Achemical formula is a way of presenting information about the chemical proportions ofatoms that constitute a particularchemical compound ormolecule, usingchemical element symbols, numbers, and sometimes also other symbols, such as parentheses, dashes, brackets, commas andplus (+) andminus (−) signs. These are limited to a single typographic line of symbols, which may includesubscripts and superscripts. A chemical formula is not achemical name since it does not contain any words. Although a chemical formula may imply certain simplechemical structures, it is not the same as a full chemicalstructural formula. Chemical formulae can fully specify the structure of only the simplest of molecules andchemical substances, and are generally more limited in power than chemical names and structural formulae.
The simplest types of chemical formulae are calledempirical formulae, which use letters and numbers indicating the numericalproportions 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 forglucose isCH2O (twice as manyhydrogen atoms ascarbon andoxygen), while its molecular formula isC6H12O6 (12 hydrogen atoms, six carbon and oxygen atoms).
Sometimes a chemical formula is complicated by being written as acondensed formula (or condensed molecular formula, occasionally called a "semi-structural formula"), which conveys additional information about the particular ways in which the atoms arechemically bonded together, either incovalent bonds,ionic bonds, 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 forethanol, which isCH3−CH2−OH orCH3CH2OH. 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 differentsubstituents.
Since a chemical formula must be expressed as a single line of chemicalelement 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 asisomers. For example, glucose shares itsmolecular formulaC6H12O6 with a number of othersugars, includingfructose,galactose andmannose. Linear equivalent chemicalnames exist that can and do specify uniquely any complex structural formula (seechemical nomenclature), 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 inchemical equations to describechemical reactions 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, thusbalancing 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 constituentelement by itschemical symbol 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 ofethanol may be writtenC2H6O 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 isboron carbide, whose formula ofCBn 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 simplemolecules, chemical formulae often employ ways to suggest the structure of the molecule. These types of formulae are variously known asmolecular formulae andcondensed formulae. A molecular formula enumerates the number of atoms to reflect those in the molecule, so that the molecular formula forglucose isC6H12O6 rather than the glucose empirical formula, which isCH2O. 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 may be represented by the condensed chemical formulaCH3CH2OH, anddimethyl ether by the condensed formulaCH3OCH3. 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 simpleorganic compounds.
Condensed chemical formulae may also be used to representionic compounds that do not exist as discrete molecules, but nonetheless do contain covalently bound clusters within them. Thesepolyatomic ions are groups of atoms that are covalently bound together and have an overall ionic charge, such as thesulfate[SO4]2− ion. Each polyatomic ion in a compound is written individually in order to illustrate the separate groupings. For example, the compounddichlorine hexoxide has an empirical formulaClO3, and molecular formulaCl2O6, but in liquid or solid forms, this compound is more correctly shown by an ionic condensed formula[ClO2]+[ClO4]−, which illustrates that this compound consists of[ClO2]+ ions and[ClO4]− 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 ofchemical nomenclature. For example, one systematic name for glucose is (2R,3S,4R,5R)-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.
Inchemistry, theempirical formula 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 forionic compounds, such asCaCl2, and for macromolecules, such asSiO2. An empirical formula makes no reference toisomerism, structure, or absolute number of atoms. The termempirical refers to the process ofelemental analysis, a technique ofanalytical chemistry used to determine the relative percent composition of a pure chemical substance by element.
For example,hexane has a molecular formula ofC6H14, and (for one of its isomers, n-hexane) a structural formulaCH3CH2CH2CH2CH2CH3, implying that it has a chain structure of 6carbon atoms, and 14hydrogen atoms. However, the empirical formula for hexane isC3H7. Likewise the empirical formula forhydrogen peroxide,H2O2, is simplyHO, expressing the 1:1 ratio of component elements.Formaldehyde andacetic acid have the same empirical formula,CH2O. This is also the molecular formula for formaldehyde, but acetic acid has double the number of atoms.
Like the other formula types detailed below, an empirical formula shows the number of elements in a molecule, and determines whether it is abinary compound,ternary compound,quaternary compound, or has even more elements.
Molecular formula
Isobutane structural formula Molecular formula:C4H10 Condensed formula:(CH3)3CH
n-Butane structural formula Molecular formula:C4H10 Condensed formula:CH3CH2CH2CH3
Molecular formulae simply indicate the 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 isCH2O (ratio 1:2:1), while its molecular formula isC6H12O6 (number of atoms 6:12:6). For water, both formulae areH2O. A molecular formula provides more information about a molecule than its empirical formula, but is more difficult to establish.
In addition to indicating the number of atoms of each elementa molecule, a structural formula indicates how the atoms are organized, and shows (or implies) thechemical bonds between the atoms. There are multiple types of structural formulas focused on different aspects of the molecular structure.
The two diagrams show two molecules which arestructural isomers of each other, since they both have the same molecular formulaC4H10, but they have different structural formulas as shown.
Theconnectivity 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 ofisomers) might have completely different chemical and/or physical properties if the atoms are connected differently or in different positions. In such cases, astructural formula is useful, as it illustrates which atoms are bonded to which other ones. From the connectivity, it is often possible to deduce the approximateshape of the molecule.
