Chemical formula



A chemical formula is a concise way of expressing information about the empirical formula.

This system for writing chemical formulas was invented by the 19th-century Swedish chemist Jons Jakob Berzelius.

Molecular and structural formula

For example methane, a simple molecule consisting of one hydrogen atoms has the chemical formula:

H4

and oxygen atoms has the chemical formula:

O6.

A chemical formula supplies information about the types and spatial arrangement of 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 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 and correct method is to write H2C=CH2 or less commonly H2C::CH2. 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 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 in the following way: (CH3)3CH. However, this implies a different structure from other molecules that can be formed using the same atoms (isomers). The formula (CH3)3CH implies a chain of three carbon atoms, with the middle carbon atom bonded to another carbon:


and the remaining bonds on the carbons all leading to hydrogen atoms. However, the same number of atoms (10 hydrogens and 4 carbons, or C4H10) may be used to make a straight chain: CH3CH2CH2CH3.

The alkene 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).

Polymers

For hydrocarbon molecule that is described as: CH3(CH2)50CH3, is a molecule with 50 repeating units. If the number of repeating units is unknown or variable, the letter n may be used to indicate this: CH3(CH2)nCH3.

Ions

For polyatomic ion may also be shown in this way. For example: hydronium, H3O+ or sulfate, SO42-.

Isotopes

Although stable isotope chemistry than to conventional chemistry, different isotopes may be indicated with a left-hand superscript in a chemical formula. For example, the phosphate ion containing radioactive phosphorus-32 is 32PO43-. Also a study involving stable isotope ratios might include 18O:16O.

A left-hand subscript is sometimes used to indicate redundantly the atomic number. For example, 8O2 for dioxygen, and 168O2 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.

Empirical formula

In analytical chemistry used to determine the relative percent composition of a pure chemical substance by element.

For example hydrogen peroxide, H2O2, is simply HO expressing the 1:1 ratio of component elements.

Non-stoichiometric formulas

Main article: Non-stoichiometric compound

Chemical formulas most often use integers for each element. However, there is a whole 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 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

Chemical formula used for a series of compounds that differ from each other by a constant unit is called general formula. Such a series is called the Hill system is a common convention for writing and sorting formulas.

Hill System

Main article: Hill system

The Hill system is a system of writing chemical formulas such that the number of chemical elements subsequently, in alphabetical order. When the formula contains no carbon, all the elements, including hydrogen, are listed alphabetically.

References

Ralph S. Petrucci, William S. Harwood and F. Geoffrey Herring, General Chemistry, 8th Edition (Prentice-Hall 2002), chapter 3.

See also

 
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Chemical_formula". A list of authors is available in Wikipedia.