Why air surrounding earth always remains stick towards earth instead of flowing towards space like a gas?
The force that keeps the air (or the atmosphere) from flowing towards space is the force of gravity. The molecules of the air can escape the gravity of the Earth if their average speed is greater than the escape velocity (which is near 11 km⁄s for Earth). But not all molecules of the air are equally likely to escape. The light molecules (for example, helium) move faster than the heavier molecules (for example, oxygen) and they are more likely to reach escape velocity and escape to space. And this is the reason why the light molecules such as helium or hydrogen are rare in the air. But the most part of the air always remains stick towards Earth due to the gravity.