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A chemical formula is a way of presenting information about the chemical proportions of
atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of atom ...

atom
s that constitute a particular
chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entity, molecular entities) composed of atoms from more than one chemical element, element held together by chemical bonds. A homonuclear molecule, m ...
or molecule, using
chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical elements cannot be broken down into simp ...
symbols, numbers, and sometimes also other symbols, such as parentheses, dashes, brackets, commas and ''plus'' (+) and ''minus'' (−) signs. These are limited to a single typographic line of symbols, which may include subscripts and superscripts. A chemical formula is not a
chemical nameA chemical nomenclature is a set of rules to generate systematic names for chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entity, molecular entities) composed of atoms from more t ...
, and it contains no words. Although a chemical formula may imply certain simple chemical structures, it is not the same as a full chemical
structural formula The structural formula of a chemical compound is a graphic representation of the molecular structure (determined by structural chemistry methods), showing how the atoms are possibly arranged in the real three-dimensional space. The chemical bondi ...
. Chemical formulae can fully specify the structure of only the simplest of
molecule A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecule is an electrically Electricity is the set of physical phenomena associated with the presence and motion I ...

molecule
s and
chemical substance A chemical substance is a form of matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of atoms, which ...
s, and are generally more limited in power than chemical names and structural formulae. The simplest types of chemical formulae are called ''
empirical formula In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: their composition, structure, properties, behavior and the changes they underg ...
e'', which use letters and numbers indicating the numerical ''proportions'' of atoms of each type. Molecular formulae indicate the simple numbers of each type of atom in a molecule, with no information on structure. For example, the empirical formula for
glucose Glucose is a simple with the . Glucose is the most abundant , a subcategory of s. Glucose is mainly made by and most during from water and carbon dioxide, using energy from sunlight, where it is used to make in s, the most abundant carbohydr ...

glucose
is CH2O (twice as many
hydrogen Hydrogen is the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same ...

hydrogen
atoms as
carbon Carbon (from la, carbo "coal") is a with the C and 6. It is lic and —making four s available to form s. It belongs to group 14 of the periodic table. Carbon makes up only about 0.025 percent of Earth's crust. Three occur naturally, ...

carbon
and
oxygen Oxygen is the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same ...

oxygen
), while its molecular formula is C6H12O6 (12 hydrogen atoms, six carbon and oxygen atoms). Sometimes a chemical formula is complicated by being written as a
condensed formula The structural formula of a chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entity, molecular entities) composed of atoms from more than one chemical element, element held togeth ...
(or condensed molecular formula, occasionally called a "semi-structural formula"), which conveys additional information about the particular ways in which the atoms are chemically bonded together, either in
covalent bond A covalent bond is a chemical bond that involves the sharing of electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs, and the stable balance of attractive and repulsive forces between atoms, when they s ...
s,
ionic bond Ionic bonding is a type of chemical bond A chemical bond is a lasting attraction between atoms, ions or molecules that enables the formation of chemical compounds. The bond may result from the Coulomb's law, electrostatic force of attraction be ...
s, or various combinations of these types. This is possible if the relevant bonding is easy to show in one dimension. An example is the condensed molecular/chemical formula for
ethanol Ethanol (also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic Organic may refer to: * Organic, of or relating to an organism, a living entity * Organic, of or relating to an anatomical organ (anatomy), ...

ethanol
, which is CH3-CH2-OH or CH3CH2OH. However, even a condensed chemical formula is necessarily limited in its ability to show complex bonding relationships between atoms, especially atoms that have bonds to four or more different
substituent A substituent is one or a group of atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that ca ...
s. Since a chemical formula must be expressed as a single line of chemical element symbols, it often cannot be as informative as a true structural formula, which is a graphical representation of the spatial relationship between atoms in chemical compounds (see for example the figure for butane structural and chemical formulae, at right). For reasons of structural complexity, a single condensed chemical formula (or semi-structural formula) may correspond to different molecules, known as
isomer In chemistry, isomers are molecules or polyatomic ions with identical molecular formulas — that is, same number of atoms of each element (chemistry), element — but distinct arrangements of atoms in space. Isomerism is existence or possibil ...

