Which of the following is the smallest chemical unit that retains the properties of a pure substance?

• A. Atom
• B. Ion
• C. Proton
• D. Molecule

Answer: (A)

Verified step by step guidance

1. Understand the question: It asks for the smallest chemical unit that retains the properties of a pure substance.
2. Recall definitions: An atom is the smallest unit of an element that maintains the chemical properties of that element.
3. Consider ions: Ions are atoms or molecules with a charge, but they may not represent the pure substance itself since they are charged species.
4. Consider protons: Protons are subatomic particles within an atom and do not retain the properties of the substance alone.
5. Consider molecules: Molecules are groups of atoms bonded together and represent compounds or elements, but atoms are smaller and still retain elemental properties.