Which gas forms about 78% of the Earth’s atmosphere?

Nitrogen forms about 78% of the Earth's atmosphere because it is an inert gas, meaning it is chemically stable and does not easily react with other elements. This stability allows nitrogen to accumulate and remain in the atmosphere for long periods of time. Additionally, nitrogen is constantly being produced by processes such as volcanic activity and biological nitrogen fixation, where certain bacteria convert atmospheric nitrogen into a form that can be used by plants. These factors contribute to the high abundance of nitrogen in our atmosphere, making it the most prevalent gas.