H2 + Cl2 => 2HCl
I would first list all the # of atoms of each molecule on the reactant side
(Remember that numbers on the right side of each element are the # of atoms in each unit particle).
H = 2
Cl = 2
Next, you want to match the number of atoms on the product. (Remember that the number on the left side of the unit particle shows the number of molecules of each unit particle. Also, the number applies to all the elements of the unit particle.
2HCl
