GCSE›AQA GCSE Chemistry›Mind mapsThe Evolution of the AtmosphereAQA GCSE Chemistry: Mind mapStudy pack PDFAlso for this subtopic:Revision notesFlashcardsSubtopic testCover factsEarly atmosphereFormed about 4.6 billion years agoFrom outgassing by volcanoesMainly COX2\ce{CO2}COX2 and water vapourSmall amounts of ammonia and methaneLittle or no oxygenLosing water and CO2Earth cooled → water vapour condensed into oceansCOX2\ce{CO2}COX2 dissolved in the oceansFormed carbonates in sedimentary rocksCarbon locked in fossil fuelsOxygen arrivesPhotosynthesis by algae and plantsOxygen built up slowly over billions of yearsGreat Oxygenation Event about 2.4 billion years agoOxygen formed the ozone layer, blocking UVAtmospherehow it evolvedN2O2ArAmmonia to nitrogenUV and lightning broke down ammoniaDenitrifying bacteria made NX2\ce{N2}NX2Nitrogen is inert, so stableToday's air78% nitrogen21% oxygenAbout 1% argon0.04% carbon dioxideBroadly stable for about 200 million yearsExam tipsContrast: early reducing, now oxidisingGive both CO2 routes: rocks and fossil fuelsExplain why oxygen accumulated: photosynthesis