There are a handful of traps that catch nearly everyone here. Meet them once, on purpose, and they stop being traps.
Trap One: “a Molecule Must Be a Compound”
This is the big one. A molecule just means two or more atoms bonded together — it says nothing about whether they're the same atom or different ones. O₂ is a molecule, but it's still an element, because both atoms are oxygen. Same with N₂ in the air, or H₂. Don't let the fact that something travels as a pair fool you into calling it a compound — look inside the molecule: same element all the way through = element, no matter how many atoms are bonded.
Trap Two: a Compound Is Nothing Like a Mixture
People imagine water is “just hydrogen and oxygen mixed”. It isn't, and the difference is enormous. Hydrogen is an explosive gas and oxygen is the gas that feeds fire — yet bond them in a fixed ratio and you get water, which puts fires out. A compound is a brand-new substance with its own properties, born when atoms bond. A mixture is the opposite: nothing new is made, and every ingredient keeps its own properties. That's the deep tell — in a mixture each part keeps its properties; in a compound the elements lose theirs and the compound has its own.
Trap Three: the Elements “keep Their Properties” Inside a Compound
Here's the killer example. Sodium is a metal so reactive it bursts into flame in water; chlorine is a poisonous green gas once used as a weapon. Bond them one-to-one and you get table salt, NaCl — which you sprinkle on chips and eat. The salt is nothing like either ingredient, because bonding rebuilds the matter into something new. So you can't reason “it contains chlorine, so it must be dangerous”. Once elements are locked in a compound, their old properties are gone.
And the Quiet One: “pure” Doesn't Mean “good for You”
In everyday talk “pure” means clean or healthy. In chemistry pure only means one substance, all the same particle — nothing about safety. Pure mercury and pure chlorine are both pure substances and both would hurt you; a wholesome mixture like fresh air isn't pure at all. Keep the chemistry meaning separate: pure = single substance, full stop.