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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.
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.
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).
[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.
The helium-3, in liquid state at 3.2 K, can be evaporated into the superfluid helium-4, where it acts as a gas due to the latter's properties as a Bose–Einstein condensate. This evaporation pulls energy from the overall system, which can be pumped out in a way completely analogous to normal refrigeration techniques.
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 influence of the evolution of life has to be taken into account rather soon in the history of the atmosphere because hints of earliest life forms appeared as early as 3.5 billion years ago. [48] How Earth at that time maintained a climate warm enough for liquid water and life, if the early Sun put out 30% lower solar radiance than today, is ...
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 ...