Explain why atoms have different isotopes. In other words, how is it that helium can exist in three different forms?
The number of protons, massive, positively charged nuclear particles defines the identity of the nucleus.
The number of protons, massive, positively charged nuclear particles, gives
The nucleus may contain various numbers of neutrons, massive, neutrally charged nuclear particles, which gives to the existence of isotopes.
To answer your question, the helium atom has
This video discusses how to determine the number of subatomic particles in C-12 , C-14 , H-1 , H-2 and H-3 .