Writing Chemical Equations

In a chemical reaction, we always write the reactants on the left hand side of the arrow and the products on the right hand side of the arrow.

Reactants Products

For example, lets take a look at a chemical reaction between calcium and oxygen. The word equation is:

Calcium + OxygenCalcium oxide

This means that…

  • Calcium and oxygen are our reactants
  • Calcium oxide is our product

We can then turn this word equation into a symbol equation.

Symbol Equations

Continuing from the example above, we must look at the periodic table to find the symbols.


Looking at the periodic table, we can look at group 2 to find calcium:

  • Now we know that the symbol for calcium is Ca

We now look at group 6 to find oxygen:

  • Now we know that the symbol for oxygen is O

As we now have the reactant symbols, we can start writing our symbol equation.

Ca + O2

  • Keep in mind that oxygen atoms always come in pairs, that’s why we write the little 2 (subscript) beside the oxygen symbol, as O2.


It’s not possible to find calcium oxide on the periodic table because it’s a compound. In this case, calcium and oxygen have reacted together to form a compound. This means that calcium oxide is made up of calcium and oxygen.

In order to know what symbols to write calcium oxide, it is helpful to remember the endings of chemical formulae.

Word EndingSymbol Ending
Ends with ide
E.g. oxide, fluoride, bromide
Ends with a single element symbol
E.g. O, F, Br

Let’s look at some examples which will help you write chemical formulas:

CompoundChemical Formula
Potassium hydroxideKOH
Sodium sulphateK2SO4
Calcium carbonateCaCO3
Sodium nitrateNaNO3
Calcium oxideCaO

Let’s go back to our earlier example:

Calcium + Oxygen Calcium oxide

  • We can see that the product ends in ide (oxide) – Which means that the symbol ends with a single element, in this case O
  • This means the chemical formula for the product (magnesium oxide) is CaO

Therefore, the symbol equation is:

Ca + O2 CaO

However, the equation is not yet complete, we still need to balance it.