The atmosphere is mainly composed of Nitrogen (78%). But on earth for life to be supported oxygen needs to be present. Oxygen is the next most abundant element in the atmoshpere (about 21%). The next most abundant element is Argon (0.9%), then it is Carbon - 0.03%. Then there are several small minute volumes of other elements in the atmosphere that are irrelevant 

Not quite irrelevant. For example, 90% of the atmosphere's ozone is in the stratosphere and forms only a tiny proportion of the overall atmosphere, but it protects life on Earth from most of the harmful ultra-violet light that the Sun fires in our direction. Carbon dioxide is also present in very small quantities, but is vital to plant life as well as being linked with global warming. It's not really useful to consider the atmosphere as just the sum of its elements; after all, we wouldn't consider a glass of water to be just a mix of hydrogen and oxygen.