How do isotopes of an element differ? In what ways are they the same?
Isotopes differ in the number of neutrons, fundamental, massive, neutral nuclear particles, their atoms contain.
Let's take the element hydrogen as an exemplar, which so far as I know is the most abundant element in the universe. Most hydrogen nuclei are
A few hydrogen atoms, are substituted by an extra massive nuclear particle, the neutron, which we would represent as
And thus all isotopes of the same element necessarily contain