Here's a breakdown:
* Pure elements: These are substances made up of only one type of atom, like gold (Au) or oxygen (O2). So, in this sense, a pure element would be composed of only one type of molecule - the molecule of that element.
* Polymers: These are large molecules made up of repeating subunits called monomers. For example, polyethylene is composed of repeating ethylene molecules. While it has a single repeating unit, it's not a single molecule.
* Some compounds: While most compounds are made of multiple elements, some are made of only one element bonded in a specific way. For example, ozone (O3) is made up only of oxygen atoms.
So, the answer depends on how strictly you define "composed" and "molecule."
* If you mean a single molecule, then the answer is a pure element.
* If you mean a substance made up of only one type of molecule, then the answer is a pure element or a compound with only one type of atom.
