Writing Chemical Equations

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

Reactants Products

For example, let’s look at the 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

So, calcium and oxygen react to form calcium oxide. We can then turn this word equation into a symbol equation.

Symbol Equations

Continuing from the example above, let’s 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 can look at group 6 to find oxygen:

  • Now we know that the symbol for oxygen is O

Having identified the symbols for the reactants, we can now 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.


Calcium oxide is not found on the periodic table as it is a compound made up of calcium and oxygen atoms that have reacted together.

To determine the chemical formula of 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 sulphateNa2SO4
Calcium carbonateCaCO3
Sodium nitrateNaNO3
Calcium oxideCaO

Let’s return 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 that the chemical formula for the product (calcium oxide) is CaO.

Therefore, the symbol equation is:

Ca + O2 CaO

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

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