Final answer:
The correct statement regarding the relationship between molecules and compounds is 'All compounds are molecules, but not all molecules are compounds.' Compounds require different elements combined, while molecules can consist of the same element and therefore may not be a compound. So the correct option is A.
Step-by-step explanation:
The correct statement is A: All compounds are molecules, but not all molecules are compounds. This is because a compound is a substance made up of two or more different elements that are chemically combined in a fixed ratio. For example, water (H2O) is a compound consisting of two hydrogen atoms and one oxygen atom chemically bonded. However, a molecule is simply the smallest unit of a chemical substance that can exist independently while retaining its chemical properties. Molecules can be composed of multiple atoms of the same element, such as O2 (a molecule of oxygen), which means they are not compounds because they do not contain different elements.
Considering this definition, statement B is incorrect because some molecules are not compounds as they can consist of only one type of atom. Statement C is also incorrect because something can be both a molecule and a compound if it is a molecule containing different atoms. Lastly, statement D is false because molecules and compounds are not exactly the same; compounds must contain two or more different elements, while molecules may consist of one or more atoms of the same or different elements.