isomer
s. For example glucose shares its
molecular formula A chemical formula is a way of presenting information about the chemical proportions of atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes ...
C6H12O6 with a number of other
sugar Sugar is the generic name for sweet-tasting, soluble carbohydrate is a disaccharide A disaccharide (also called a double sugar or ''biose'') is the sugar formed when two monosaccharides are joined by glycosidic linkage. Like monosacc ...

sugar
s, including
fructose Fructose, or fruit sugar, is a ketonic simple sugar Monosaccharides (from Greek language, Greek ''wikt:μόνος, monos'': single, ''sacchar'': sugar), also called simple sugars, are the simplest form of sugar and the most basic units (monomers ...

fructose
,
galactose Galactose (, '' galacto-'' + ''-ose The suffix In linguistics Linguistics is the science, scientific study of language. It encompasses the analysis of every aspect of language, as well as the methods for studying and modeling them. ...
and
mannose Mannose is a sugar monomer of the hexose, aldohexose series of carbohydrates. It is a C-2 epimer of glucose. Mannose is important in human metabolism, especially in the glycosylation of certain proteins. Several Congenital disorder of glycosylati ...

mannose
. Linear equivalent chemical ''names'' exist that can and do specify uniquely any complex structural formula (see
chemical nomenclature A chemical nomenclature is a set of rules to generate systematic nameA systematic name is a name given in a systematic way to one unique group, organism, object or chemical substance A chemical substance is a form of matter In classical ph ...
), but such names must use many terms (words), rather than the simple element symbols, numbers, and simple typographical symbols that define a chemical formula. Chemical formulae may be used in
chemical equation A chemical equation is the symbolic representation of a chemical reaction A chemical reaction is a process that leads to the chemical transformation of one set of chemical substance A chemical substance is a form of matter In classical ...
s to describe
chemical reaction A chemical reaction is a process that leads to the chemical transformation of one set of chemical substance A chemical substance is a form of matter In classical physics and general chemistry, matter is any substance that has mass and t ...

chemical reaction
s and other chemical transformations, such as the dissolving of ionic compounds into solution. While, as noted, chemical formulae do not have the full power of structural formulae to show chemical relationships between atoms, they are sufficient to keep track of numbers of atoms and numbers of electrical charges in chemical reactions, thus balancing chemical equations so that these equations can be used in chemical problems involving conservation of atoms, and conservation of electric charge.


Overview

A chemical formula identifies each constituent
element Element may refer to: Science * Chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all ...
by its
chemical symbol Chemical symbols are the abbreviations used in chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, beh ...
and indicates the proportionate number of atoms of each element. In empirical formulae, these proportions begin with a key element and then assign numbers of atoms of the other elements in the compound, by ratios to the key element. For molecular compounds, these ratio numbers can all be expressed as whole numbers. For example, the empirical formula of
ethanol Ethanol (also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic Organic may refer to: * Organic, of or relating to an organism, a living entity * Organic, of or relating to an anatomical organ (anatomy), ...

ethanol
may be written C2H6O because the molecules of ethanol all contain two carbon atoms, six hydrogen atoms, and one oxygen atom. Some types of ionic compounds, however, cannot be written with entirely whole-number empirical formulae. An example is
boron carbide Boron carbide (chemical formula approximately B4C) is an extremely hard boron Boron is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, a ...
, whose formula of CBn is a variable non-whole number ratio with n ranging from over 4 to more than 6.5. When the chemical compound of the formula consists of simple
molecule A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecule is an electrically Electricity is the set of physical phenomena associated with the presence and motion I ...

molecule
s, chemical formulae often employ ways to suggest the structure of the molecule. These types of formulae are variously known as ''molecular formulae'' and '' condensed formulae''. A molecular formula enumerates the number of atoms to reflect those in the molecule, so that the molecular formula for
glucose Glucose is a simple with the . Glucose is the most abundant , a subcategory of s. Glucose is mainly made by and most during from water and carbon dioxide, using energy from sunlight, where it is used to make in s, the most abundant carbohydr ...

