At room temperature, which substance exists as diatomic molecules?

• A. Oxygen
• B. Ozone
• C. Water
• D. Sodium chloride

Answer: (A)

The idea being tested is whether a substance exists as diatomic molecules under typical room-temperature conditions. A diatomic molecule is two atoms held together by covalent bonds. Oxygen forms O2, two oxygen atoms bonded together, which is why it’s commonly found as a diatomic gas in the air. Ozone, on the other hand, is made of three oxygen atoms (O3), so it’s not diatomic. Water is a molecule with two hydrogen and one oxygen atom, so it isn’t diatomic either—the total number of atoms is three. Sodium chloride is an ionic compound, forming a lattice of Na+ and Cl− ions rather than two atoms bonded covalently. Therefore, oxygen is the substance that exists as diatomic molecules at room temperature.