy releasing reactions are called exothermic reactions. On a molecular level, reactions can also be initiated by the addition or removal of electrons using electromagnetic radiation (light).

Introduction

Laboratory, Institute of Biochemistry, University of Cologne

Chemistry is about the electrical or electrostatical interactions of matter. These interactions might be between two substances, or between matter (electrons) and energy, especially in conjunction with the First Law of Thermodynamics. Traditionally, chemistry involves interactions between the electrons of substances in chemical reactions, where one or more substances are changed into other substances. Sometimes these reactions are facilitated and enhanced in efficiency by a catalyst, which might be another chemical substance (such as sulfuric acid catalyzing the electrolysis of water), or a non-material phenomenon (such as electromagnetic radiation in photochemical reactions). Traditional chemistry also deals with the analysis of chemicals both in and apart from a reaction, as in spectroscopy.

Ordinary matter consists of atoms and the subatomic components that make up atoms; protons, electrons and neutrons. Atoms can be combined to produce more complex forms of matter such as ions, molecules or crystals. The structure of the world we experience, and the properties of the matter we interact with, are determined by the properties of chemical substances and their interactions. Steel is harder than iron due to the incorporation of carbon resulting 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.

Substances tend to be classified in terms of their energy or phase as well as their chemical compositions. The three phases of matter are Solid, Liquid, and Gas. Solids at room temperature have low kinetic energy and are fixed structures which can resist gravity and other weak forces attempting to rearrange them, due to their tight bonds. Liquids have weaker bonds, with no structure, and they flow with gravity. Gases have no bonds and act as free particles. Water (H2O) is a liquid at room temperature because its molecules are bound by intermolecular forces called hydrogen bonds. Hydrogen sulphide (H2S) on the other hand is a gas at room temperature and pressure, as its molecules are bound by weaker hydrogen bonds dipole-dipole interactions. Due to the higher electro negativity of oxygen compared to sulphur, the individual atoms in water tend to have higher charges than in hydrogen sulphide, leading to stronger hydrogen bonding in H2O than in H2S. Hydrogen bonds in water have enough energy to keep the water molecules from separating from each other but not enough to stop them from 'sliding around', making it a liquid at temperatures between 0°C and 100°C at sea level. Lowering the temperature or energy allows tighter organized bonds to form, creating a solid, and releasing energy. In this, water (H2O) behaves anomalously, as its largest density as a fluid is at 4 ˚C, not at its freezing point. Increasing the energy heat of fusion will melt the ice although the temperature will not change until all the ice is melted. Increasing the temperature of the water will cause boiling (see heat of vaporization) when there is enough energy to break the weak polar bonds at 100 °C, allowing the H2O molecules to disperse enough to be a gas. Note that in each case, energy is required to break the hydrogen bonds, as well as to to further separate the molecules from each other.

History of chemistry

For more information, see: History of chemistry.


The roots of chemistry can be traced to the phenomenon of burning. Fire was a mystical force that transformed one substance into another and was of primary interest to mankind. It was fire that led to the discovery of iron and glass. After gold was discovered, and became used as a precious metal, many people were interested to find a method that could convert other substances into gold. This led to the protoscience called Alchemy. Alchemists discovered many chemical processes that led to the development of modern chemistry. Chemistry as we know it today was invented by Antoine Lavoisier with his law of Conservation of mass in 1783. The discoveries of the chemical elements has a long history culminating in the creation of the periodic table of elements by Dmitri Mendeleev. The Nobel Prize in Chemistry, created in 1901 gives an excellent overview of chemical discovery in the past 100 years.

The chemical industry represents an important economic activity. The global top 50 chemical producers in 2004 had sales of 587 billion dollars with a profit margin of 8.1% and research and development spending of 2.1% of total chemical sales.[1]

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 is the analysis of material samples to gain an understanding of their chemical composition and structure. Analytical chemistry incorporates standardized experimental methods in chemistry. These methods may be used in all subdiciplines of chemistry, excluding purely theoretical 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 is the study of the structure, properties, composition, mechanisms, and reactions of organic compounds. An organic compound is formally defined as any compound bearing one or more covalent bonds between two or more carbon atoms.
  • Nuclear chemistry is the study of the chemistry of radioactive materials, the chemical effects of radiation and all chemistry associated with nuclear equipment and processes. Modern transmutation is a large component of nuclear chemistry, and the table of nuclides is an important result and tool for this field.

Other fields include Astrochemistry, Atmospheric chemistry, Chemical Engineering, Chemo-informatics, Electrochemistry, Environmental chemistry, Flow chemistry or Rheology, Geochemistry, Green chemistry, History of chemistry, Materials science, Medicinal chemistry, Molecular Biology, Molecular genetics, Nanotechnology, Organometallic chemistry, Petrochemistry, Pharmacology, Photochemistry, Phytochemistry, Polymer chemistry, Solid-state chemistry, Sonochemistry, Supramolecular chemistry, Surface chemistry, and Thermochemistry.