glucose
is C6H12O6 rather than the glucose empirical formula, which is CH2O. However, except for very simple substances, molecular chemical formulae lack needed structural information, and are ambiguous. For simple molecules, a condensed (or semi-structural) formula is a type of chemical formula that may fully imply a correct structural formula. For example,
ethanol Ethanol (also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic Organic may refer to: * Organic, of or relating to an organism, a living entity * Organic, of or relating to an anatomical organ (anatomy), ...

ethanol
may be represented by the condensed chemical formula CH3CH2OH, and
dimethyl ether Dimethyl ether (DME, also known as methoxymethane) is the organic compound , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen chemical bond, bonds. D ...
by the condensed formula CH3OCH3. These two molecules have the same empirical and molecular formulae (C2H6O), but may be differentiated by the condensed formulae shown, which are sufficient to represent the full structure of these simple
organic compound In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior and the changes they undergo during ...
s. Condensed chemical formulae may also be used to represent
ionic compound structure of sodium chloride, NaCl, a typical ionic compound. The purple spheres represent sodium cations, Na+, and the green spheres represent chloride anions, Cl−. The yellow stipples show the electrostatic forces. In chemistry, an ionic compo ...
s that do not exist as discrete molecules, but nonetheless do contain covalently bound clusters within them. These
polyatomic ion A polyatomic ion, also known as a molecular ion, is a covalently bonded A covalent bond is a chemical bond A chemical bond is a lasting attraction between atoms, ions or molecules that enables the formation of chemical compounds. The b ...
s are groups of atoms that are covalently bound together and have an overall ionic charge, such as the sulfate ion. Each polyatomic ion in a compound is written individually in order to illustrate the separate groupings. For example, the compound
dichlorine hexoxide Dichlorine hexoxide is the chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entity, molecular entities) composed of atoms from more than one chemical element, element held together ...

dichlorine hexoxide
has an empirical formula , and molecular formula , but in liquid or solid forms, this compound is more correctly shown by an ionic condensed formula , which illustrates that this compound consists of ions and ions. In such cases, the condensed formula only need be complex enough to show at least one of each ionic species. Chemical formulae as described here are distinct from the far more complex chemical systematic names that are used in various systems of
chemical nomenclature A chemical nomenclature is a set of rules to generate systematic nameA systematic name is a name given in a systematic way to one unique group, organism, object or chemical substance A chemical substance is a form of matter In classical ph ...
. For example, one systematic name for glucose is (2''R'',3''S'',4''R'',5''R'')-2,3,4,5,6-pentahydroxyhexanal. This name, interpreted by the rules behind it, fully specifies glucose's structural formula, but the name is not a chemical formula as usually understood, and uses terms and words not used in chemical formulae. Such names, unlike basic formulae, may be able to represent full structural formulae without graphs.


Empirical formula

In
chemistry Chemistry is the scientific Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity or awareness, of someone or something, such as facts A fact is an occurrence in the real world. T ...

chemistry
, the
empirical formula In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: their composition, structure, properties, behavior and the changes they underg ...
of a chemical is a simple expression of the relative number of each type of atom or ratio of the elements in the compound. Empirical formulae are the standard for
ionic compound structure of sodium chloride, NaCl, a typical ionic compound. The purple spheres represent sodium cations, Na+, and the green spheres represent chloride anions, Cl−. The yellow stipples show the electrostatic forces. In chemistry, an ionic compo ...
s, such as , and for macromolecules, such as . An empirical formula makes no reference to
isomer In chemistry, isomers are molecules or polyatomic ions with identical molecular formulas — that is, same number of atoms of each element (chemistry), element — but distinct arrangements of atoms in space. Isomerism is existence or possibil ...

isomer
ism, structure, or absolute number of atoms. The term ''empirical'' refers to the process of
elemental analysis Elemental analysis is a process where a sample of some material (e.g., soil, waste or drinking water, bodily fluids, minerals In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "dis ...
, a technique of
analytical chemistry Analytical chemistry studies and uses instruments and methods used to separate, identify, and quantify matter. In practice, separation, identification or quantification may constitute the entire analysis or be combined with another method. Sepa ...
used to determine the relative percent composition of a pure chemical substance by element. For example,
hexane Hexane () is an organic compound, a straight-chain alkane with six carbon atoms and has the molecular formula C6H14. Hexane is a significant constituent of gasoline. It is a colorless liquid, odorless when pure, and with boiling points approxim ...

