How to arrange coefficients in chemical equations?

In chemical reactions, the sum of the coefficients before the reaction equation is equal to the sum of the coefficients after the reaction equation. To put it simply, then: “nothing goes anywhere and is not taken out of nowhere.” So, for example, in the reaction of water formation from oxygen and hydrogen: H2 + O2 = H2O – we have two hydrogen atoms and two oxygen atoms, which in the sum give a compound of water, where there are two hydrogen atoms and only one oxygen. Therefore, in front of water, in the reaction, we put a two: H2 + O2 = 2H2O – now we have two oxygen atoms and four hydrogens, so at the beginning we also put a two in front of hydrogen: 2H2 + O2 = 2H2O.