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Based on molecular orbital theory, He 2 should not exist, and a chemical bond cannot form between the atoms. However, the van der Waals force exists between helium atoms as shown by the existence of liquid helium , and at a certain range of distances between atoms the attraction exceeds the repulsion.
In Earth's atmosphere, the ratio of 3 He to 4 He is 1.343(13) × 10 −6. [5] However, the isotopic abundance of helium varies greatly depending on its origin. In the Local Interstellar Cloud , the proportion of 3 He to 4 He is 1.62(29) × 10 −4 , [ 6 ] which is ~121 times higher than in Earth's atmosphere.
[132] [133] The concentration is low and fairly constant despite the continuous production of new helium because most helium in the Earth's atmosphere escapes into space by several processes. [134] [135] [136] In the Earth's heterosphere, a part of the upper atmosphere, helium and hydrogen are the most abundant elements.
At pressure of 1 atmosphere, the transition occurs at approximately 2.17 K. The lowest pressure at which He-I and He-II can coexist is the vapor−He-I−He-II triple point at 2.1768 K (−270.9732 °C) and 5.0418 kPa (0.049759 atm), which is the "saturated vapor pressure " at that temperature (pure helium gas in thermal equilibrium over the ...
Noble gas cluster ions exist for different noble gases. Singly charged cluster ions containing xenon exist with the formula He n Xe + m, where n and m ≥ 1. [74] Many different He n Kr + exist with n between 1 and 17, with higher values possible. He n Kr + 2 and He n Kr + 3 also exist for many values of n. He 12 Kr + 2 and He 12 Kr +
Liquid helium is a physical state of helium at very low temperatures at standard atmospheric pressures.Liquid helium may show superfluidity.. At standard pressure, the chemical element helium exists in a liquid form only at the extremely low temperature of −269 °C (−452.20 °F; 4.15 K).
Since the atmosphere of the Earth has a mass of about 5.14 × 10 18 kilograms (1.133 × 10 19 lb), [24] the mass of 3 He in the Earth's atmosphere is the product of these numbers, or about 37,000 tonnes (36,000 long tons; 41,000 short tons) of 3 He. (In fact the effective figure is ten times smaller, since the above ppm are ppmv and not ppmw.
The surface-based homosphere includes the troposphere, stratosphere, mesosphere, and the lowest part of the thermosphere, where the chemical composition of the atmosphere does not depend on molecular weight because the gases are mixed by turbulence. [28]