How many stable isotopes of He exist?
There are nine known isotopes of helium.
They range from helium-2 to helium-10.
Only helium-3 and helium-4 are stable.
All the radioisotopes are short-lived.
The longest-lived is helium-6, with a half-life of 810 ms.
The least stable isotope is helium-5, with a half-life of 7.6×10⁻²² s.
Helium-2 may have an even shorter half-life.