Final answer:
An atom is the smallest unit of an element, while a molecule is a group of two or more atoms bonded together. Atoms form molecules, which can be either elements or compounds depending on the types of atoms involved.
Step-by-step explanation:
The difference between an atom and a molecule is that an atom is the smallest unit of matter that has the properties of a chemical element, while a molecule is a group of two or more atoms held together by chemical bonds. Specifically, option A is correct: An atom is the smallest unit of an element, and a molecule is a combination of two or more atoms. For example, a molecule of water (H₂O) is made up of two hydrogen atoms and one oxygen atom. Atoms contain subatomic particles such as protons, neutrons, and electrons, forming the basic building blocks of all matter. Molecules, however, can be composed of multiple atoms from the same element (as in oxygen gas, O₂) or from different elements (as in carbon dioxide, CO₂), making them the smallest units of compounds that retain the chemical properties of those compounds.