The major gas in the Earth's atmosphere is nitrogen (N2).

Explanation:
The Earth's atmosphere is composed of several gases, but the most abundant gas is nitrogen. Here is an explanation of why nitrogen constitutes the major portion of the atmosphere:

1. Nitrogen Composition:
- Nitrogen makes up around 78% of the Earth's atmosphere.
- It occurs in a diatomic form, meaning that two nitrogen atoms combine to form a nitrogen molecule (N2).

2. Abundance:
- Nitrogen is highly abundant due to various natural processes, such as volcanic activity, biological processes, and the decay of organic matter.
- The nitrogen cycle plays a crucial role in maintaining the balance of nitrogen in the atmosphere, biosphere, and other Earth systems.
- Nitrogen is also a component of many minerals and rocks, which can release nitrogen into the atmosphere through weathering processes.

3. Role in Life:
- Nitrogen is an essential element for life on Earth.
- It is a key component of proteins, nucleic acids (DNA and RNA), and other vital organic molecules.
- Plants and other organisms require nitrogen to grow, and it is often a limiting nutrient in ecosystems.
- Nitrogen fixation, the process by which certain bacteria convert atmospheric nitrogen into forms usable by plants, is crucial for maintaining the nitrogen balance in ecosystems.

4. Stable and Inert:
- Nitrogen is relatively stable and inert, meaning it does not react easily with other substances.
- This stability allows it to remain in the atmosphere for long periods without being consumed or altered significantly.
- Compared to other gases like oxygen (O2) and carbon dioxide (CO2), nitrogen is less reactive and less influenced by biological and chemical processes.

5. Role in Climate:
- While nitrogen itself does not directly contribute to climate change, it plays a role in regulating the Earth's climate system.
- Nitrogen compounds, such as nitrous oxide (N2O), can act as greenhouse gases and contribute to global warming.
- Excessive nitrogen inputs from human activities, such as the use of nitrogen-based fertilizers, can lead to environmental issues like eutrophication and air pollution.

In conclusion, nitrogen is the major component of the Earth's atmosphere due to its abundance, stability, and essential role in supporting life. Understanding the composition and dynamics of the Earth's atmosphere is crucial for studying climate change, atmospheric processes, and the interconnectedness of Earth's systems.