A molecule is the convergence of two or more atoms.
It depends upon the molecule, of course. Hydrogen, for example, exists almost entirely in its molecular form h2.
But when you bring an atom of sodium and combine it with an atom of chloride, you get table salt. NaCl. Either one of those elements separate is toxic to humans but who can live without salt? When Carbon (C), oxygen(O) and hydrogen (H) get together as part of the carbon-8 molecule they can form things like methane, gasoline, alcohol and a variety of other substances.
If you combine oxygen with iron you get rust.