Which of the following lists contains only diatomic elements?

• A. Hydrogen, Nitrogen, Oxygen, Fluorine
• B. Helium, Neon, Argon, Krypton
• C. Carbon, Silicon, Boron, Phosphorus
• D. Sodium, Magnesium, Aluminum, Silicon

Answer: (A)

Diatomic elements are those that normally exist as molecules made of two atoms of the same element in standard conditions. In the first list, hydrogen, nitrogen, oxygen, and fluorine all form diatomic molecules H2, N2, O2, and F2. The other lists include elements that are not diatomic in their standard state: noble gases (helium, neon, argon, krypton) are monatomic; carbon, silicon, boron, and phosphorus typically exist as solids with covalent networks or discrete non-diatomic molecules; and sodium, magnesium, aluminum are metals (with silicon being a metalloid). Therefore, the first list is the one containing only diatomic elements.