Which of the following are the smallest chemical units that can combine to form molecules?

• A. Neutrons
• B. Ions
• C. Protons
• D. Atoms

Answer: (D)

Verified step by step guidance

1. Understand the question: It asks for the smallest chemical units that can combine to form molecules.
2. Recall the definitions of the options: Neutrons and protons are subatomic particles found inside atoms; ions are atoms or molecules with a net electric charge due to loss or gain of electrons.
3. Recognize that molecules are formed by the combination of atoms through chemical bonds.
4. Atoms are the smallest units of matter that retain the properties of an element and can chemically combine to form molecules.
5. Therefore, the correct answer is atoms, as they are the fundamental units that combine to create molecules.