hexane
has a molecular formula of , or structurally , implying that it has a chain structure of 6
carbon Carbon (from la, carbo "coal") is a with the C and 6. It is lic and —making four s available to form s. It belongs to group 14 of the periodic table. Carbon makes up only about 0.025 percent of Earth's crust. Three occur naturally, ...

carbon
atoms, and 14
hydrogen Hydrogen is the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same ...

hydrogen
atoms. However, the empirical formula for hexane is . Likewise the empirical formula for
hydrogen peroxide Hydrogen peroxide is a chemical compound A chemical compound is a chemical substance A chemical substance is a form of matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by hav ...

hydrogen peroxide
, , is simply HO expressing the 1:1 ratio of component elements.
Formaldehyde Formaldehyde ( , also ) (systematic nameA systematic name is a name given in a systematic way to one unique group, organism, object or chemical substance A chemical substance is a form of matter In classical physics and general chemist ...
and
acetic acid Acetic acid , systematically named ethanoic acid , is a colourless liquid organic compound with the chemical formula CH3COOH (also written as CH3CO2H, C2H4O2, or HC2H3O2). Vinegar is no less than 4% acetic acid by volume, making acetic acid ...

acetic acid
have the same empirical formula, . This is the actual chemical formula for formaldehyde, but acetic acid has double the number of atoms.


Molecular formula

Molecular formulae indicate the simple numbers of each type of atom in a molecule of a molecular substance. They are the same as empirical formulae for molecules that only have one atom of a particular type, but otherwise may have larger numbers. An example of the difference is the empirical formula for glucose, which is CH2O (''ratio'' 1:2:1), while its molecular formula is C6H12O6 (''number of atoms'' 6:12:6). For water, both formulae are H2O. A molecular formula provides more information about a molecule than its empirical formula, but is more difficult to establish. A molecular formula shows the number of elements in a molecule, and determines whether it is a
binary compound In materials chemistry, a binary phase or binary compound is a chemical compound containing two different elements. Some binary phase compounds are molecular, e.g. carbon tetrachloride (CCl4). More typically binary phase refers to extended solids ...
,
ternary compound Ternary (from Latin ''ternarius'') is an adjective meaning "composed of three items". It can refer to: Mathematics and logic * Ternary numeral system, a base-3 counting system ** Balanced ternary, a positional numeral system, useful for compar ...
,
quaternary compound In chemistry, a quaternary compound is a compound consisting of exactly four chemical elements. In another use of the term in organic chemistry, a quaternary compound is or has a cation consisting of a central positively charged atom with four sub ...
, or has even more elements.


Condensed formula

The connectivity of a molecule often has a strong influence on its physical and chemical properties and behavior. Two molecules composed of the same numbers of the same types of atoms (i.e. a pair of
isomer In chemistry, isomers are molecules or polyatomic ions with identical molecular formulas — that is, same number of atoms of each element (chemistry), element — but distinct arrangements of atoms in space. Isomerism is existence or possibil ...

isomer
s) might have completely different chemical and/or physical properties if the atoms are connected differently or in different positions. In such cases, a
structural formula The structural formula of a chemical compound is a graphic representation of the molecular structure (determined by structural chemistry methods), showing how the atoms are possibly arranged in the real three-dimensional space. The chemical bondi ...
is useful, as it illustrates which atoms are bonded to which other ones. From the connectivity, it is often possible to deduce the approximate shape of the molecule. A condensed chemical formula may represent the types and spatial arrangement of bonds in a simple chemical substance, though it does not necessarily specify
isomer In chemistry, isomers are molecules or polyatomic ions with identical molecular formulas — that is, same number of atoms of each element (chemistry), element — but distinct arrangements of atoms in space. Isomerism is existence or possibil ...

