Earth's atmosphere slowly leaks to space. Which gas escapes fastest?

Answer: Hydrogen

Hydrogen ✓ — Correct! Hydrogen atoms are so light that their thermal motion alone can carry them past Earth's escape velocity in the exosphere. Earth loses about 90 tonnes of atmosphere per day, dominated by hydrogen — the same slow drain that stripped Mars's atmosphere over billions of years.

Oxygen — Not quite. Oxygen is 16× heavier than hydrogen, so thermal motion isn't enough to launch it out of Earth's gravity. Some oxygen does escape via a slower 'polar fountain' (ions flung out along magnetic field lines), but the rate is far below hydrogen's.

Carbon dioxide — Not quite. CO₂ is 44× heavier than hydrogen and sits lower in the atmosphere, so it's not escaping to space in any meaningful amount. CO₂ traps outgoing infrared heat, but the molecules themselves stay down here with us.