name implies, is one unit – whether atom, ion or molecule – corresponding to a given formula. One formula unit of NaCl for example is one Na⁺ion and one Cl^– ion; one formula unit of MgBr₂ is one Mg²⁺ ion and two Br^– ions and one formula unit of water is one H₂ O molecule.

Now let us balance the chemical equation using a systematic method. For this take the example of hydrogen reacting with oxygen to form water.

Step 1: Write the equation with the correct chemical formulae for each reactant and product.

Eg: In the reaction of hydrogen with oxygen to yield water, you can write chemical equations as follow:
H₂+ O₂ --> H₂O...............(5)

Step 2: After writing the molecular formulae of the substances the equation is to be balanced. For this we should not touch the ratio of atoms in the molecules of the susbtances but we may assign suitable numbers as the coefficients before the formulae.

In the above equation write ‘2’ before the molecular formula of hydrogen and also ‘2’ before the molecular formula of water. Observe whether atoms of both the elements hydrogen and oxyen are same (or) different on both the sides. They are in the same number on both sides. Therefore, the equation is balanced.
2H₂ + O₂-> 2H₂O.............(6)

Step 3: Sometimes there is a possibility that the coefficients of all the substances getting divided with a suitable number. Since we require the lowest ratio of coefficient for reactants and products the above operation is to be done. If no common factor is there, there is no need to divide the equation. The above equation needs no division of the coefficients of the substances.

Step 4: Verify the equation for the balancing of atoms on both sides of the equations. The above equation (6) is a balanced equation.

Let us work out some more examples to see how equations are balanced.

Eg-1: Combustion of propane (C₃H₈)
Propane, C ₃ H₈ is a colourless, odourless gas often used as a heating and cooking fuel. Write the chemical equation for the combustion reaction of propane. The reactants are propane and oxygen and the products are