isomer
s or complex structures. For example,
ethane Ethane ( or ) is an Organic compound, organic chemical compound with chemical formula . At Standard conditions for temperature and pressure, standard temperature and pressure, ethane is a colorless, odourless gas. Like many hydrocarbons, ethane ...
consists of two carbon atoms single-bonded to each other, with each carbon atom having three hydrogen atoms bonded to it. Its chemical formula can be rendered as CH3CH3. In
ethylene Ethylene (IUPAC The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations that represents chemists in individual countries. It is a member of the International Science C ...

ethylene
there is a double bond between the carbon atoms (and thus each carbon only has two hydrogens), therefore the chemical formula may be written: CH2CH2, and the fact that there is a double bond between the carbons is implicit because carbon has a valence of four. However, a more explicit method is to write H2C=CH2 or less commonly H2C::CH2. The two lines (or two pairs of dots) indicate that a
double bond In chemistry, a double bond is a between two s involving four s as opposed to two in a . Double bonds occur most commonly between two carbon atoms, for example in s. Many double bonds exist between two different elements: for example, in a group ...

double bond
connects the atoms on either side of them. A
triple bond A triple bond in chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior and the changes they un ...

triple bond
may be expressed with three lines (HC≡CH) or three pairs of dots (HC:::CH), and if there may be ambiguity, a single line or pair of dots may be used to indicate a single bond. Molecules with multiple
functional group In organic chemistry, a functional group is a substituent or moiety (chemistry), moiety in a molecule that causes the molecule's characteristic chemical reactions. The same functional group will undergo the same or similar chemical reactions re ...
s that are the same may be expressed by enclosing the repeated group in round brackets. For example,
isobutane Isobutane, also known as ''i''-butane, 2-methylpropane or methylpropane, is a chemical compound A chemical compound is a chemical substance A chemical substance is a form of matter In classical physics and general chemistry, matter is a ...

isobutane
may be written (CH3)3CH. This condensed structural formula implies a different connectivity from other molecules that can be formed using the same atoms in the same proportions (
isomers In chemistry, isomers are molecules or polyatomic ions with identical molecular formulas — that is, same number of atoms of each element (chemistry), element — but distinct arrangements of atoms in space. Isomerism is existence or possibili ...

isomers
). The formula (CH3)3CH implies a central carbon atom connected to one hydrogen atom and three
CH3 groups
CH<sub>3</sub> groups
. The same number of atoms of each element (10 hydrogens and 4 carbons, or C4H10) may be used to make a straight chain molecule, ''n''-
butane Butane () or ''n''-butane is an alkane with the formula C4H10. Butane is a gas at room temperature and atmospheric pressure. Butane is a highly flammable, colorless, easily liquefy, liquefied gas that quickly vaporizes at room temperature. The n ...

butane
: CH3CH2CH2CH3.


Law of composition

In any given chemical compound, the elements always combine in the same proportion with each other. This is the
law of constant composition In chemistry, the law of definite proportion, sometimes called Proust's law, or law of constant composition states that a given chemical compound always contains its component elements in fixed ratio (by mass) and does not depend on its source and ...
. The law of constant composition says that, in any particular chemical compound, all samples of that compound will be made up of the same elements in the same proportion or ratio. For example, any water molecule is always made up of two hydrogen atoms and one oxygen atom in a 2:1 ratio. If we look at the relative masses of oxygen and hydrogen in a water molecule, we see that 94% of the mass of a water molecule is accounted for by oxygen and the remaining 6% is the mass of hydrogen. This mass proportion will be the same for any water molecule.


