In Earth science, which gas makes up the largest percentage of the Earth atmosphere by volume at sea level?

Introduction / Context:
The composition of the Earth atmosphere is a fundamental fact in both geography and science. Although oxygen is essential for human and animal respiration, it is not actually the most abundant gas in the air around us. Knowing the relative proportions of the main gases is important for understanding weather, climate, and various environmental processes. This question tests whether you correctly recall which gas dominates the atmosphere by volume near the Earth surface.

Given Data / Assumptions:
- The question asks for the gas that makes up most of the Earth atmosphere by volume at sea level.
- Options include oxygen, helium, nitrogen, and carbon dioxide.
- The standard composition of dry air near the surface is assumed.
- Minor gases and water vapour are not the focus of this question.

Concept / Approach:
Dry air near the Earth surface is composed mainly of nitrogen and oxygen, with smaller amounts of argon and trace gases such as carbon dioxide. Nitrogen accounts for about seventy eight percent by volume, while oxygen accounts for about twenty one percent. Carbon dioxide is present at less than one percent, and helium is a trace gas. Therefore, the correct answer is nitrogen, which is the most abundant gas in the atmosphere by volume at sea level.

Step-by-Step Solution:
1. Recall the approximate composition of dry air at sea level. 2. Note that nitrogen is around seventy eight percent and oxygen is around twenty one percent by volume. 3. Recognise that carbon dioxide and helium occur only in trace amounts compared with nitrogen and oxygen. 4. Compare the percentages and identify the gas with the highest proportion. 5. Conclude that nitrogen is the gas that makes up most of the Earth atmosphere.

Verification / Alternative check:
Standard data given in school textbooks, encyclopedias, and scientific references all agree on the primary composition of the atmosphere. Charts showing pie diagrams of atmospheric composition consistently depict nitrogen as the largest segment, followed by oxygen. Carbon dioxide is usually represented as a very small slice, and helium is almost invisible on such scales. Weather and climate discussions often start by mentioning that nitrogen is the dominant component, which confirms this fact from multiple independent sources.

Why Other Options Are Wrong:
Oxygen: This is the second most abundant gas in the atmosphere, important for respiration, but it does not exceed nitrogen in volume.
Helium: Helium is a light, noble gas present only in very small trace amounts and does not make up a large fraction of the atmosphere.
Carbon dioxide: This gas is significant for the greenhouse effect and photosynthesis but is present at less than one percent by volume in the lower atmosphere.

Common Pitfalls:
Because oxygen is so vital for breathing, people sometimes assume that it must be the most abundant gas, leading them to choose it incorrectly. Others may overestimate the amount of carbon dioxide due to discussions about global warming. To avoid these errors, it is best to memorise the approximate composition of dry air and remember that nitrogen is the dominant component by volume, followed by oxygen, with other gases making up only small fractions.

Final Answer:
Nitrogen is the gas that makes up most of the Earth atmosphere by volume at sea level.