Although nitrogen makes up about 78% of the Earth's atmosphere, it is often considered inert because it is relatively unreactive under standard conditions. There are a few reasons for this:

1. Strong Triple Bond: Nitrogen atoms are bonded together by a very strong triple bond, which requires a significant amount of energy to break. This means that nitrogen molecules are very stable and do not easily react with other substances.

2. High Electronegativity: Nitrogen has a relatively high electronegativity, meaning that it has a strong attraction for electrons. This makes it difficult for nitrogen to share electrons with other atoms, which limits its reactivity.

3. Lack of Polarity: Nitrogen molecules are nonpolar, meaning that they do not have a significant positive or negative charge. This makes it difficult for them to interact with polar molecules or charged ions.

However, it's important to note that nitrogen is not completely inert. Under certain conditions, such as high temperatures, high pressures, or in the presence of certain catalysts, nitrogen can react with other substances. For example, nitrogen can react with oxygen to form nitrogen oxides, which are components of smog and acid rain. It can also react with hydrogen to form ammonia, which is used in the production of fertilizers and explosives.

In summary, nitrogen is considered inert under standard conditions due to its strong triple bond, high electronegativity, and nonpolarity. However, it can undergo reactions under certain specific conditions.