Molecular is the describing word that belongs to a molecule. A molecule is the smallest piece of a substance you can have that is still that substance. Split a water molecule any smaller and you no longer have water: you have hydrogen and oxygen, which behave differently. Most molecules are two or more atoms holding together. A very few substances can exist as single atoms (the gases that almost never join with anything), and chemists still talk about them in the same breath. You will meet molecular constantly in science class: molecular structure, molecular biology, molecular weight. It is school-and-lab English, not kitchen English. When someone says “at the molecular level,” they mean “down at the tiny scale of these units,” often as a way of saying “very, very small and basic.” The thing itself is the molecule; molecular only describes it.