What layer in the atmosphere do air molecules spread out and heat up?

Air molecules spread out and heat up primarily in the troposphere, which is the lowest layer of the Earth's atmosphere. This layer extends from the surface up to about 8 to 15 kilometers (5 to 9 miles) high, depending on geographic location and weather conditions. The troposphere is where most weather phenomena occur, and it is heated by the Earth's surface, causing air to rise and cool as it expands.