7.3 The Earth atmosphere
The two main gases are both elements and account for about 99 percent of the gases in the atmosphere. They are:
about 4/5 or 80 percent nitrogen (a relatively unreactive gas)
about 1/5 or 20 percent oxygen (the gas that allows animals and plants to respire and for fuels to burn)
The remaining gases, such as carbon dioxide, water vapour and noble gases such as argon, are found in much smaller proportions.
7.3 The Earth’s Atmosphere in the past.
Scientists believe that the Earth was formed about 4.5 billion years ago. Its early atmosphere was probably formed from the gases given out by volcanoes. It is believed that there was intense volcanic activity for the first billion years of the Earth's existence.
The early atmosphere was probably mostly carbon dioxide with little or no oxygen. There were smaller proportions of water vapour, ammonia and methane. As the Earth cooled down, most of the water vapour condensed and formed the oceans.
Mars and Venus today
It is thought that the atmospheres of Mars and Venus today, which contain mostly carbon dioxide, are similar to the early atmosphere of the Earth.