Atom is the most basic unit of matter. They are the general term used to describe pieces of matter. But a different kind of atom makes up an element. A combination of atom of different elements creates a molecule. For example, you have water; it has a chemical formula of H2O. If you separate H from O, you have two atoms. Two atoms for hydrogen and one atom for oxygen. However, they are of different elements. And when you combine them to form H2O, you create a molecule. Another example of molecules are air. Air pressure is gravity pulling down on air molecules. Like us, air has also mass and the pressure is brought down by the earth’s gravity causing an increase in weight exerted on you as you descend lower into the atmosphere. So, as you enter into the other layers of atmosphere above the troposphere, the air pressure starts to decrease. Below the atmosphere is the hydrosphere. This is where all liquid forms are located. And since the seawater has a greater mass than air, it has the greatest pressure.
