Helium is the second most abundant element in the universe, making up about 24% of the total mass of the universe. Helium is a noble gas. The most stable isotopes of helium are Helium-3 3He and Helium-4 4He. Besides these isotopes, there are seven other unstable isotopes.


Symbol He
Atomic number 2
Atomic mass 4.00260 g/mol
Melting point -272.2 °C
Boiling point -268.9 °C
Density 0.178×10-3 g/cm3 at STP
Isotopes 2

Frequently Asked Questions[edit]

What is a diproton?[edit]

Diproton is one of helium’s isotope 2He, which is highly unstable with two protons and no neutrons. Diproton usually decays into deuterium and is mostly noted in the nuclear fusion reactions in the stars. In the proton-proton chain reaction, two protons in the hydrogen plasma collide and via quantum tunneling, a few would form diproton.

Why inhaling helium changes the human timbre?[edit]

When a human speaks, the modulated sound produced by the vibrating vocal folds travels to the vocal tract where it is filtered and amplified, to create the human’s unique timbre. Under normal conditions, the medium inside this vocal tract is air. At the sea level, the sound travels at the same speed of 343 m/s inside the tract. But when helium is inhaled, the medium changes to helium and the speed of sound to 1007 m/s. As the frequency remains the same, and the speed increases, according to the wave equation v = fλ, the wavelength increases. This changes the voice timbre.

If wind instruments are filled with helium, will the sound change?[edit]

As the medium inside the instrument is helium, the speed of sound will be higher inside. And so the wavelength of the sound should change accordingly with the increased speed and corresponding change in wavelength.