The major constituent of Earth’s atmosphere by percentage is nitrogen (N2), which makes up about 78% of the atmosphere. Oxygen (O2) is the second most abundant gas, comprising approximately 21%. Together, nitrogen and oxygen make up about 99% of the Earth’s atmosphere. Other trace gases, including argon, carbon dioxide, neon, helium, methane, krypton, hydrogen, xenon, and ozone, make up the remaining 1%. These trace gases play crucial roles in various atmospheric processes, including the greenhouse effect and the overall composition and chemistry of the atmosphere.