Empirical formula

An empirical formula is a type of chemical formula that describes the ratios between chemical elements, rather than counting exact numbers of atoms or describing anything about the molecule's structure. Empirical formulas are considered the simplest way of writing what a compound is made of.

Empirical formulas were the only chemical formulas available in early chemistry and are still used today for ionic compounds and large molecules like polymers. Simpler compounds are usually written with other molecular formulas that also describe parts of their structure.

Example

The common name of phosphorus pentoxide is taken from its empirical formula of P₂O₅. The actual molecular formula for this chemical is P₄O₁₀, which is used in most contexts in modern chemistry. The ratio of phosphorus to oxygen in both formulas is 2:5.

Table salt is written with the formula NaCl, but the actual structure of salt (an ionic compound) is a cubic crystal where every sodium cation is surrounded by six chlorine anions, and every chlorine anion is surrounded by six sodium cations. A single gram of salt contains more than ten sextillion interconnected atoms of both sodium and chlorine, and because all the chemical bonds are identical, there's no way to break the larger crystals into smaller molecules. A molecular formula for a salt crystal would have different numbers based on its mass, but an empirical formula is always the same: for every sodium ion there is exactly one chlorine ion, so we use the formula NaCl.

Individual molecules of polyethylene are alkanes that can contain thousands or even millions of carbon atoms. However, the difference in chemistry between for example C₁₅₀₀₀H₃₀₀₀₂ and C₁₆₀₀₀H₃₂₀₀₂ is very small, and there's no way to guarantee exact length of any molecule. When a formula is needed, polyethylene is written C_nH_n+2 - and as the length gets very large, that approaches the empirical formula (CH₂)_n.