Chemical Formula

A chemical formula is a way of expressing information about the proportions of atoms that constitute a particular chemical compound, using a single line of chemical element symbols, numbers, and sometimes also other symbols, such as parentheses, brackets, and plus (+) and minus (–) signs.

The simplest types of such formulas are called empirical formulas, which use only letters and numbers. Sometimes the chemical formula additionally contains 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. However, a chemical formula is limited in its ability to show complex physical relationships of atoms. Since a chemical formula must be expressed as a single line of symbols, it often cannot be as informative as a structural formula, which is a graphical representation of the physical relationship of atoms to each other in chemical compounds.

A chemical formula identifies each constituent element by its chemical symbol and indicates the proportionate number of atoms of each element. In empirical formulas, these proportions begin with a key element and then assign numbers of atoms of the other elements in the compound, as 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 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 formulas. An example is boron carbide, whose formula of CB_n 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 formulas often employ ways to suggest the structure of the molecule. These types of formulas are variously known as molecular formulas and condensed formulas. For simple molecules, a condensed formula may replace a structural formula. For example, ethanol may be represented by the condensed chemical formula CH₃CH₂OH, and dimethyl ether by the condensed formula CH₃OCH₃. These two molecules have the same empirical formula (see above), but may be differentiated by the condensed formulas shown, which are sufficient to represent the full structure of these simple organic compounds.

Condensed chemical formulas 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 2− ion. Each polyatomic ion in a compound is written individually in order to illustrate the separate groupings. For example, the compound dichlorine hexoxide has a an empirical formula ClO₃, but in liquid or solid forms, this compound is more correctly shown by an ionic condensed formula +−, which to illustrate that it 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 formulas 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 formulas do not have the full power of structural formulas to show chemical relationships between atoms, they are sufficient to keep track of numbers of atoms and numbers of electical charges in chemical reactions, so that these formulas can be used in chemical problems involving conservation of atoms, and conservation of electric charge.