how to balance the chemical equation Na+O2 --------- Na2O

First of all , list the elements on both the reactant side and the product side:

LHS                               RHS

Na=1                              Na=2

O=2                                 O=1

Now i'll start with balancing O.....so i add 2 as coeffecient in front of the compound Na2O on the RHS

As a result O is balanced .....so now the eq. becomes:

Na+ O₂ --------> 2NA₂O......

as you can see the no. of atoms of Na has changed:

LHS                       RHS

Na=1                      Na=4

so to balance it we add 4 as coeffecient to Na on the LHS...

thus the eq. becomes:

4Na + O₂ -------> 2Na₂O

Hence , it is balanced.....Hope this helps! :)