A condensed (or semi-structural) formula may represent the types and spatial arrangement ofbonds in a simple chemical substance, though it does not necessarily specifyisomers or complex structures. For example,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 asCH3CH3. Inethylene 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 writeH2C=CH2 or less commonlyH2C::CH2. The two lines (or two pairs of dots) indicate that adouble bond connects the atoms on either side of them.
Atriple 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 multiplefunctional groups that are the same may be expressed by enclosing the repeated group inround brackets. For example,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). The formula(CH3)3CH implies a central carbon atom connected to one hydrogen atom and threemethyl groups (CH3). The same number of atoms of each element (10 hydrogens and 4 carbons, orC4H10) may be used to make a straight chain molecule,n-butane:CH3CH2CH2CH3.
Chemical names in answer to limitations of chemical formulae
The alkene calledbut-2-ene has two isomers, which the chemical formulaCH3CH=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 orZ) or on the opposite sides from each other (trans orE).[1]
Forpolymers in condensed chemical formulae, parentheses are placed around the repeating unit. For example, ahydrocarbon molecule that is described asCH3(CH2)50CH3, is a molecule with fifty repeating units. If the number of repeating units is unknown or variable, the lettern may be used to indicate this formula:CH3(CH2)nCH3.
Ions in condensed formulae
Forions, the charge on a particular atom may be denoted with a right-hand superscript. For example,Na+, orCu2+. The total charge on a charged molecule or apolyatomic ion may also be shown in this way, such as forhydronium,H3O+, orsulfate,SO2−4. Here + 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[B12H12]2−, which is found in compounds such ascaesium dodecaborate,Cs2[B12H12]. Parentheses ( ) can be nested inside brackets to indicate a repeating unit, as inHexamminecobalt(III) chloride,[Co(NH3)6]3+Cl−3. Here,(NH3)6 indicates that the ion contains sixammine groups (NH3) bonded tocobalt, and [ ] encloses the entire formula of the ion with charge +3.[further explanation needed]
This is strictly optional; a chemical formula is valid with or without ionization information, and Hexamminecobalt(III) chloride may be written as[Co(NH3)6]3+Cl−3 or[Co(NH3)6]Cl3. 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 isionic, rather thancovalent.
Isotopes
Althoughisotopes are more relevant tonuclear chemistry orstable isotope chemistry than to conventional chemistry, different isotopes may be indicated with a prefixedsuperscript 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 molecule18O16O.
A left-hand subscript is sometimes used redundantly to indicate theatomic number. For example,8O2 for dioxygen, and16 8O 2 for the most abundant isotopic species of dioxygen. This is convenient when writing equations fornuclear reactions, in order to show the balance of charge more clearly.
The @ symbol (at sign) indicates an atom or molecule trapped inside a cage but not chemically bound to it. For example, abuckminsterfullerene (C60) with an atom (M) would simply be represented asMC60 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 denotedM@C60 if M was inside the carbon network. A non-fullerene example is[As@Ni12As20]3−, an ion in which onearsenic (As) atom is trapped in a cage formed by the other 32 atoms.
This notation was proposed in 1991[3] with the discovery offullerene cages (endohedral fullerenes), which can trap atoms such asLa to form, for example,La@C60 orLa@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 inASCII, which most modern character encoding schemes are based on), and the visual aspects suggesting the structure of an endohedral fullerene.
Chemical formulae most often useintegers for each element. However, there is a class of compounds, callednon-stoichiometric compounds, that cannot be represented by small integers. Such a formula might be written usingdecimal fractions, as inFe0.95O, or it might include a variable part represented by a letter, as inFe1−xO, wherex 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 ageneral formula. It generates ahomologous series of chemical formulae. For example,alcohols may be represented by the formulaCnH2n + 1OH (n ≥ 1), giving the homologsmethanol,ethanol,propanol for 1 ≤n ≤ 3.
Hill system
TheHill 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 ofcarbonatoms in amolecule is indicated first, the number ofhydrogen atoms next, and then the number of all otherchemical elements subsequently, inalphabetical order of thechemical symbols. 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 intolexicographical order—it is possible tocollate chemical formulae into what is known as Hill system order.
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").[5]
The following example formulae are written using the Hill system, and listed in Hill order:
^Chai, Yan; Guo, Ting; Jin, Changming; Haufler, Robert E.; Chibante, L. P. Felipe; Fure, Jan; Wang, Lihong; Alford, J. Michael; Smalley, Richard E. (1991). "Fullerenes wlth Metals Inside".Journal of Physical Chemistry.95 (20):7564–7568.doi:10.1021/j100173a002.
^abWiggins, Gary. (1991).Chemical Information Sources. New York: McGraw Hill. p. 120.
Petrucci, Ralph H.; Harwood, William S.; Herring, F. Geoffrey (2002)."3".General chemistry: principles and modern applications (8th ed.). Upper Saddle River, N.J: Prentice Hall.ISBN978-0-13-014329-7.LCCN2001032331.OCLC46872308.