Chemical names in answer to limitations of chemical formulae

The alkene called but-2-ene has two isomers, which the chemical formula CH3CH=CHCH3 does not identify. The relative position of the two methyl groups must be indicated by additional notation denoting whether the methyl groups are on the same side of the double bond (''cis'' or ''Z'') or on the opposite sides from each other (''trans'' or ''E''). As noted above, in order to represent the full structural formulae of many complex organic and inorganic compounds,
chemical nomenclature A chemical nomenclature is a set of rules to generate systematic nameA systematic name is a name given in a systematic way to one unique group, organism, object or chemical substance A chemical substance is a form of matter In classical ph ...
may be needed which goes well beyond the available resources used above in simple condensed formulae. See
IUPAC nomenclature of organic chemistry In chemical nomenclature A chemical nomenclature is a set of rules to generate systematic nameA systematic name is a name given in a systematic way to one unique group, organism, object or chemical substance A chemical substance is a form of ...
and
IUPAC nomenclature of inorganic chemistry 2005 Nomenclature of Inorganic Chemistry, IUPAC Recommendations 2005 is the 2005 version of '' Nomenclature of Inorganic Chemistry'' (which is informally called the Red Book). It is a collection of rules for naming inorganic compounds, as recommended by ...
for examples. In addition, linear naming systems such as
International Chemical Identifier The IUPAC The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations that represents chemists in individual countries. It is a member of the International Science Council (I ...
(InChI) allow a computer to construct a structural formula, and
simplified molecular-input line-entry system : break cycles, then write as branches off a main backbone The simplified molecular-input line-entry system (SMILES) is a specification in the form of a line notation for describing the structure of chemical species A chemical substance is a ...
(SMILES) allows a more human-readable ASCII input. However, all these nomenclature systems go beyond the standards of chemical formulae, and technically are chemical naming systems, not formula systems.


Polymers in condensed formulae

For
polymer A polymer (; Greek ''poly- Poly, from the Greek :wikt:πολύς, πολύς meaning "many" or "much", may refer to: Businesses * China Poly Group Corporation, a Chinese business group, and its subsidiaries: ** Poly Property, a Hong Kong inc ...

polymer
s in condensed chemical formulae, parentheses are placed around the repeating unit. For example, a
hydrocarbon In organic chemistry Organic chemistry is a branch of chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, prop ...
molecule that is described as CH3(CH2)50CH3, is a molecule with fifty repeating units. If the number of repeating units is unknown or variable, the letter ''n'' may be used to indicate this formula: CH3(CH2)''n''CH3.


Ions in condensed formulae

For
ion An ion () is an atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ...
s, the charge on a particular atom may be denoted with a right-hand superscript. For example, Na+, or Cu2+. The total charge on a charged molecule or a
polyatomic ion A polyatomic ion, also known as a molecular ion, is a covalently bonded A covalent bond is a chemical bond A chemical bond is a lasting attraction between atoms, ions or molecules that enables the formation of chemical compounds. The b ...
may also be shown in this way. For example:
H3O+
H<sub>3</sub>O<sup>+</sup>
or
SO42−
SO<sub>4</sub><sup>2−</sup>
. Note that + and - are used in place of +1 and -1, respectively. For more complex ions, brackets are often used to enclose the ionic formula, as in 12H12sup>2−, which is found in compounds such as Cs2 12H12 Parentheses ( ) can be nested inside brackets to indicate a repeating unit, as in o(NH3)6sup>3+Cl3. Here, (NH3)6 indicates that the ion contains six NH3 groups bonded to
cobalt Cobalt is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical element ...

cobalt
, and encloses the entire formula of the ion with charge +3. This is strictly optional; a chemical formula is valid with or without ionization information, and Hexamminecobalt(III) chloride may be written as o(NH3)6sup>3+Cl3 or o(NH3)6l3. Brackets, like parentheses, behave in chemistry as they do in mathematics, grouping terms together they are not specifically employed only for ionization states. In the latter case here, the parentheses indicate 6 groups all of the same shape, bonded to another group of size 1 (the cobalt atom), and then the entire bundle, as a group, is bonded to 3 chlorine atoms. In the former case, it is clearer that the bond connecting the chlorines is
ionic Ionic or Ionian may refer to: Arts and entertainment * Ionic meter, a poetic metre in ancient Greek and Latin poetry * Ionian mode, a musical mode or a diatonic scale Places and peoples * Ionian, of or from Ionia, an ancient region in western An ...

ionic
, rather than
covalent A covalent bond is a chemical bond A chemical bond is a lasting attraction between atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and take ...
.


