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.