Question

How many stable isotopes of He exist?

Answer 1

Hint: In the above question, it is asked about stable isotopes of He. Isotopes can be defined as elements having the same atomic number but different mass. This occurs due to the presence of the same number of protons but different numbers of neutrons in an element.

Complete step by step solution
Helium has nine isotopes which range from Helium-2 to Helium-10. But only helium-3 and helium-4 are stable. Rest all are radioactive and are present only for a short duration of time.
Hence, we can conclude that only 2 stable isotopes of He exist.

Additional Information
Isotope separation is a significant technological challenge, particularly in case of heavy elements such as uranium or plutonium. Lighter elements such as lithium, carbon, nitrogen, and oxygen can be easily separated by gas diffusion of their compounds such as CO and NO. The separation of hydrogen and deuterium is unusual because it is based on chemical rather than physical properties, for example it can be separated only in the Girdler sulfide process.
A neutral atom has the same number of electrons and protons. Thus different isotopes of a given element all have the same number of electrons and hence, they share similar electronic structure. As we know that the chemical behaviour of an atom is largely determined by its electronic structure, different isotopes exhibit nearly identical chemical behaviour.

Note
We have a total 80 stable isotopes in a periodic table. The largest number of stable isotopes seen for an element is 10, that is, for tin. No element has nine or eight stable isotopes.