Question

Explain why atoms have different isotopes. In other words, how is it that helium can exist in three different forms?

Answer 1

The number of protons, massive, positively charged nuclear particles defines the identity of the nucleus.

Explanation:

The number of protons, massive, positively charged nuclear particles, gives `Z`, the atomic number, which defines the identity of the nucleus.

`Z=1, "hydrogen",Z=2, "helium",Z=3, "lithium",...........Z=21, "scandium".`

The nucleus may contain various numbers of neutrons, massive, neutrally charged nuclear particles, which gives to the existence of isotopes.

For hydrogen, `Z=1` by defintion. That is there is a proton in all hydrogen nuclei. Some hydrogen nuclei contain 1 neutron to give the deuterium isotope, `""^2H`; an even smaller number of hydrogen nuclei contain 2 neutrons, to give the tritium isotope, `""^3H`; Most atoms have a number of isotopes in different proportions.

To answer your question, the helium atom has `Z=2`; i.e. there are 2 nuclear protons, which is what defines the atom as helium. The most common isotope of helium is `""^4He`. `""^3He` and `""^5He` are known with vanishingly small abundances, and half lives.

This video discusses how to determine the number of subatomic particles in C-12 , C-14 , H-1 , H-2 and H-3 .