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

Sep 8, 2016

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.

$Z = 1 , \text{hydrogen",Z=2, "helium",Z=3, "lithium",...........Z=21, "scandium} .$

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 .