Question

Carbon dioxide gas is heavier than both nitrogen and oxygen. why does not it form lower layer in the atmosphere?

Answer 1

Carbon dioxide is present in the atmosphere in 0.04 % only.  But Oxygen and Nitrogen are present in 21% and 78% quantities.  CO2 is heavier than O2 and N2.  

However at room temperatures and higher, the gases have a lot of mobility.  The molecules have high velocity and due to winds too, they mix, flow in all directions.  So all the Carbon dioxide does not settle at the lowest altitude.  The Carbon dioxide gets generated and gets depleted by the trees.  This cycle continues.

Thus the concentration of CO2 remains about the same in the layers in the atmosphere.  

But if some people remain for a long duration in a totally sealed room, then the CO2 concentration increases and settles at the lower altitudes and that is dangerous to life.

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If one goes into a tunnel or a cave, in the interiors the CO2 concentration is higher.  So one has to be more careful.

Answer 2

Though carbon dioxide is heavier than air, but it has a tendency to mix with other gases of air and since the rest gases are found at higher altitude so the carbon dioxide could not be able to be in the lower layer of atmospheric air. ... Hence, the concentration of Oxygen is greater than CO2.

Hope it helps..