Short answers, pulled from the story.
By mole fraction, dry air is 78.08 percent nitrogen, 20.95 percent oxygen, 0.93 percent argon, and 0.04 percent carbon dioxide, with small amounts of trace gases. Air also holds a variable amount of water vapor, averaging around 1 percent at sea level and about 0.4 percent over the entire atmosphere.
Earth's atmosphere divides into five main layers: the troposphere from 0 to about 12 kilometers, the stratosphere up to about 50 kilometers, the mesosphere to about 80 kilometers, the thermosphere up to 500 to 1000 kilometers, and the exosphere beyond. They are distinguished mainly by their temperature behavior with altitude.
The Kármán line at 100 kilometers is often used as the conventional definition of the edge of space, and by international convention it marks where human travelers are considered astronauts. About 99.99997 percent of the atmosphere's mass lies below this line.
The sky looks blue because of Rayleigh scattering, in which shorter blue wavelengths scatter more easily than longer red ones, so you see scattered blue light. Sunsets appear red because the Sun's low rays pass through more atmosphere, scattering out the blue and leaving the red.
Free oxygen began to accumulate about 2.4 billion years ago during the Great Oxygenation Event, after roughly a billion years of cyanobacterial photosynthesis recorded in stromatolite fossils from 2.7 billion years ago. Oxygen could not accumulate until its production exceeded the reducing materials, such as ferrous iron and sulfur, that removed it.
The atmosphere has a total mean mass of about 5.1480 times 10 to the 18 kilograms, roughly 1/1,200,000 the mass of Earth. Three quarters of that mass lies within about 11 kilometers of the surface, and 99 percent lies below 30 kilometers.