Rivision of chapter, Elements, compounds, and mixture

Matter is classified into pure substances (elements and compounds) and impure substances (mixtures). Elements are pure substances made of only one type of atom, while compounds are combinations of two or more elements chemically bonded in fixed ratios. Molecules are the smallest units of compounds that exist independently, classified as monoatomic (one atom), diatomic (two atoms), triatomic (three atoms), tetra-atomic (four atoms), or polyatomic (more than four atoms). Noble gases (helium, neon, argon, krypton, xenon, radon) are elements that do not participate in chemical reactions.