Isotopes

Although
isotope Isotopes are two or more types of atoms that have the same atomic number 300px, The Rutherford–Bohr model of the hydrogen atom () or a hydrogen-like ion (). In this model it is an essential feature that the photon energy (or frequency) of ...
s are more relevant to
nuclear chemistry Nuclear chemistry is the sub-field of chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make up matter to the chemical compo ...
or
stable isotope The term stable isotope has a meaning similar to stable nuclide Stable nuclides are nuclide A nuclide (or nucleide, from nucleus, also known as nuclear species) is a class of atoms characterized by their number of proton A proton is a subat ...
chemistry than to conventional chemistry, different isotopes may be indicated with a prefixed
superscript A subscript or superscript is a character (such as a number or letter) that is set slightly below or above the normal line of type, respectively. It is usually smaller than the rest of the text. Subscripts appear at or below the baseline, whil ...

superscript
in a chemical formula. For example, the phosphate ion containing radioactive phosphorus-32 is [32PO4]3−. Also a study involving stable isotope ratios might include the molecule 18O16O. A left-hand subscript is sometimes used redundantly to indicate the
atomic number The atomic number or proton number (symbol ''Z'') of a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. ...
. For example, 8O2 for dioxygen, and for the most abundant isotopic species of dioxygen. This is convenient when writing equations for
nuclear reaction In nuclear physics and nuclear chemistry, a nuclear reaction is a process in which two atomic nucleus, nuclei, or a nucleus and an external subatomic particle, collide to produce one or more new nuclides. Thus, a nuclear reaction must cause a t ...
s, in order to show the balance of charge more clearly.


Trapped atoms

The @ symbol (
at sign The at sign, , is normally read aloud as "at"; it is also commonly called the at symbol, commercial at or address sign. It is used as an accounting Accounting or Accountancy is the measurement ' Measurement is the number, numerical quantif ...

at sign
) indicates an atom or molecule trapped inside a cage but not chemically bound to it. For example, a
buckminsterfullerene Buckminsterfullerene is a type of fullerene with the formula C60. It has a cage-like fused-ring structure (truncated icosahedron) that resembles a Ball (association football), soccer ball, made of twenty hexagons and twelve pentagons. Each carbo ...

buckminsterfullerene
(C60) with an atom (M) would simply be represented as MC60 regardless of whether M was inside the fullerene without chemical bonding or outside, bound to one of the carbon atoms. Using the @ symbol, this would be denoted M@C60 if M was inside the carbon network. A non-fullerene example is s@Ni12As20sup>3−, an ion in which one As atom is trapped in a cage formed by the other 32 atoms. This notation was proposed in 1991 with the discovery of
fullerene A fullerene is an allotrope of carbon whose molecule consists of carbon Carbon (from la, carbo "coal") is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical el ...

fullerene
cages (
endohedral fullerene Rendering of a buckminsterfullerene containing a noble gas atom (M@C60). Endohedral fullerenes, also called endofullerenes, are fullerene A fullerene is an allotrope of carbon whose molecule consists of carbon Carbon (from la, carbo ...

endohedral fullerene
s), which can trap atoms such as
La
La
to form, for example, La@C60 or La@C82. The choice of the symbol has been explained by the authors as being concise, readily printed and transmitted electronically (the at sign is included in
ASCII ASCII ( ), abbreviated from American Standard Code for Information Interchange, is a character encoding Character encoding is the process of assigning numbers to graphical Graphics (from Greek Greek may refer to: Greece Anything of, ...
, which most modern character encoding schemes are based on), and the visual aspects suggesting the structure of an endohedral fullerene.


Non-stoichiometric chemical formulae

Chemical formulae most often use
integer An integer (from the Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''communicare'', meaning "to share" or "to ...
s for each element. However, there is a class of compounds, called
non-stoichiometric compound Non-stoichiometric compounds are chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entity, molecular entities) composed of atoms from more than one chemical element, element held t ...
s, that cannot be represented by small integers. Such a formula might be written using
decimal fraction The decimal numeral system A numeral system (or system of numeration) is a writing system A writing system is a method of visually representing verbal communication Communication (from Latin ''communicare'', meaning "to share") is t ...
s, as in Fe0.95O, or it might include a variable part represented by a letter, as in Fe1–xO, where x is normally much less than 1.


