Chemical symbols and formulae
Chemical symbols
Each elementA substance made of one type of atom only. is represented by its own chemical symbol. The periodic tableA tabular representation of all known elements in order based on atomic number, eg all the noble gases are found on the right of the periodic table. shows the names and symbols of the elements. A chemical symbol:
- consists of one or two letters
- always starts with a capital letter, with any other letter in lower case
For example, the element mercury is shown as Hg. You must not show it as HG, hg or hG.
Learn more on atoms, elements and compounds in this podcast.
Listen to the full series on BBC Sounds.
Formulae of elements
A chemical formulaA combination of symbols that indicates the chemical composition of a substance. represents an element or compoundA substance formed by the chemical union of two or more elements. in balanced chemical equationA chemical equation written using the symbols and formulae of the reactants and products, so that the number of units of each element present is the same on both sides of the arrow.. The formulae for most elements is just their symbol.
Some non-metalElement that is a poor conductor of electricity and heat, and which forms acidic oxides. elements exist as simple moleculeA collection of two or more atoms held together by chemical bonds. with two atomThe smallest part of an element that can exist. joined together. We say that these elements are diatomicA molecule made up of only two atoms. The seven diatomic elements are iodine, bromine, chlorine, fluorine, oxygen, nitrogen and hydrogen.. To show this, their formulae contain a subscriptA letter or number written below the line. number 2. For example:
- hydrogen, H2
- nitrogen, N2
- oxygen, O2
- fluorine, F2
- bromine, Br2
- chlorine, Cl2
- iodine, I2
Formulae of simple covalent compounds
A compound contains two or more elements joined by chemical bonds. Simple covalent compoundCompounds, commonly formed between non-metal elements, by sharing electrons between atoms to form covalent bonds. All molecular compounds are covalent. contain non-metal atoms joined by covalent bondA bond between atoms formed when atoms share electrons to achieve a full outer shell of electrons.. Their molecular formulaChemical formula showing the actual number of atoms of each element in a molecule. show:
- the symbols for each element in a molecule
- the number of atoms of each element in a molecule
Here are some examples and their meanings:
| Name of compound | Formula | Atoms in each molecule |
| Ammonia | NH3 | 1 nitrogen, 3 hydrogen |
| Carbon dioxide | CO2 | 1 carbon, 2 oxygen |
| Methane | CH4 | 1 carbon, 4 hydrogen |
| Sulfur dioxide | SO2 | 1 sulfur, 2 oxygen |
| Water | H2O | 2 hydrogen, 1 oxygen |
| Name of compound | Ammonia |
|---|---|
| Formula | NH3 |
| Atoms in each molecule | 1 nitrogen, 3 hydrogen |
| Name of compound | Carbon dioxide |
|---|---|
| Formula | CO2 |
| Atoms in each molecule | 1 carbon, 2 oxygen |
| Name of compound | Methane |
|---|---|
| Formula | CH4 |
| Atoms in each molecule | 1 carbon, 4 hydrogen |
| Name of compound | Sulfur dioxide |
|---|---|
| Formula | SO2 |
| Atoms in each molecule | 1 sulfur, 2 oxygen |
| Name of compound | Water |
|---|---|
| Formula | H2O |
| Atoms in each molecule | 2 hydrogen, 1 oxygen |
