What gas is dominant in the composition of the atmosphere?

The Earth’s atmosphere is the shell of the Earth that is held together by gravity. There are several gases in the atmosphere. Such as: nitrogen, which is the most in the atmosphere – 78%; its oxygen in the air is about 20.95%, it is very important because it is used for breathing; water vapor which constantly changes its percentage from 0.5% to 1.2%; carbon dioxide, which is used by plants for photosynthesis. There is constant pressure in the atmosphere, which is numerically the force that acts on 1 unit of surface area, atmospheric pressure is measured in millimeters of mercury.