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: any of two or more species of atoms of a chemical element with the same atomic number and nearly identical chemical behavior but with differing atomic mass or mass number and different physical properties These example sentences are selected automatically from various online news sources to reflect current usage of the word 'isotope.' Views expressed in the examples do not represent the opinion of Merriam-Webster or its editors. The atmosphere of Earth is the layer of gases, commonly known as air, that surrounds the planet Earth and is retained by Earth's gravity.The exosphere is the outermost layer of Earth's atmosphere (i.e. It extends from the exobase, which is located at the top of the thermosphere at an altitude of about 700 km above sea level, to about 10,000 km (6,200 mi; 33,000,000 ft) where it merges into the solar wind.This layer is mainly composed of extremely low densities of hydrogen, helium and several heavier molecules including nitrogen, oxygen and carbon dioxide closer to the exobase.The atoms and molecules are so far apart that they can travel hundreds of kilometers without colliding with one another.Thus, the exosphere no longer behaves like a gas, and the particles constantly escape into space.However, the aurora borealis and aurora australis sometimes occur in the lower part of the exosphere, where they overlap into the thermosphere.

Various industrial pollutants also may be present as gases or aerosols, such as chlorine (elemental or in compounds), fluorine compounds and elemental mercury vapor.

Air also contains a variable amount of water vapor, on average around 1% at sea level, and 0.4% over the entire atmosphere.

Air content and atmospheric pressure vary at different layers, and air suitable for use in photosynthesis by terrestrial plants and breathing of terrestrial animals is found only in Earth's troposphere and in artificial atmospheres.

Sulfur compounds such as hydrogen sulfide and sulfur dioxide (SO Earth's atmosphere Lower 4 layers of the atmosphere in 3 dimensions as seen diagonally from above the exobase.

Layers drawn to scale, objects within the layers are not to scale.

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