Final answer:
The Archaean atmosphere lacked free oxygen and was composed of reducing gases like methane, ammonia, and hydrogen. Sunlight split these gases apart, allowing hydrogen to escape into space, paving the way for a more oxidized atmosphere and later the rise of oxygen levels due to photosynthetic life.
Step-by-step explanation:
The gas that was missing from the Archaean atmosphere was free oxygen (O₂). During the Archaean era, Earth's atmosphere was very different from today's atmosphere. It was primarily composed of reducing gases like methane (CH₄), ammonia (NH₃), hydrogen (H₂), and water vapor, with virtually no free oxygen present. Ultraviolet light from the Sun played a significant role in altering this early atmosphere. It split apart the molecules of reducing gases, and most of the lightweight hydrogen atoms eventually escaped into space due to their low molecular weight. Consequently, Earth's atmospheric chemistry transformed, becoming more oxidized, which set the stage for the later proliferation of oxygen-producing photosynthetic life that would drastically increase the levels of oxygen in the atmosphere.