Molecules in air in the atmosphere are attracted by gravitational force of the earth. Explain why all of them do not fall into the earth just like an apple falling from a tree.

In the case of apple falling from the tree, the mass of the apple is very large as compared to the air molecules underneath the apple. Thus, the vertical motion of apple dominates and due to gravitational force, it falls towards the earth.

Whereas an air molecule is surrounded by many air molecules having relatively equal mass. also due to random thermal motion of molecules their velocity is not directed downwards. Therefore, the air molecules do not fall towards earth. The density of air molecules close to the surface of earth is greater than other due to the gravitational force.