Which of the following gases contains the highest number of atoms at s.t.p?

Analyze the question: We need the total number of atoms (not molecules) in each option. So we must consider atomicity, meaning how many atoms are in one molecule of each gas.

Concept: Number of atoms = moles × atoms per molecule × Avogadro's number (6.02 × 10²³). Since Avogadro's number is the same for all, we compare (moles × atoms per molecule).

Solution:

• A. Neon is monatomic (Ne): 6 × 1 = 6 mol of atoms
• B. Oxygen is diatomic (O₂): 3 × 2 = 6 mol of atoms
• C. Chlorine is diatomic (Cl₂): 2 × 2 = 4 mol of atoms
• D. Ethane is C₂H₆, which has 2 + 6 = 8 atoms per molecule: 1 × 8 = 8 mol of atoms

Comparison: D (8) > A = B (6) > C (4)

Number of atoms in D = 8 × 6.02 × 10²³ ≈ 4.8 × 10²⁴ atoms, which is the highest.

Answer: D. 1 mole of ethane

Tip: When a question asks for the number of atoms, always multiply moles by the atomicity. Noble gases are monatomic, common gases like O₂, N₂, Cl₂ and H₂ are diatomic, and for compounds, count every atom in the formula. Many students pick neon because 6 moles looks biggest, but that is the trap!