Any molecule can be represented by a formula that lists all the atoms in that molecule. Each element is represented by its atomic symbol in the Periodic Table – e.g. H for hydrogen, Ca for calcium. If more than one atom of a particular element is present, then it’s indicated by a number in subscript after the atomic symbol — for example, H2O means there are 2 atoms of hydrogen and one of oxygen.
If there is more than one of whole groups of atoms, then that’s shown with a bracket around them. For example, calcium hydroxide has one calcium (Ca) for every two hydroxides (OH), so is written as Ca(OH)2.
If a charge is present, it’s indicated in superscript, with a sign (+/-) and a number if more than one charge is present. For example, calcium ions have two positive charges so are written Ca2+.
Sometimes a formula can be written in a way that gives an indication of the structure. A chemical equation shows a reaction taking place. On the left-hand side are the reactants, the molecules that take part in the reaction. On the right-hand side are the products, the molecules that are created in the reaction. Each side must contain the same number of each kind of atom. An arrow between them indicates the direction the reaction is expected to occur:
Ca + Cl2 → CaCl2
Calcium + chlorine → Calcium chloride
If there are two or more of any molecules, that’s indicated with a number in front of the molecule:
CH4 + 2O2 → CO2 + 2H2O
One methane molecule + two oxygen molecules → one carbon dioxide molecule and two water molecules
An equation may sometimes specify which phase each molecule is in – whether it’s solid, liquid, or gas, or if it’s dissolved. This is written in brackets after the molecule – (s) for solid, (l) for liquid, (g) for gas, and (aq) for ‘aqueous’, meaning dissolved in water. For example, solid calcium carbonate reacts with carbonic acid in water to form calcium bicarbonate, which is much more soluble so becomes dissolved in water:
CaCO3 (s) + H2CO3 (aq) → Ca(HCO3)2 (aq)
Solid calcium carbonate + dissolved carbonic acid → dissolved calcium bicarbonate
All chemical reactions can actually proceed in either direction. Most of the time, one direction is expected to take place more strongly, so the arrow is written in that direction. However, many common reactions happen in both directions simultaneously, creating an equilibrium where the reactions are taking place but there is no net change in concentration from one side to the other. This is indicated with the symbol ⇌.
For example, carbonic acid in water is continually breaking down into bicarbonate and hydrogen ions – but the ions are also continually joining back together.
H2CO3 ⇌ HCO3– + H+
Carbonic acid ⇌ bicarbonate + hydrogen ions
