What term best describes the relationship between O2 and O3?

• A. Isotopes
• B. Allotropes
• C. Compounds
• D. Polymorphs

Answer: (B)

Allotropes describe different structural forms of the same element. Oxygen can exist as O2, a diatomic molecule, or O3, ozone, with different bonding and geometry. This makes them distinct substances with different properties, even though they are both just oxygen atoms. Isotopes would be atoms of the same element with different numbers of neutrons, which isn’t what’s shown here. Compounds are made from two or more elements, which isn’t the case for O2 and O3. Polymorphs are different crystal forms of the same compound, usually solids, not different molecular forms of the same element. So the relationship between O2 and O3 is best described as allotropes.