A chemical equation is always balanced with respect which one of the following 

i) Number of atoms             

ii) Number of molecules 

iii) Number of moles           

iv) Mass

During chemical change, there is no change in the kind of atoms and number of atoms present before the reaction and after the reaction. But the arrangement of atoms is different in the reactants and the products. 

Hence, chemical equation should be balanced on the basis of i) Number of atoms ii) Mass (law of conservation of mass).