What region contains the majority of molecules in the atmosphere?

The majority of molecules in the atmosphere are found in the troposphere, which is the lowest layer of Earth's atmosphere, extending from the surface up to about 8 to 15 kilometers (5 to 9 miles) high. This region contains approximately 75% of the atmosphere's mass and is where most weather phenomena occur. The concentration of gases, such as nitrogen and oxygen, is highest in this layer, making it crucial for supporting life on Earth.