The helium-4 nucleus on its own is known as an alpha particle, the particle emitted in alpha-type radioactive decay. Neon is the only element other than helium that has never been observed to bond with other elements in stable compounds however, at temperatures close to absolute zero and under extreme pressure, helium can form unstable eximer molecules with elements like sodium, fluorine, and nitrogen. This structure structure that yields the most stable and inert element on the periodic table, a monoatomic gas that always exists in pure elemental form in its natural state. 99.999% of all helium atoms exists as stable isotope helium-4, composed of two protons, two neutrons, and two electrons in a single shell. Odorless, colorless, and nontoxic, helium is the lightest of the noble gases, and the second lightest of all elements in the universe after hydrogen. This inertness is characteristic of the elements known as “noble gases” (Group 18 on the periodic table, which includes neon and argon), whose extreme stability and unwillingness to react with other elements is due to the completeness of their outer valence shells. Helium remained unknown by scientists for so long due to its extreme chemical inertness, making it difficult to detect via conventional methods. In space, however, helium is far more prevalent it is the second most abundant element in the universe after hydrogen, making up 24% of its total observable mass, and is primarily produced in the cores of hydrogen-burning stars that fuse protons into helium nuclei to produce light and heat. It is the sixth most abundant gas in the atmosphere at 5.2ppm and one of the only elements light enough to possess escape velocity, rising and exiting the atmosphere at a rate roughly equal to its formation on earth. Limited to the scope of the planet, helium is a relatively scarce element, produced only in via the radioactive decay of elements such as uranium and thorium. Scottish chemist William Ramsay was the first to successfully isolate the element in 1895 by treating a sample of the uranium ore cleveite, thereby proving its existence on earth. Several months thereafter, English chemists Joseph Norman Lockyer and Edward Franklin observed the same spectral line in sunlight and came to the same conclusion the two proposed the name “Helium” for the element after Helios, the Greek god of the sun, appropriate for the first element to be discovered in space rather than on earth. Using a spectrometer to analyze the chromosphere of the sun during a solar eclipse in 1868, French astronomer Jules Janssen noticed an unusual yellow line in the spectrum that he suspected indicated the presence of a yet-undiscovered element. American Elements: The Materials Science Company™ | Certified bulk & lab quantity manufacturer of metals, chemicals, nanoparticles & other advanced materials
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