When balancing chemical equations why is it permitted to change the numbers in front of the chemical formulas but not the number within the chemical formula?

Answer 1

The chemical formula of a molecule shows how many atoms are in the molecule so you cannot change it since that would mean it is a different molecule then.

To balance the equations you have to have the same amount of atoms in the left and right hand side. So you try to balance the numbers of each molecule on each side so that there are for example 3 hydrogen atoms overall on each side.

Example:

H2 + O2 ---> H2O

This equation is not balanced since there are two H and two O on left side but only one O and two H on the right side. To balance you cannot change the formula from H2O (water) to H2O2 (hydrogen peroxide) because this is a different compound. But you can balance it like this:

2H2 + O2 ---> 2H2O

Notice that now there are 4H on left hand side (2*2) and 2O, which is the same like the right hand side where you have 4H(2*2) and 2O (2*1 because there is no number after O so there is only 1 atom of it in the molecule)