(a) Air is a mixture of gases, water vapour and dust particles. It has many gases such as nitrogen, oxygen, carbon dioxide, carbon monoxide, water vapour and even dust particles.
(b) The relative amount of gases present in our atmosphere is as follows:
Oxygen - 20.9%
Nitogen - 78%
Carbon dioxide = 0.03%
The increasing order these gases based on their abundance is
carbon dioxide < Oxygen < Nitrogen.