Diatomic elements form stable two-atom molecules at room temperature

Steps:

• Identify diatomic elements as those existing as X₂ molecules in gaseous state: hydrogen (H₂), nitrogen (N₂), oxygen (O₂), fluorine (F₂), chlorine (Cl₂).
• Exclude noble gases like helium (He) and neon (Ne), which are monatomic due to full electron shells.
• Examine options for exactly three diatomic elements from this group.
• Verify room temperature (25°C) conditions, where these are gases.

Why C is correct:

• Nitrogen (N₂), oxygen (O₂), and fluorine (F₂) are halogens or pnictogens forming stable diatomic molecules per molecular orbital theory for unpaired electrons.

Why the others are wrong:

• A: Helium exists as monatomic He atoms.
• B: Neon exists as monatomic Ne atoms.
• D: Helium exists as monatomic He atoms.

Final answer: C