Final answer:
Oxygen gas (O2) is a molecule but not a compound because it consists of two atoms of the same element. Carbon dioxide (CO2), water (H2O), and methane (CH4) are all compounds because they are composed of different elements.
Step-by-step explanation:
The question asks which of the following is a molecule but NOT a compound: A) O2, B) CO2, C) H2O, D) CH4. A molecule is two or more atoms bonded together, while a compound is a molecule composed of two or more different elements. O2 (oxygen gas) is a molecule consisting of two oxygen atoms and is not a compound because it is composed of the same element. CO2 (carbon dioxide), H2O (water), and CH4 (methane) are all compounds because they are composed of different elements.