SCIENCE PHOTO LIBRARY SCIENCE PHOTO LIBRARY
Air in a flask. The Earth's atmosphere mostly consists of the elemental gases nitrogen (N2) and oxygen (O2), both vital for life on Earth, with trace amount of other gases and elements. Oxygen is used directly in respiration, while nitrogen is found in many organic compounds. Nitrogen is a non-metal in group 15 of the periodic table. It forms 78 percent of the atmosphere, and liquefies at minus 196 degrees Celsius. Oxygen is a non-metal in group 16 of the periodic table. It forms 20 percent of the atmosphere, and liquefies at minus 219 degrees Celsius.
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