Helium is a chemical element with symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table. Its boiling and melting points are the lowest among all the elements. Helium is the second least reactive noble gas, after neon, and thus the second least reactive of all elements. It is inert and monatomic in all standard conditions. Since, helium has relatively low molar (atomic) mass, its thermal conductivity, specific heat, and sound speed in the gas phase are all greater than any other gas except hydrogen. For similar reasons, and also due to the small size of helium atoms, heliums diffusion rate through solids is three times that of air and around 65% that of hydrogen. Helium is the least water-soluble monatomic gas, and its refraction index is closer to unity than that of any other gas. The speed of sound in helium is nearly three times the speed of sound in air. Helium is the second most abundant as well as the lightest element in the observable universe, being present at about 24% of the total elemental mass, which is more than 12 times the mass of all the heavier elements, combined. On Earth it is relatively rare 5.2 ppm by volume in the atmosphere. Helium is a finite resource, and once released into the atmosphere, it readily escapes into space. This gas has a negative Joule-Thomson coefficient at normal ambient temperatures, meaning it heats up when allowed to freely expand.Once it reaches below Joule-Thomson inversion temperature ( of about 32 to 50k at 1 atmosphere), it freely expands and cools. Once pre-cooled below this temperature, helium can be liquefied through expansion cooling.
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