Worlds that support life have water in abundance, and during the congealing period after a big bang hydrogen and oxygen in many states can be found freely floating around the intensely hot proto-planets. As planets congeal, the pressure results in heat, but after time this dissipates. Meanwhile, the surface boils. Condensation occurs, forming seas upon the surface, but as nature abhors a vacuum, freely floating molecules do not all settle. What causes an atmosphere to exist, and what factors affect the composition of an atmosphere. Even in the absence of heat that would cause molecules of whatever nature to vaporize, an atmosphere builds. The Earth's atmosphere continues to build today, but are the oceans boiling?
Water vapor is in abundance in the Earth's atmosphere, yet arrived there not due to the action of intense heat but to fill a void. Place a vacuum against the surface of a pool of water and watch what happens - water vapor. The water pool will not completely disburse because its normal state at the condensation temperature is a liquid. But the constant motion of molecules means that the molecules at the surface have nothing to bump against in a vacuum, so like a car without brakes, off they go. At a certain point the air-borne molecules start bumping into each other and bumping against the surface of the water pool, and the situation stabilizes. So, does this mean that the atmosphere of a planet is constantly disbursing into space? Yes and no. Deep space is bitterly cold, and when moving away from the surface of a planet air- borne molecules slow down the bumping action. The situation stabilizes, again. However, some small loss is a constant factor, so that after billions of years some small quantity of the planet's substance has dissipated.
Atmospheres, as any meteorologist knows, are composed of more than just free oxygen and water vapor - an atmosphere reflects in its composition the planet it wraps. Every metal and every molecule combination on the open surface of the planet can be found in the atmosphere. This is demonstrated by the sense of smell, which is in fact nothing more than contact of the nose with tiny particles floating in the air. In fact, as volcanic eruptions send substances from the core of the Earth airborne, the atmosphere usually reflects the planet in its entirety. However, just as the oceans differ from the land, so the atmosphere differs also, from both land and sea.
Land is composed of elements or molecular combinations that are either not water soluble, tend to cling to other molecules to form a heavy settling substance, or are not exposed to enough water to leave its solid state. Under constant rain, soil erodes, but likewise clumps and clings to other soil particles and thus again settles out. Metals washed constantly with a liquid are found in that liquid, thus the concern for lead poisoning when drinking water stands in lead pipes. Many factors affect whether a substance is found on land, in the sea, or in the air. If it clumps and clings it will eventually be too heavy for anything but land or the sea floor. If it is a liquid at the temperatures normal for the Earth it will find its way into the water systems, there to be evenly disbursed if water soluble or if not soluble to form a separate layer in the water body such as oil on top or liquid mercury below.
An atmosphere is composed of those elements which can remain free or clump only to form tiny molecules, so big and no larger. Water vapor is composed of two parts hydrogen and one part oxygen, and the three elements form a tight band with little tendency to clump or cling unless other factors present. Similarly, any combination of elements that is discrete will remain airborne. These tiny elements or discrete groupings of elements can include heavy metals, as the winds that carry radioactivity across the land and sea after a nuclear explosion attest. Elements capable of being radioactive are some of the heaviest known to man, yet there they are, wafting aloft.
The composition of atmospheres is dependent on wind action and air currents also. Some elements or groupings would move lower within the atmosphere due to their relative weight, and some rise, due to being light, were the atmospheric soup not constantly stirred. The Albatross, a giant bird of no small weight, soars almost endlessly on air currents above the waves, its wings not moving for hours at a time. Atmospheric currents are affected by the warmth or coolness of the land or sea mass underneath, the density of air masses nearby, the pressure of any air masses moving toward or away from the spot, and the temperature of the air mass itself as it is warmed by the Sun or cooled on the dark side of the Earth - constantly stirred.
Thus, one should take care what they spew into the air - as it does not simply blow away.