We are introducing the way that chemists write "recipes," which is really what a chemical equation is. It works just like other recipes. You mix in the right ratios and get the right amount out. You can cut the recipe…reduce all the ingredients (reactants) by the same proportion and make less of the product. If you do so, you may have some of the ingredients left over. It all works like that. We call this "stoichiometry," which sounds like something hard to understand, but it really isn't. The trick is that all of our recipes are written in a new unit you haven't heard of: "moles." which is just a count of atoms or molecules (or anything else). And we have to convert from what we can measure (grams) to what we need (moles).
What is the evidence is there that there are atoms? Why do we think they exist? There are several lines of evidence. By the way, the image above is of Mr Potato Head from Toy Story™ and is used without permission.