Compared to the Earth’s early atmosphere , the modern atmosphere is oxygen-rich. It contains:

less water vapour

less carbon dioxide

more oxygen

Question Name the most abundant gas in the modern atmosphere. Reveal answer up down Nitrogen.

Water vapour

The Earth cooled after it formed. When it became cool enough, water vapour in the atmosphere condensed . It fell as rain, creating the oceans.

Carbon dioxide

Carbon dioxide is a very soluble gas. It dissolves readily in water. As the oceans formed, carbon dioxide dissolved and the amount of it in the atmosphere decreased.

Oxygen

Plants make their own food by photosynthesis . In this process, carbon dioxide from the atmosphere reacts with water. Glucose is produced, with oxygen as a by-product .

Photosynthesis by primitive plants and algae caused the removal of carbon dioxide from the air, and the release of oxygen. This had two effects:

the amount of carbon dioxide decreased further

the amount of oxygen in the atmosphere gradually increased