Molecular formulas describe the number and type of atoms in a single molecule of a compound. The constituent elements are represented by their chemical symbols, and the number of atoms of each element present in each molecule is shown as a subscript following that element's symbol. For organic compounds, carbon and hydrogen are listed as the first elements in the molecular formula, and they are followed by the remaining elements in alphabetical order. For ionic compounds, the cation precedes the anion in the molecular formula. For example, for butane, the molecular formula is C4H10.
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, dashes, brackets, 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, and it contains no words. Although a chemical formula may imply certain simple chemical structures, it is not the same as a full chemical structural formula. Chemical formulas are more limiting than chemical names and stuctural formulas.
The simplest types of chemical formulas are called empirical formulas, which use only letters and numbers indicating atomic proportional ratios (the numerical proportions of atoms of one type to those of other types). Molecular formulas indicate the simple numbers of each type of atom in a molecule of a molecular substance, and are thus sometimes the same as empirical formulas (for molecules that only have one atom of a particular type), and at other times require larger numbers than do empirical formulas. An example of the difference is the empirical formula for glucose, which is CH2O, while its molecular formula requires all numbers to be increased by a factor of six, giving C6H12O6.
However, as can be seen by comparing the molecular formula to the structural formula Figure 1, the former lacks information about the arrangement of atoms; because of this, one molecular formula can describe a number of different chemical structures. A structural formula is not as compact and easy to communicate, but it provides information that the molecular formula does not about the relative positioning of atoms and the bonding between atoms. Compounds that share a chemical formula but have different chemical structures are known as isomers, and they can have quite different physical properties.
The molecular formula for a compound can be the same as or a multiple of the compound's empirical formula, as the empirical formula represents the simplest whole-integer ratio of atoms in a compound. Combustion analysis can be used to determine the empirical formula of a compound, and if the molecular weight of the compound is known, this information can also be used to determine the molecular formula.
Examples of Molecular Formulas:
- The compound dichlorine hexoxide has an empirical formula ClO3, and molecular formula Cl2O6
- The compound glucose has the empirical formula CH2O, and the molecular formula C6H12O6