Rank the gases in atmospheric air from greatest to least in percentage: O2, CO2, nitrogen, water.

The correct ranking of gases in atmospheric air from greatest to least in percentage is based on their typical concentration in the atmosphere.

Nitrogen is the most abundant gas in the atmosphere, making up about 78% of the air we breathe. Following nitrogen, oxygen is the second most prevalent, comprising roughly 21% of the atmosphere. Water vapor is variable but typically accounts for around 0.4% to 2% depending on the humidity, and carbon dioxide, while increasing due to human activity, is currently about 0.04%.

Therefore, ranking these gases from greatest to least in percentage yields nitrogen first, followed by oxygen, then water vapor, and finally carbon dioxide. Understanding the composition of atmospheric gases is essential for various applications in fields such as environmental science, medicine, and engineering.