Chemical symbols and formulae

Chemical symbols

Each is represented by its own chemical symbol. The 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.

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Formulae of elements

A chemical represents an element or in . The formulae for most elements is just their symbol.

Some elements exist as simple with two joined together. We say that these elements are . To show this, their formulae contain a 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 contain non-metal atoms joined by . Their 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

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