Compact notation for chemical compounds
-\overset{\displaystyle H \atop |}{\underset{\underset{\underset{\underset{\underset{\underset{| \atop \displaystyle H}{C}}-H ;/chem ;
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 ("ratio" 1:2:1), while its molecular formula is ("number of atoms" 6:12:6). 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 (or implies) 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 (i.e. a pair of isomers) 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 (or semi-structural) 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 (and thus each carbon only has two hydrogens), 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 (or two pairs of dots) 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 (isomers). The formula implies a central carbon atom connected to one hydrogen atom and three methyl groups (). The same number of atoms of each element (10 hydrogens and 4 carbons, or ) may be used to make a straight chain molecule, "n"-butane: .
Chemical names in answer to limitations of chemical formulae.
The alkene called but-2-ene has two isomers, which the chemical formula 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 may be needed which goes well beyond the available resources used above in simple condensed formulae. See IUPAC nomenclature of organic chemistry and IUPAC nomenclature of inorganic chemistry 2005 for examples. In addition, linear naming systems such as International Chemical Identifier (InChI) allow a computer to construct a structural formula, and simplified molecular-input line-entry system (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 polymers in condensed chemical formulae, parentheses are placed around the repeating unit. For example, a hydrocarbon molecule that is described as , 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: .
Ions in condensed formulae.
For ions, the charge on a particular atom may be denoted with a right-hand superscript. For example, , or . The total charge on a charged molecule or a polyatomic ion may also be shown in this way, such as for hydronium, , or sulfate, . 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 , which is found in compounds such as caesium dodecaborate, . Parentheses ( ) can be nested inside brackets to indicate a repeating unit, as in Hexamminecobalt(III) chloride, . Here, indicates that the ion contains six ammine groups () bonded to 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 or . 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, rather than covalent.
Isotopes.
Although isotopes are more relevant to nuclear chemistry or stable isotope chemistry than to conventional chemistry, different isotopes may be indicated with a prefixed superscript in a chemical formula. For example, the phosphate ion containing radioactive phosphorus-32 is . Also a study involving stable isotope ratios might include the molecule .
A left-hand subscript is sometimes used redundantly to indicate the atomic number. For example, for dioxygen, and O for the most abundant isotopic species of dioxygen. This is convenient when writing equations for nuclear reactions, in order to show the balance of charge more clearly.
Trapped atoms.
The @ symbol (at sign) indicates an atom or molecule trapped inside a cage but not chemically bound to it. For example, a buckminsterfullerene () with an atom (M) would simply be represented as 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 if M was inside the carbon network. A non-fullerene example is , an ion in which one arsenic (As) atom is trapped in a cage formed by the other 32 atoms.
This notation was proposed in 1991 with the discovery of fullerene cages (endohedral fullerenes), which can trap atoms such as La to form, for example, or . 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, 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 integers for each element. However, there is a class of compounds, called non-stoichiometric compounds, that cannot be represented by small integers. Such a formula might be written using decimal fractions, as in , or it might include a variable part represented by a letter, as in , 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 series of chemical formulae. For example, alcohols may be represented by the formula ("n" ≥ 1), giving the homologs methanol, ethanol, propanol for 1 ≤ "n" ≤ 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 atoms in a molecule is indicated first, the number of hydrogen atoms next, and then the number of all other chemical elements subsequently, in alphabetical order of the chemical 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 into lexicographical order—it is possible to collate 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 in 1900. It is the most commonly used system in chemical databases and printed indexes to sort lists of compounds.
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:
References.
 ;templatestyles src="Reflist/styles.css" / ;