formula, in chemistry, an expression showing the chemical composition of a
compound. Formulas of compounds are used in writing the equations (see
chemical equations) that represent
chemical reactions. Compounds are combinations in fixed proportions of the chemical
elements. The smallest unit of an element is the
atom.
Formulas for Compounds
The formula of a well-known compound, water, is H2O. Water is made up of molecules, and the formula shows that each molecule consists of two atoms of hydrogen, H, bonded to an atom of oxygen, O. The subscript 2 indicates that there are two atoms of hydrogen in the molecule; where no subscript appears, as after the O, the subscript 1 is implied. It should be kept in mind that not all compounds are molecular. For example, sodium chloride, NaCl, is an ionic rather than a molecular compound. Solid sodium chloride consists of a collection of sodium ions and chloride ions arranged in a regular, three-dimensional pattern called a crystalline structure. One cannot say that a certain sodium ion and a certain chloride ion are grouped together into a unit, since each sodium ion is equally associated with all its neighboring chloride ions and each chloride ion is equally associated with all its neighboring sodium ions. The formula NaCl, therefore, cannot be taken as showing the composition of some particular unit, such as a molecule. Rather, it shows the proportion of the atoms of each element making up the compound-in this case, one atom of sodium to every atom of chlorine; such a formula is called an empirical formula.
Molecular and Empirical Formulas
If a compound is molecular, the molecular formula is preferred to the empirical formula since it gives more information. A molecule of glucose, for example, consists of 6 carbon atoms, 12 hydrogen atoms, and 6 oxygen atoms. Its molecular formula, C6H12O6, displays this information explicitly; the empirical formula is CH2O. From the formula one can also deduce the proportion of the atoms of each element making up the compound: one atom of carbon to every two atoms of hydrogen to every one atom of oxygen (6 : 12 : 6=1 : 2 : 1). The empirical formula of glucose, CH2O, shows only the proportion, not the actual number of atoms.
Many compounds may have the same empirical formula. For example, formaldehyde, each molecule of which consists of one carbon atom, two hydrogen atoms, and one oxygen atom, has the molecular formula CH2O, which is identical to the empirical formula of glucose. Another example is furnished by ethyne (acetylene), whose molecular formula is C2H2, and benzene, whose molecular formula is C6H6. Both have the same empirical formula, CH.
In addition to showing the actual number of atoms, molecular formulas are also more useful than empirical formulas in that they explicitly show radicals. For example, the molecular formula for the compound aluminum sulfate, Al2(SO4)3, shows that it contains three sulfate radicals (SO4). The empirical formula, Al2S3O12, does not show this. When only one radical is present in the molecule, the parentheses and subscript are omitted, e.g., CuSO4 for cupric sulfate. Other groups are also shown in molecular formulas, e.g., the water molecules in the mineral chalcanthite (blue vitriol), which consists of cupric sulfate atoms to each of which are attached five water molecules. Its molecular formula is CuSO4·5H2O, its empirical formula CuSO9H10.
Structural Formulas
In many cases, especially with organic compounds, even the molecular formula does not provide enough information to identify a compound, so that structural formulas are needed. For example, both ethanol (ethyl alcohol) and dimethyl ether have the molecular formula C2H6O (see isomer). Their structural formulas are:
In these formulas each line represents a single covalent
chemical bond. A double bond is represented by a double line and a triple bond by a triple line. In ethene (ethylene), C
2H
4, the carbon atoms are joined by a double bond. The structural formula of ethene is:
(In many representations of structural formulas, the angles of the lines indicating bonds do not necessarily have meaning.) In ethyne (acetylene), C
2H
2, the carbon atoms are joined by a triple bond. The structural formula of ethyne is:
Semistructural Formulas
Structural formulas are often simplified so that they can be written on a single line; the simplified formulas are often called semistructural formulas. The semistructural formula for ethanol is CH3CH2OH, or more simply C2H5OH. In such a semistructural formula the OH is written explicitly to indicate that the oxygen has a hydrogen bonded to it. The C2H5 indicates that the two carbon atoms are bonded to one another. The semistructural formula for dimethyl ether may be written CH3OCH3. Here the O is placed between the two carbon atoms to show that the carbons are bonded to the oxygen. A carbon often has three hydrogens bonded to it, and the H3 is written after the C. In some cases the H3 is written before the C for clarity; thus the formula for dimethyl ether might be written H3COCH3.
Electron Dot Diagrams
Dots are used in a type of formula called the electron dot diagram, where each pair of dots represents a pair of shared electrons in a covalent bond. The diagrams for ethane (CH3CH3), ethene, and ethyne are:
, where tbsp is the U.S. tablespoon (as seen in 
. The derivation of the formula proceeds as:
, the formula with prescribed units is