General forms for organic compounds

A chemical formula used for a series of compounds that differ from each other by a constant unit is called a ''general formula''. It generates a
homologous seriesIn organic chemistry, a homologous series is a sequence In mathematics, a sequence is an enumerated collection of objects in which repetitions are allowed and order theory, order matters. Like a Set (mathematics), set, it contains Element (mathemati ...
of chemical formulae. For example,
alcohols In chemistry, alcohol is an organic compound that carries at least one hydroxyl functional group (−OH) bound to a Saturated and unsaturated compounds, saturated carbon atom. The term alcohol originally referred to the primary alcohol ethan ...

alcohols
may be represented by the formula C''n''H(2n + 1)OH (''n'' ≥ 1), giving the homologs
methanol Methanol, also known as methyl alcohol, amongst other names, is a chemical A chemical substance is a form of matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. ...

methanol
,
ethanol Ethanol (also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic Organic may refer to: * Organic, of or relating to an organism, a living entity * Organic, of or relating to an anatomical organ (anatomy), ...

ethanol
,
propanol There are two isomer In , isomers are s or s with identical s — that is, same number of of each — but distinct arrangements of atoms in space. Isomerism is existence or possibility of isomers. Isomers do not necessarily share similar ...

propanol
for ''n''=1–3.


Hill system

The Hill system (or Hill notation) is a system of writing empirical chemical formulae, molecular chemical formulae and components of a condensed formula such that the number of
carbon Carbon (from la, carbo "coal") is a with the C and 6. It is lic and —making four s available to form s. It belongs to group 14 of the periodic table. Carbon makes up only about 0.025 percent of Earth's crust. Three occur naturally, ...

carbon
atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of atom ...

atom
s in a
molecule A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. A molecule is an electrically Electricity is the set of physical phenomena associated with the presence and motion I ...

molecule
is indicated first, the number of
hydrogen Hydrogen is the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same ...

hydrogen
atoms next, and then the number of all other
chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical elements cannot be broken down into simp ...
s subsequently, in
alphabetical order Alphabetical order is a system whereby character string In computer programming Computer programming is the process of designing and building an executable computer program to accomplish a specific computing result or to perform a parti ...
of the
chemical symbols Chemical symbols are the abbreviations used in chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, beh ...
. When the formula contains no carbon, all the elements, including hydrogen, are listed alphabetically. By sorting formulae according to the number of atoms of each element present in the formula according to these rules, with differences in earlier elements or numbers being treated as more significant than differences in any later element or number—like sorting text strings into
lexicographical order In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers (arithmetic and number theory), formulas and related structures (algebra), shapes and spaces in which they are contained (geometry), and quantities and ...
—it is possible to
collate Collation is the assembly of written information into a standard order. Many systems of collation are based on number, numerical order or alphabetical order, or extensions and combinations thereof. Collation is a fundamental element of most office ...
chemical formulae into what is known as Hill system order. The Hill system was first published by Edwin A. Hill of the
United States Patent and Trademark Office The United States Patent and Trademark Office (USPTO) is an agency in the U.S. Department of Commerce that issues patent A patent is a type of intellectual property that gives its owner the legal right to exclude others from making, using ...
in 1900. It is the most commonly used system in chemical databases and printed indexes to sort lists of compounds.Wiggins, Gary. (1991). ''Chemical Information Sources.'' New York: McGraw Hill. p. 120. A list of formulae in Hill system order is arranged alphabetically, as above, with single-letter elements coming before two-letter symbols when the symbols begin with the same letter (so "B" comes before "Be", which comes before "Br"). The following example formulae are written using the Hill system, and listed in Hill order: * BrI * BrClH2Si * CCl4 * CH3I * C2H5Br * H2O4S


See also

* Dictionary of chemical formulae * Element symbol * Nuclear notation * Periodic table *IUPAC nomenclature of inorganic chemistry * Formula unit


References

*


External links


Hill notation example
from the University of Massachusetts Lowell libraries, including how to sort into Hill system order
Molecular formula calculation applying Hill notation
The library calculating Hill notation i
available on npm
{{DEFAULTSORT:Chemical Formula Chemical formulas, Chemical nomenclature Notation