The Earth's atmosphere · GCSE Chemistry

Today's atmosphere

For about 200 million years, the proportions of gases in the atmosphere have been much the same as today.
It is about four-fifths (about 80%) nitrogen and about one-fifth (about 20%) oxygen, with small amounts of other gases including carbon dioxide, water vapour and noble gases (mainly argon).

The Earth is about 4.6 billion years old, and evidence about the early atmosphere is limited, so theories have changed over time.
One theory: in the first billion years, intense volcanic activity released gases that formed the early atmosphere, and water vapour condensed to form the oceans.
The early atmosphere may have been like those of Mars and Venus today: mainly carbon dioxide, with little or no oxygen. Volcanoes also released nitrogen, which gradually built up, and there may have been small amounts of methane and ammonia.

When the oceans formed, carbon dioxide dissolved in the water, and carbonates were precipitated, forming sediments.
Algae and plants removed carbon dioxide by photosynthesis.
Carbon was locked up in sedimentary rocks and fossil fuels: limestone formed from the shells and skeletons of marine organisms; coal formed from plants; crude oil and natural gas formed from plankton.

Algae first produced oxygen by photosynthesis about 2.7 billion years ago: carbon dioxide + water → glucose + oxygen.
Over the next billion years plants evolved and the percentage of oxygen gradually increased to a level that let animals evolve.

Atmosphere: The layer of gases surrounding the Earth.
Nitrogen: The most common gas in today's atmosphere, about 80%.
Early atmosphere: The atmosphere of the young Earth, mainly carbon dioxide from volcanoes.
Volcanic activity: Eruptions that released gases such as carbon dioxide, water vapour and nitrogen.
Sedimentary rock: Rock formed from layers of sediment, such as limestone.
Fossil fuel: A fuel formed from the remains of ancient organisms, such as coal, oil and natural gas.
Algae: Simple plant-like organisms that were the first to produce oxygen by photosynthesis.