The number of atoms on each side of a chemical equation must be equal.

To balance a chemical equation the following rules must be followed:

–After you have written a chemical equation check to see if the equation is balanced (have the same number of atoms of each element on each side).

–If it is not balanced then the smallest possible number must be placed in front of the reactants and/or products to ensure it is balanced on either side.

Let’s look at the following equations:

C_(s) + O_{2 (g) →}CO_{2 (g)}

The equation above is balanced as you can see there are an equal number of carbon and oxygen atoms on either side.

H_{2 (g)} + O_{2 (g)} →  H₂O_(l)

This equation above is not balanced. Let’s look at the left and right sides of the equation and count the number of hydrogen and oxygen atoms. There are 2 hydrogen atoms and 2 oxygen atoms on the left side of the equation and there are 2 hydrogen atoms and one oxygen atom on the right side.

If we follow the rules from above, we must add the smallest possible number in front of the products and/or the reactants to ensure it is balanced on either side.

The balanced equation is:

2H_{2 (g)} + O_{2 (g)} →2H₂O_(l)

Here is another example:

Fe_(s) + Cl_{2 (g)} →FeCl_3(s)

The equation above is not balanced.

The balanced equation looks something like this:

2Fe_(s) + 3Cl_2(g) →2FeCl_3(s)