<P> The abundance of chemical elements in the universe is dominated by the large amounts of hydrogen and helium which were produced in the Big Bang . Remaining elements, making up only about 2% of the universe, were largely produced by supernovae and certain red giant stars . Lithium, beryllium and boron are rare because although they are produced by nuclear fusion, they are then destroyed by other reactions in the stars . The elements from carbon to iron are relatively more common in the universe because of the ease of making them in supernova nucleosynthesis . Elements of higher atomic number than iron (element 26) become progressively more rare in the universe, because they increasingly absorb stellar energy in being produced . Elements with even atomic numbers are generally more common than their neighbors in the periodic table, also due to favorable energetics of formation . </P> <P> The abundance of elements in the Sun and outer planets is similar to that in the universe . Due to solar heating, the elements of Earth and the inner rocky planets of the Solar System have undergone an additional depletion of volatile hydrogen, helium, neon, nitrogen, and carbon (which volatilizes as methane). The crust, mantle, and core of the Earth show evidence of chemical segregation plus some sequestration by density . Lighter silicates of aluminum are found in the crust, with more magnesium silicate in the mantle, while metallic iron and nickel compose the core . The abundance of elements in specialized environments, such as atmospheres, or oceans, or the human body, are primarily a product of chemical interactions with the medium in which they reside . </P> <Table> Ten most common elements in the Milky Way Galaxy estimated spectroscopically <Tr> <Th> Z </Th> <Th> Element </Th> <Th> Mass fraction (ppm) </Th> </Tr> <Tr> <Td> </Td> <Td> Hydrogen </Td> <Td> 739,000 </Td> </Tr> <Tr> <Td> </Td> <Td> Helium </Td> <Td> 240,000 </Td> </Tr> <Tr> <Td> 8 </Td> <Td> Oxygen </Td> <Td> 10,400 </Td> </Tr> <Tr> <Td> 6 </Td> <Td> Carbon </Td> <Td> 4,600 </Td> </Tr> <Tr> <Td> 10 </Td> <Td> Neon </Td> <Td> 1,340 </Td> </Tr> <Tr> <Td> 26 </Td> <Td> Iron </Td> <Td> 1,090 </Td> </Tr> <Tr> <Td> 7 </Td> <Td> Nitrogen </Td> <Td> 960 </Td> </Tr> <Tr> <Td> 14 </Td> <Td> Silicon </Td> <Td> 650 </Td> </Tr> <Tr> <Td> 12 </Td> <Td> Magnesium </Td> <Td> 580 </Td> </Tr> <Tr> <Td> 16 </Td> <Td> Sulfur </Td> <Td> 440 </Td> </Tr> </Table> <Tr> <Th> Z </Th> <Th> Element </Th> <Th> Mass fraction (ppm) </Th> </Tr>

What is the most common gas in the universe
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