What were the main components of the atmosphere when the Earth formed and what are the main components today?

When the Earth formed around 4.5 billion years ago, its atmosphere was primarily composed of hydrogen and helium, along with trace amounts of methane, ammonia, water vapor, and carbon dioxide. Over time, volcanic activity released gases, leading to the development of a secondary atmosphere dominated by nitrogen and carbon dioxide, with water vapor condensing to form oceans. Today, the atmosphere is approximately 78% nitrogen, 21% oxygen, and 1% other gases, including argon and carbon dioxide, reflecting significant changes due to biological processes and geological activity.