OverviewA chemical formula identifies each constituent by its 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 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 , 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 s, chemical formulae often employ ways to suggest the structure of the molecule. These types of formulae are variously known as ''molecular formulae'' and '. A molecular formula enumerates the number of atoms to reflect those in the molecule, so that the molecular formula for 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, may be represented by the condensed chemical formula CH3CH2OH, and 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 s. Condensed chemical formulae may also be used to represent s that do not exist as discrete molecules, but nonetheless do contain covalently bound clusters within them. These 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 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 . 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 formulaIn , the 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 s, such as , and for macromolecules, such as . An empirical formula makes no reference to ism, structure, or absolute number of atoms. The term ''empirical'' refers to the process of , a technique of used to determine the relative percent composition of a pure chemical substance by element. For example, has a molecular formula of , or structurally , implying that it has a chain structure of 6 atoms, and 14 atoms. However, the empirical formula for hexane is . Likewise the empirical formula for , , is simply HO expressing the 1:1 ratio of component elements. and have the same empirical formula, . This is the actual chemical formula for formaldehyde, but acetic acid has double the number of atoms.
Molecular formulaMolecular 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 , , , or has even more elements.
Condensed formulaThe 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 s) might have completely different chemical and/or physical properties if the atoms are connected differently or in different positions. In such cases, a is useful, as it illustrates which atoms are bonded to which other ones. From the connectivity, it is often possible to deduce the approximate . A condensed chemical formula may represent the types and spatial arrangement of in a simple chemical substance, though it does not necessarily specify s or complex structures. For example, 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 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 connects the atoms on either side of them. A 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 s that are the same may be expressed by enclosing the repeated group in . For example, 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 (). The formula (CH3)3CH implies a central carbon atom connected to one hydrogen atom and three . 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''-: CH3CH2CH2CH3.
Law of compositionIn any given chemical compound, the elements always combine in the same proportion with each other. This is the . 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 formulaeThe 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, may be needed which goes well beyond the available resources used above in simple condensed formulae. See and for examples. In addition, linear naming systems such as (InChI) allow a computer to construct a structural formula, and (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 formulaeFor s in condensed chemical formulae, parentheses are placed around the repeating unit. For example, a 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 formulaeFor 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 may also be shown in this way. For example: or . 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 12H12">12H12sup>2−, which is found in compounds such as ]. Parentheses ( ) can be nested inside brackets to indicate a repeating unit, as in . Here, (NH3)6 indicates that the ion contains six bonded to , 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 3)6">o(NH3)6sup>3+Cl3− or 3)6">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 , rather than .
IsotopesAlthough s are more relevant to or chemistry than to conventional chemistry, different isotopes may be indicated with a prefixed 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 . For example, 8O2 for dioxygen, and for the most abundant isotopic species of dioxygen. This is convenient when writing equations for s, in order to show the balance of charge more clearly.
Trapped atomsThe @ symbol () indicates an atom or molecule trapped inside a cage but not chemically bound to it. For example, a (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 12As20">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 cages (s), which can trap atoms such as 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 , which most modern character encoding schemes are based on), and the visual aspects suggesting the structure of an endohedral fullerene.
Non-stoichiometric chemical formulaeChemical formulae most often use s for each element. However, there is a class of compounds, called s, that cannot be represented by small integers. Such a formula might be written using 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 compoundsA 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 of chemical formulae. For example, may be represented by the formula C''n''H(2n + 1)OH (''n'' ≥ 1), giving the homologs , , for ''n''=1–3.
Hill systemThe 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 s in a is indicated first, the number of atoms next, and then the number of all other s subsequently, in of the . 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 —it is possible to chemical formulae into what is known as Hill system order. The Hill system was first published by of the 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
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