Short Answer: UV-C from the Sun is absorbed by the oxygen up high, and that allows ozone to be made up high. Ozone decays back to oxygen before it can fall very far... Ozone is not the heaviest gas, iodine heptafluoride (for example) is much heavier. Ozone is heavier than other gases commonly found in the atmosphere... Ozone is present in all layers of the atmosphere. It is concentrated in the stratosphere (which is not the "top of the atmosphere" there are more layers above it), because almost all the UV-C from the Sun is stopped there by oxygen and nitrogen, where the atmosphere's density starts really going up. Oxygen molecules broken into atoms, can yield the production of some ozone, monatomic oxygen is very short lived, so ozone is formed very quickly or not at all. As you move lower in the atmosphere, water vapor (and other contaminants) rapidly destroy ozone, so this allows ozone concentrations to decrease to near zero at Earth's surface. Oxygen molecules are broken apart by 215nm or shorter UV light (UV-C). So are nitrogen, and other atmospheric constituents. So beyond a certain depth into the atmosphere, there is insufficient 215nm or shorter light to break oxygen which can then make ozone. Then it takes a very long time (months) for gasses to diffuse in quantity large distances vertically, and ozone gas does degrade with time... so it appears this heavy gas hovers high above us. When in fact it is created, and disppears before it can fall very far.