Chemical Formula: Definition and Examples

Symbols and subscripts

A chemical formula is a compact notation that lists the atoms in a substance. Each element appears as its symbol, the one found in the periodic table, and a whole number written below the line right after it gives the quantity. That number is the subscript. When there is only one atom the subscript is left out: water is H2O, two hydrogen atoms and one oxygen, and nobody writes H2O1. Carbon dioxide, CO2, follows the same rule. In fact the term “chemical formula” covers a whole family of notations that say what a compound is made of and, in some types, how.

Practical example: to read H2SO4, go from left to right. H2 is two hydrogen atoms, S is one sulfur atom with its subscript of 1 omitted, and O4 is four oxygen atoms.

Empirical, molecular and structural formulas

One compound can be described at three levels of detail, and the table lines them up. The molecular formula always holds the empirical one a whole number of times, and the structural formula adds the bonds between the atoms.

In glucose the ratio of carbon to hydrogen to oxygen is 1 to 2 to 1, so the empirical formula is CH2O. The molecular formula, C6H12O6, is the one PubChem records. Going from the empirical to the molecular formula takes one more piece of data, the molecular weight.

Practical example: the molecular formula of butane is C4H10. Divide the subscripts by two and you get C2H5, the empirical formula, which is the smallest ratio between the atoms. The molecule holds two of those units, a fact the empirical formula cannot show by itself.

When the molecular formula is not enough

Ethanol and dimethyl ether are two different compounds. PubChem lists the same molecular formula for both, C2H6O, and their empirical formula is the same too. The word isomers names exactly this: compounds with the same chemical formula and different structures. Faced with such a pair, the molecular formula is not enough, and the structural formula settles it by recording the bonds between the atoms. That is why the condensed formulas of the two compounds come out different even though the atom count is identical.

Ionic compounds

Ionic compounds contain no separate molecules, so their composition is given by the empirical formula, which here is also the standard one. CaCl2 is an example. In sodium chloride the Na+ and Cl− ions are in a 1 to 1 ratio, and NaCl writes down that ratio.

The smallest grouping represented by an empirical formula is called a formula unit. It applies to ionic compounds and to covalent ones. Some macromolecules, such as SiO2, also use the empirical formula as their standard one.

Hill order

In chemical databases you may find H2O4S where you expected H2SO4. The cause is Hill order: in carbon compounds, carbon is written first and then hydrogen, while in compounds without carbon all elements, hydrogen included, follow alphabetical order. PubChem therefore records sulfuric acid as H2O4S and sodium chloride as ClNa, whereas the usual way to write them is H2SO4 and NaCl.