Chemical formula
A chemical formula is a way of presenting information about the chemical proportions of atoms that constitute a particular chemical compound or molecule, using chemical element 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 name since it does not contain any words. Although a chemical formula may imply certain simple chemical structures, it is not the same as a full chemical structural formula. Chemical formulae can fully specify the structure of only the simplest of molecules and chemical substances, and are generally more limited in power than chemical names and structural formulae.
The simplest types of chemical formulae are called empirical formulae, 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 is , while its molecular formula is .
Sometimes a chemical formula is complicated by being written as a condensed formula, which conveys additional information about the particular ways in which the atoms are chemically bonded together, either in covalent 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 for ethanol, which is or. 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 substituents.
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. For reasons of structural complexity, a single condensed chemical formula may correspond to different molecules, known as isomers. For example, glucose shares its molecular formula with a number of other sugars, including fructose, galactose and mannose. Linear equivalent chemical names exist that can and do specify uniquely any complex structural formula, but such names must use many terms, rather than the simple element symbols, numbers, and simple typographical symbols that define a chemical formula.
Chemical formulae may be used in chemical equations to describe chemical 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, 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 by its chemical 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 of ethanol may be written 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, whose formula of 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 molecules, 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 is rather than the glucose empirical formula, which is. However, except for very simple substances, molecular chemical formulae lack needed structural information, and are ambiguous.
For simple molecules, a condensed 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 formula, and dimethyl ether by the condensed formula. These two molecules have the same empirical and molecular formulae, but may be differentiated by the condensed formulae shown, which are sufficient to represent the full structure of these simple organic compounds.
Condensed chemical formulae may also be used to represent ionic compounds that do not exist as discrete molecules, but nonetheless do contain covalently bound clusters within them. These polyatomic ions 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 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. For example, one systematic name for glucose is -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.
Types
Empirical formula
In chemistry, the empirical 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 for ionic compounds, such as, and for macromolecules, such as. An empirical formula makes no reference to isomerism, structure, or absolute number of atoms. The term empirical refers to the process of elemental analysis, a technique of analytical chemistry used to determine the relative percent composition of a pure chemical substance by element.For example, hexane has a molecular formula of, and a structural formula, implying that it has a chain structure of 6 carbon atoms, and 14 hydrogen atoms. However, the empirical formula for hexane is. Likewise the empirical formula for hydrogen peroxide,, is simply, expressing the 1:1 ratio of component elements. Formaldehyde and acetic acid have the same empirical formula,. 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 a binary compound, ternary compound, quaternary compound, or has even more elements.
Molecular formula
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 is , while its molecular formula is . For water, both formulae are. A molecular formula provides more information about a molecule than its empirical formula, but is more difficult to establish.Structural formula
In addition to indicating the number of atoms of each elementa molecule, a structural formula indicates how the atoms are organized, and shows the chemical 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 are structural isomers of each other, since they both have the same molecular formula, but they have different structural formulas as shown.
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 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 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 formula may represent the types and spatial arrangement of bonds in a simple chemical substance, though it does not necessarily specify isomers 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 as. In ethylene there is a double bond between the carbon atoms, therefore the chemical formula may be written:, 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 or less commonly. The two lines indicate that a double bond connects the atoms on either side of them.
A triple bond may be expressed with three lines or three pairs of dots, 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 groups that are the same may be expressed by enclosing the repeated group in round brackets. For example, isobutane may be written. 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 implies a central carbon atom connected to one hydrogen atom and three methyl groups. The same number of atoms of each element may be used to make a straight chain molecule, n-butane:.