Which of the following substances has a molecule as its smallest representative particle?

• A. Sodium chloride (NaCl)
• B. Oxygen (O_2)
• C. Diamond (C)
• D. Sodium (Na)

Answer: (B)

Verified step by step guidance

1. Understand the concept of the 'smallest representative particle' for different types of substances. For ionic compounds like sodium chloride (NaCl), the smallest unit is a formula unit, not a molecule, because they form a lattice of ions.
2. For covalent network solids like diamond (C), the smallest representative particle is a giant network of atoms bonded together, not discrete molecules.
3. For metallic substances like sodium (Na), the smallest representative particle is a metal atom arranged in a metallic lattice, not molecules.
4. For molecular substances like oxygen (O_2), the smallest representative particle is a molecule composed of two oxygen atoms bonded together.
5. Therefore, identify which substance is molecular by recognizing that oxygen (O_2) exists as discrete molecules, making it the correct answer.