What is Helium atom ?

   

 Helium (from Greek) is a chemical element with the symbol He and atomic number 2.  It is a colorless, odorlesss, tasteless, non-toxic, inert, monatomic gas, the first in the noble gas group in the periodic table.  Its boiling point is the lowest among all the elements.  Helium is the second lightest and second most abundant element in the observable universe (hydrogen in the lightest and most abundant).  

     It is present at about 24% of the total elemental mass, which is more than 12 times the mass of all the heavier elements combined.  Its abundance is similar to this in both the sun and in jupiter.  This is due to the very high nuclear binding energy (per nucleon)of helium-4, with respect to the next three elements  after helium.

     Helium is named for the Greek Titan of the sun, Helios.  It was first detected as an unknown, yellow spectral line signature in sunlight, during a solar eclipse in 1868 by georges Rayet, Captain C.T. Haig, Norman R. Pogson, and Lieutenant John Herschel, and was subsequently confirmed by French astronomer, Jules Janssen.  Janssen is often jointly credited with detecting the element, along with norman Lockyer.  

     On Earth, it is relatively rare 5.2 ppm by volume in the atmosphere.  Most terrestrial helium present today is created by the natural radioactive decay of heavy radioactive elements (thorium and uranium, although there are other examples), as the alpha particles emitted by such decays consist of helium-4 nuclei.  This radiogenic helium is trapped with natural gas in concentrations as great as 7% by volume, from which it is extracted commercially by a low-temperature separation process called fractional distillation.