Which of the following gases is monoatomic?

Step 1: Analyze the question A monoatomic gas exists as single, separate atoms rather than as molecules made of two or more atoms bonded together.

Step 2: Explain the concept

• Noble gases (Group 18/0: helium, neon, argon, krypton, xenon, radon) have complete outer electron shells (stable octet, or duplet for helium). Because of this, they do not need to bond with other atoms, so they exist as single atoms.
• Many other common gases (hydrogen, nitrogen, oxygen, fluorine, chlorine) are diatomic. Their atoms share electrons to achieve a stable configuration.

Step 3: Examine each option

• A. Argon (Ar): A noble gas with 8 electrons in its outer shell (2, 8, 8). It is stable and exists as single atoms, so it is monoatomic.
• B. Chlorine (Cl₂): Has 7 outer electrons and shares one pair to form Cl₂, so it is diatomic.
• C. Nitrogen (N₂): Forms a triple bond between two atoms, so it is diatomic.
• D. Oxygen (O₂): Forms a double bond between two atoms, so it is diatomic.

Step 4: Answer Argon is the only monoatomic gas in the list.

Tip: Remember that the diatomic gases are H₂, N₂, O₂, F₂, Cl₂ (some people recall "HONClBrIF"). Any noble gas (He, Ne, Ar, Kr, Xe, Rn) is monoatomic. When you see a noble gas in the options for a "monoatomic" question, it is almost always the answer.