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What is the air made of today, and how did the atmosphere change over billions of years?

The composition of today's atmosphere, the early atmosphere, and how the proportions of carbon dioxide and oxygen changed over geological time.

A focused answer to WJEC GCSE Chemistry topic 1.4 on the atmosphere, covering the composition of air today, the volcanic early atmosphere, and how the levels of carbon dioxide and oxygen changed to give the atmosphere we have now.

Generated by Claude Opus 4.89 min answer

Reviewed by: AI editorial process; not yet individually human-reviewed

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  1. What this topic is asking
  2. The atmosphere today
  3. The early atmosphere
  4. How the atmosphere changed
  5. Try this

What this topic is asking

WJEC wants you to state what air is made of today and to explain how the early atmosphere changed into the one we breathe now. This is part of topic 1.4 The ever-changing Earth in Unit 1 of WJEC GCSE Chemistry (3430).

The atmosphere today

The early atmosphere

How the atmosphere changed

The build-up of oxygen also allowed more complex animals to evolve and created the ozone layer, which screens out harmful ultraviolet radiation.

Some of the carbon removed by photosynthesis did not return to the air but was buried as the remains of plants and tiny sea organisms. Over millions of years, heat and pressure turned these remains into the fossil fuels coal, oil and natural gas. This is why burning fossil fuels today releases carbon dioxide that was locked away long ago: it effectively reverses part of the slow process that built up the oxygen-rich atmosphere, returning ancient carbon to the air far faster than nature removed it.

Try this

Q1. State the percentage of oxygen in the atmosphere today. [1 mark]

  • Cue. About 21%21\%.

Q2. Name the process by which early plants and algae added oxygen to the atmosphere. [1 mark]

  • Cue. Photosynthesis.

Exam-style practice questions

Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

WJEC 20193 marksState the approximate proportions of the two main gases in today's atmosphere and name one other gas present in a small amount.
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A topic 1.4 recall question. Today's atmosphere is about 78% nitrogen and about 21% oxygen (1 mark for nitrogen proportion, 1 mark for oxygen proportion). One other gas present in a small amount is carbon dioxide (about 0.04%0.04\%), argon (a noble gas, about 1%1\%) or water vapour (1 mark). Markers reward the two main proportions and a valid minor gas. A common error is to swap the nitrogen and oxygen figures.

WJEC 20224 marksExplain how the amount of carbon dioxide in the atmosphere decreased and the amount of oxygen increased after the Earth formed.
Show worked answer →

A topic 1.4 Explain question. The early atmosphere was rich in carbon dioxide from volcanoes (1 mark). As the Earth cooled, water vapour condensed to form the oceans, and large amounts of carbon dioxide dissolved in them and were later locked into sedimentary rocks and shells (1 mark). When plants and algae evolved, photosynthesis removed carbon dioxide and released oxygen (1 mark), so over time carbon dioxide fell and oxygen rose to today's level (1 mark). Markers reward the volcanic source, the role of the oceans/rocks, and photosynthesis. A common error is to forget photosynthesis as the source of oxygen.

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