Answer

Co2 is generally havier then oxygen and nitrogen . It does not form lower layer in the atmosphere because:

• CO2 has capability to absorb IR radiations and are heated and thus gets lifted up. Vibrational energy of C=O bonds lies in IR while this doesn't happens in N2 and O2.
• 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. 
• Out in the open it is being constantly mixed by all sorts of major air movements. These include major air currents—the westerlies, air cells, storms, areas of warm air moving up and areas of cold air moving down(convection currents).
• Dispersion is another major way gases disperse, that is, move from areas of concentration to areas of lesser concentration.
• Both oxygen and nitrogen are usually dioxygen and dinitrogen in the atmosphere. So carbon dioxide is not that much heavier than the bulk of the atmosphere. There are many trace gases in the atmosphere that are heavier than carbon dioxide and yet they remain mixed as the atmosphere is constantly moving.