(a) Solids are generally very heavy while gases are light. Explain.
(b) Carbon dioxide gas is heavier than both nitrogen and oxygen. Why does not it form lower layer in the atmosphere ?

Explanation

(a) In the solids, the particles are very closely packed. As a result, the number of particles per unit volume is quite large. Therefore, the solids are normally quite heavy. In the gases, the particles are loosly packed. The number of particles per unit volume is comparatively small. Therefore, gases are light .
(b) The diffusion of a gas is not affected by gravity. This means that carbon dioxide `(CO_(2))` remains uniformaly mixed in air. Therefore, the gas does not form the lower layer in the atmosphere.