Chemical symbols and molecules

To avoid having to write ‘hydrogen atom’ in every sentence, chemists use short, internationally recognised symbols.

Some symbols you should know

Element	Symbol	Element	Symbol
Hydrogen	H	Sodium	Na
Carbon	C	Chlorine	Cl
Oxygen	O	Iron	Fe
Nitrogen	N	Copper	Cu

Note that the first letter is always capitalised, and any second letter is always lower-case: Na (sodium), not NA or na.

Combining atoms to form molecules

A molecule is a group of atoms bonded together. Its chemical formula indicates which atoms it is made up of, and in what quantities, using a small superscript number:

H2O   -> 2 atomes d'hydrogène (H) + 1 atome d'oxygène (O)  : l'eau
CO2   -> 1 atome de carbone (C) + 2 atomes d'oxygène (O)   : le dioxyde de carbone
O2    -> 2 atomes d'oxygène (O)                            : le dioxygène, celui qu'on respire

When no subscript is written, this means there is only one: in H₂O, there is indeed only one oxygen atom, not zero.

An important distinction

Do not confuse an atom of oxygen (O), which almost never exists on its own in nature, with the oxygen molecule (O₂), which is what you actually breathe in the air. Similarly, the carbon dioxide (CO₂) you exhale has nothing to do with carbon monoxide (CO): a single oxygen atom makes all the difference, completely altering the properties of the gas; the latter is dangerous to breathe, whilst the former is our usual respiratory waste product.