On average, the mass of the earth's atmosphere is composed of 76.7 percent nitrogen, in the form of N2.
The remaining 23.3 percent of the atmosphere's mass is composed near-entirely of oxygen in the form of O2.
Extremely small amounts of argon, carbon dioxide, neon, and helium are also found in the atmosphere, but these constituents are so light that they are nearly untraceable by mass.
Pollutants--like methane, carbon dioxide and sulfur--account for very little of the atmosphere's mass, but even small changes in its chemical composition can be catastrophic.
The average composition of the atmosphere is monitored by many academic and government organizations, including NASA and many meteorological foundations.