The following gases are diatomic EXCEPT

Analyze the question: We need the gas that is NOT diatomic, meaning its molecules are not made of two atoms.

Concept: A diatomic molecule contains two atoms bonded together (e.g., X₂). Common diatomic elements include H₂, N₂, O₂, F₂, Cl₂, Br₂, and I₂. Noble gases (Group 18/0) have complete outer electron shells, so they are stable on their own and exist as single atoms. This is called being monatomic.

Solution:

• A. Fluorine: F₂ (diatomic)
• B. Helium: He (monatomic, a noble gas)
• C. Hydrogen: H₂ (diatomic)
• D. Nitrogen: N₂ (diatomic)
• E. Oxygen: O₂ (diatomic)

Helium is the only noble gas in the list. It has a full outer shell (2 electrons in its first shell), so it does not bond with other helium atoms and exists as single atoms.

Answer: B. Helium.

Tip: Remember the diatomic elements with the phrase "HONClBrIF" (pronounced "Hon-cl-brif"): Hydrogen, Oxygen, Nitrogen, Chlorine, Bromine, Iodine, Fluorine. Any noble gas (He, Ne, Ar, Kr, Xe, Rn) is monatomic. If an "EXCEPT" question lists a noble gas, it is almost always the answer.