Video narration
Here's the key: a reaction can give a high percent yield and still produce substantial by-products. Percent yield tells you how much desired product you actually obtained compared with the theoretical amount. Atom economy comes from the balanced equation: it tells you what share of the reactant material can become the target, whether or not your experiment reaches that amount. Notice the numerator is not the mass of every product. It is only the desired product; the denominator includes every reactant, counted in the balanced proportions. So if the equation sends atoms into other products, those atoms still count in the input mass, but not in the target mass. That makes atom economy useful in green chemistry, but it is not a complete environmental verdict. By-products are not automatically hazardous; their amount and properties matter. Solvents, energy use, hazards, and practical yield shape the broader picture too. So where did the rest go? Into other products-and sometimes waste. Atom economy tells you how much of the reactant material the desired product keeps.