In Earth's atmosphere, which gas is the most abundant by volume in the air we breathe at sea level?

Introduction / Context:
The atmosphere surrounding the Earth is a mixture of several gases, and understanding their relative abundance is a basic concept in general science, geography, and environmental studies. Knowing which gas is most abundant in the air helps explain phenomena such as combustion, breathing, and climate. Many people assume oxygen dominates because it is essential for life, but atmospheric composition data tells a different story. This question tests knowledge of the gas that makes up the largest share of the air by volume.

Given Data / Assumptions:
- The focus is on the composition of the Earth atmosphere at sea level under normal conditions.
- Options include nitrogen, argon, oxygen, and helium.
- We are asked to identify the most abundant gas by volume.
- Trace gases like carbon dioxide are not listed because they form a much smaller fraction of the atmosphere.

Concept / Approach:
Dry air at sea level is composed of approximately 78 percent nitrogen, 21 percent oxygen, and about 0.9 percent argon, with other gases present in much smaller amounts. Nitrogen therefore makes up the largest share of the atmosphere. Oxygen, while vital for respiration and combustion, is the second most abundant gas. Argon is a noble gas present in less than 1 percent, and helium is present only in tiny trace amounts. Thus, the most abundant gas in the Earth atmosphere is nitrogen, not oxygen or any other gas listed.

Step-by-Step Solution:
Step 1: Recall the approximate percentages of major atmospheric gases: nitrogen about 78 percent, oxygen about 21 percent, argon about 0.9 percent. Step 2: Compare these values and note that nitrogen clearly has the highest percentage. Step 3: Recognise that helium is a trace gas found in very small amounts in the atmosphere. Step 4: Confirm that oxygen, though important for life, is only the second most abundant gas. Step 5: Select nitrogen as the correct answer.

Verification / Alternative check:
Standard geography and science textbooks present a composition chart for air, listing nitrogen around 78 percent, oxygen around 21 percent, and argon under 1 percent. Environmental and meteorological references use the same basic percentages when discussing atmospheric gases. This agreement among multiple sources confirms nitrogen as the most abundant gas in the atmosphere. No credible source suggests that oxygen, argon, or helium surpass nitrogen in percentage by volume in the lower atmosphere.

Why Other Options Are Wrong:
- Oxygen is vital for breathing but makes up only about 21 percent of the atmosphere and is therefore second in abundance.
- Argon is a noble gas present at less than 1 percent and cannot be the most abundant gas.
- Helium is present only in trace amounts and is far from being the major component of the air.

Common Pitfalls:
Because oxygen is often emphasised in discussions about respiration and fire, some learners mistakenly think it is the main component of air. Others may not fully remember the approximate percentages and may guess incorrectly. A handy memory aid is that air is roughly 4 parts nitrogen to 1 part oxygen, with a small amount of argon and traces of other gases. Keeping this ratio in mind makes it easy to select nitrogen as the most abundant gas in the atmosphere.

Final Answer:
The most abundant gas in the atmosphere is Nitrogen.