Fundamental concepts

Nomenclature

For more information, see: IUPAC nomenclature.


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.

Atoms

For more information, see: Atoms and Molecules.


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

(PD) Drawing: Lawrence Berkeley National Laboratory
A common concept of an atom's structure
© Drawing: Courtesy of HowStuffWorks.com
Modern concept of the quantum atom

Elements

For more information, see: Elements and Periodic table of elements.


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 the element carbon, and all atoms with 92 protons in their nuclei are atoms of uranium. The most convenient presentation of the chemical elements is in the periodic table of elements, which groups elements by atomic number. Due to its ingenious arrangement, groups, or columns, and periods, or rows, of elements in the table either share several chemical properties, or follow a certain trend in characteristics such as atomic radius, electronegativity, electron affinity, and etc. Lists of the elements by name, by symbol, and by atomic number are also available. In addition, several isotopes of an element may exist.

Ions

For more information, see: Ion.


An ion is a charged species, or an atom or a molecule that has lost or gained one or more electrons. Positively charged cations (e.g. sodium cation Na+) and negatively charged anions (e.g. chloride Cl-) can form 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-).

Compounds

For more information, see: Chemical compound.


A compound is a substance with a fixed ratio of chemical elements which determines 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, with the oxygen between the hydrogens, and an angle of 104.5° between them. Compounds are formed and interconverted by chemical reactions.

Molecules

For more information, see: Molecule and Atoms and Molecules.


A molecule is a combination of two or more atoms in a definite arrangement held together by chemical bonds. It is the smallest indivisible portion of a pure compound or element that retains a set of unique chemical properties.

Substance

For more information, see: Chemical substance.


A chemical substance can be an element, compound or a mixture of compounds, elements or compounds and elements. Most of the matter we encounter in our daily life are one or another kind of mixtures, e.g. air, alloys, biomass etc.

Bonding

Electron atomic and molecular orbitals
For more information, see: Chemical bond.


A chemical bond is the multipole balance between the positive charges in the nuclei and the negative charges oscillating about them. More than simple attraction and repulsion, the energies and distributions characterize the availability of an electron to bond to another atom. These potentials create the interactions which holds together atoms in molecules or crystals. In many simple compounds, Valence Bond Theory, the Valence Shell Electron Pair Repulsion model (VSEPR), 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, primarily based on principles of quantum chemistry such as the molecular orbital theory, are necessary. See diagram on electronic orbitals.

States of matter

For more information, see: 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 phase of matter is defined by the phase transition, 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 is considered 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 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 has received attention for its relevance to systems in biology.

Chemical reactions

For more information, see: Chemical reaction.

A Chemical reaction is a process that results in the interconversion of chemical substances. 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. For example, substances that react with oxygen to produce other substances are said to undergo oxidation; similarly a group of substances called acids or alkalis can react with one another to neutralize each other's effect, a phenomenon known as neutralization. Substances can also be dissociated or synthesized from other substances by various different chemical processes.

Quantum chemistry

For more information, see: Quantum chemistry.

Quantum chemistry mathematically describes the fundamental behavior of matter at the molecular scale. It is, in principle, possible to describe all chemical systems using this theory. In practice, only the most simple chemical systems may qualitatively 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 primarily necessary for most chemistry, as the important implications of the theory (principally the orbital approximation) can be understood and applied in simpler terms.

In quantum mechanics (several applications in computational chemistry and quantum chemistry), the Hamiltonian, or the physical state, of a particle can be expressed as the sum of two operators, one corresponding to kinetic energy and the other to potential energy. The Hamiltonian of a particle with no electric charge and no spin is described by the Schrödinger wave equation of a particle without forces being exerted on it.

Solutions of the Schrödinger equation for the hydrogen atom gives the form of the wave function for atomic orbitals, and the relative energy of say the 1s,2s,2p and 3s orbitals. The orbital approximation can be extended to understand the other atoms e.g. helium, lithium and carbon.

Chemical Laws

For more information, see: Chemical law.


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 equivalent or 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 in many systems (notably biomacromolecules and minerals) the ratios tend to require large numbers, and are frequently represented as a fraction. Such compounds are known as non-stoichiometric compounds.

Etymology

Main article: Etymology of alchemy

The word chemistry comes from the earlier study of alchemy, from the old French alkemie; and the Arabic al-kimia: "the art of transformation." An alchemist was called a 'chemist' in popular speech, and later the suffix "-ry" was added to this to describe the art of the chemist as "chemistry".

Alchemy in turn is thought to possibly derive either from Greek word chemeia (χημεία) meaning "cast together", "pour together", "weld", "alloy", etc. (from khumatos, "that which is poured out, an ingot"), or from the Coptic name for Egypt kēme, or alternately, from Persian Kimia meaning "gold".

References