Why did molecular nitrogen become a major gas in Earth’s atmosphere?

Molecular nitrogen became a major component of Earth’s atmosphere primarily through the process of outgassing from the planet's interior during its early history. As Earth formed and differentiated, volatiles trapped within the solid particles that accreted to create the planet were released.

Unlike captured primordial gases, which would have included methane and ammonia, the outgassed atmosphere was composed of gases that were stable under the high-temperature conditions of the Earth's interior. In this environment, nitrogen was released primarily in the form of molecular nitrogen ($N_2$). Because nitrogen is geochemically inert and does not easily react with other elements under ordinary conditions, it remained in the atmosphere rather than being sequestered into the crust or oceans through chemical weathering or mineral formation, unlike carbon dioxide or water vapor. Consequently, nitrogen accumulated over geological time to become the most abundant gas in our atmosphere today.12