When you examine an equation and find more than one atom of an element, you’ll see a small number placed next to that element’s symbol. Let’s continue our example from “Writing Chemical Equations“:
Calcium + Oxygen → Calcium oxide
Ca + O2 → CaO
On the left side of the equation (the reactants), we have one calcium atom and two oxygen atoms. However, on the right side of the equation (the products), we have one calcium atom and one oxygen atom. So, the equation is unbalanced.
When balancing an equation you first separate the reactants from the products and list how many of each element you have.
Now we know the issue is with balancing the oxygen atoms. However, it’s crucial to remember the following rule:
When balancing chemical equations, you should only place numbers (coefficients) in front of each chemical. This means that we can’t add any small (subscript) numbers to chemicals.
These large numbers are called coefficients.
As there are two oxygen atoms on the left of the equation and only one on the right, we place a 2 in front of CaO.
Ca + O2 → 2CaO
We now count the reactants and products again.
As you can see, the equation is still unbalanced. There is only one calcium atom on the left side of the equation and two calcium atoms on the right side of the equation. So we place a 2 in front of Ca.
2Ca + O2 → 2CaO
Now count the reactants and products again.
The chemical equation is now balanced.
Chemical equations are balanced to ensure that the number of atoms of each element on both sides of the equation is the same.
This is important because the law of conservation of mass states that matter cannot be created or destroyed in a chemical reaction. Therefore, the total number of atoms of each element before the reaction must be equal to the total number of atoms of that element after the reaction.
Also, the mass of reactants must equal the mass of products. For example:
Reactants (20g) → Products (20g)