The process where bonds are broken and/or bonds are formed is a chemical reaction. As the making or breaking of bonds involves the storage or release of energy, a chemical reaction must obey the laws of thermodynamics such as energy cannot be created or destroyed, only transferred. A chemical reaction has the reactants, what atoms or molecules are combined, on the left followed by an arrow to form the products on the right, the atoms and/or molecules that are the outcome of the chemical reaction. A plus sign indicates the combination of two or more reagents or starting materials. As written, the chemical equation must balance: the number of the same type of atoms on the left must equal the number of the same type of atoms on the left.

For example consider the combustion reactions of some simple hydrocarbons:

CH₄                         +             2O₂                           [arrow]                                CO₂                        +             2H₂O

2C₂H₆                     +             7O₂                           [arrow]                               4CO₂                      +             6H₂O

C₃H₈                       +             5O₂                            [arrow]                              3CO₂                      +             4H₂O

2C₄H₁₀                   +             13O₂                         [arrow]                               8CO₂                      +             10H₂O

When dealing with chemical equations with polyatomic ions, where ions are made of more than one atom, there is a special strategy to balance the chemical equation: treat the whole polyatomic ion as if it is only one atom. Let’s take a look at this chemical equation. For example:

Na₃PO₄                 +             MgCl₂                       [arrow]                              NaCl                      +           Mg₃(PO₄)₂

The polyatomic ion in this case is PO₄. Treat this whole polyatomic ion as one atom. On the reactants side there is one PO₄ ion and on the products side, there are two PO₄ ions. The first step is to balance the polyatomic ions on both sides. To do that place a coefficient of 2 in front of Na₃PO₄. The sodium (Na) atoms double and the PO₄ is now balanced on both sides.