A chemical formula is notation that shows the number and type of atoms in a molecule. In other words, it is a written representation of a three-dimensional object. There are a few different ways of writing a chemical formula. In general, a formula includes element symbols that identify the types of atoms in the molecule. In most formulas, subscripts following an element symbol indicate the number of atoms of that element.
Here is a look at the most common types of chemical formulas: empirical formulas, molecular formulas, condensed formulas, and structural formulas.
| Chemical Formula | Description | Example (Butane) |
|---|---|---|
| Empirical formula | simplest whole number ratio of elements | C2H5 |
| Molecular formula | number and type of atoms in actual molecule | C4H10 |
| Structural formula | two-dimensional representation of three-dimensional structure | CH3CH2CH2CH3 |
Parts of a Chemical Formula
- Element Symbols: Write the one- or two-letter symbol for each element. For example, the symbol for hydrogen is H. The symbol for gold is Au.
- Order of Symbols: By convention, the cation or positive part of the molecule goes first, followed by the anion or negative part of the molecule. For example, you write H2O instead of O2H.
- Coefficient: A coefficient is number written before a formula. If present, it indicates the number of molecules.
- Subscript: A subscript is a number written after an element symbol that indicates the number of atoms of that element. For example, the “2” in H2O means each water molecule has two hydrogen atoms. The number “1” is not written. So, each water molecule contains one oxygen atom.
- Superscript: A superscript is a number written above a formula and to its right. It indicates a net electrical charge. The number is omitted for a single charge. For example, write OH– and not OH1-. When present, the number goes before the charge. For example, write SO42-.
Empirical Formula
The empirical formula indicates the smallest whole number ratio of elements in a molecule. Another name for the empirical formula is the simplest formula. While the empirical formula does not tell you exactly how many atoms of each element are in a molecule, it does indicate the mole ratio of elements.
Molecular Formula
The molecular formula is what most people mean when they talk about a chemical formula. This is the formula you use when writing chemical equations or ordering chemicals. The molecular formula gives the number and type of atoms in the molecule. Sometimes the empirical formula and molecular formula are the same. For example, H2O is both the empirical and molecular formula of water. In complex molecules, the empirical and molecular often formulas differ. For example, the empirical formula of butane is C2H5, while the molecular formula is C4H10. The subscripts in a molecular formula are always a multiple of those in the empirical formula.
Comparing Empirical and Molecular Formulas
See how you use these formulas in chemistry calculations.
Condensed Formula
The condensed formula is a type of structural formula that shows the functional groups in a molecule. Like a molecular formula, it includes both the identities and numbers of atoms. For example, the condensed formula for butane is CH3CH2CH2CH3. Just from looking at the formula, you see there is a chain of carbon atoms with hydrogen atoms attached to them.
There is also a shorter version of the condensed formula that collapses identical groups. For example, you can write the same formula for butane as CH3(CH2)2CH3. This comes in handy for describing polymers and other large molecules.
Structural Formula
A structural formula is a two-dimensional graphic representation of a three-dimensional molecule. So, it is the kind of formula you can draw, but not type on a keyboard. The exception is the condensed formula, which is a kind of structural formula that indicates the position of functional groups.
There is more than one kind of structural formula:
- Condensed formula
- Lewis dot structure
- Skeletal formula
- Newman projection
- Sawhorse projection
- Haworth projection
- Fischer projection
A structural formula uses element symbols, but may or may not include any subscripts. A skeletal formula even omits most element symbols, as the lines and the way they connect indicate carbon and hydrogen atoms.
All of the structural formulas have corresponding empirical and molecular formulas. However, most chemists use the structural formula when describing a chemical reaction because it makes visualizing the process easier.
References
- Burrows, Andrew. (20131). Chemistry: Introducing Inorganic, Organic and Physical Chemistry (2nd ed.). Oxford. ISBN 978-0-19-969185-2.
- Chai, Yan; Guo, Ting; Jin, Changming; et al. (1991). “Fullerenes With Metals Inside”. Journal of Physical Chemistry. 95 (20): 7564–7568. doi:10.1021/j100173a002
- Hill, Edwin A. (1900). “On a system of indexing chemical literature; Adopted by the Classification Division of the U.S. Patent Office”. J. Am. Chem. Soc. 22 (8): 478–494. doi:10.1021/ja02046a005
- Petrucci, Ralph H.; Harwood, William S.; Herring, F. Geoffrey (2002). General Chemistry: Principles and Modern Applications (8th ed.). Upper Saddle River, N.J: Prentice Hall. ISBN 978-0-13-014329-7.