text: i've got my chemistry question.. anyone can help? describe the changes that occured over the years (about 4000 million years) that led to the formation of today's atmosphere? ch4 + cao -> co2 + h2o + ca or something similar, basically the atmosphere billions of years ago consisted mainly of ch4. it reacted with some oxide or oxygen bearing compound to form h2o, o2 & co2- to give the atmosphere as we know it