Atmosphere atmospheres were then modified over time by various complex factors, resulting in quite different outcomes. The atmospheres of the planets Venus and Mars are primarily composed of carbon dioxide, with small quantities of nitrogen, argon, oxygen and traces of other gases. The composition of Earth's atmosphere is largely governed by the by-products of the life that it sustains. Dry air from Earth's atmosphere contains 78.08% nitrogen, 20.95% oxygen, 0.93% argon, 0.04% carbon dioxide, and traces of hydrogen, helium, and other "noble" gases (by volume), but generally a variable amount of water vapor is also present, on average about 1% at
Noble gas compound century, their inertness was ascribed to a full valence shell of electrons which render them very chemically stable and nonreactive. All noble gases have full "s" and "p" outer electron shells (except helium, which has no "p" sublevel), and so do not form chemical compounds easily. Their high ionization energy and almost zero electron affinity explain their non-reactivity. In 1933, Linus Pauling predicted that the heavier noble gases would be able to form compounds with fluorine and oxygen. Specifically, he predicted the existence of krypton hexafluoride (KrF) and xenon hexafluoride (XeF), speculated that XeF might exist as an unstable compound,