welcome to our review on forming the atmosphere the first thing we need to do is go way back into the distant past when we go back to the beginnings of the earth so we're going back 4.54 billion years because that's how old the earth actually is and when we go back 4.5 4 billion years to the early stages of our planets development what we find is it was a rather inhospitable place there was loads of volcanic activity and as a result of that it was reducing large volumes of water vapor and carbon dioxide which was our very early atmosphere was pretty much just carbon dioxide with a bit of water vapor thrown in as time went on the earth actually started to cool and as a result of that cooling that water vapor condensed to form the oceans so the water vapor was out of our atmosphere but the atmosphere then was comprised mainly of carbon dioxide and there were small amounts of ammonia and methane around as time continued to pass then eventually we had algae and plants appear and their appearance is key to the change that we see in our atmosphere that allows life like us to exist and that is that the algae and the plants carried out photosynthesis which then reduced the amount of carbon dioxide in the atmosphere because if you remember photosynthesis is carbon dioxide plus water making glucose and oxygen so when those plants and algae were making the oxygen then what happened was it reacted with the metal that was present to make metal oxides then after most of the methyl have been oxidized we had this excess of oxygen this free oxygen that then began to accumulate in the atmosphere but we can see those methyl oxides in the ground today as you look around because you can actually make out the bands of it as it formed so in the rock on the left you can see the darker red bands there of metal oxide and in the piles of the soil on the right those different colors they're the seven different colored sand is caused by different metal oxides that are present the end result of all of these changes is our present-day atmosphere and you do need to remember the percentages of the gases so nitrogen is 78% oxygen 21% carbon dioxide naught point naught 4% so those are the ones to remember I've never seen them ask about argon but it is not point nine percent in case you want to remember it I have seen them give you this pie chart and then ask you to label nitrogen oxygen and carbon dioxide recalling their percentages in the past hopefully at the end this video you can explain how the atmosphere is thought to have originally formed and how our oxygen-rich atmosphere developed over time you should also be able to recall the percentages of the gases present in our present-day atmosphere