What layer in the atmosphere has most of the gas molecules?

The layer of the atmosphere that contains most of the gas molecules is the troposphere. This lowest layer extends from the Earth's surface up to about 8 to 15 kilometers (5 to 9 miles) high, depending on latitude and weather conditions. It is where nearly all weather phenomena occur and contains approximately 75% of the atmosphere's mass. As altitude increases, the density of gas molecules decreases significantly.