Question

How do isotopes of an element differ? In what ways are they the same?

Answer 1

Isotopes differ in the number of neutrons, fundamental, massive, neutral nuclear particles, their atoms contain.

Explanation:

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 `""^(1)H`; their nuclei each contain just the one proton, just the one fundamental, massive, positively-charged nuclear particle - we would call this nucleus the `"protium isotope"`.

A few hydrogen atoms, are substituted by an extra massive nuclear particle, the neutron, which we would represent as `""^2H`, `"the deuterium isotope"`. Note that the element is still hydrogen, because `Z`, `"the atomic number"`, is still `1`. And a smaller few hydrogen nuclei are substituted by a nuclide with 2 neutrons, i.e. `""^3H`, `"the tritium isotope"`.

And thus all isotopes of the same element necessarily contain `"THE SAME NUMBER OF PROTONS,"` but `"DIFFERENT NUMBERS OF NEUTRONS"`. Most elements have several different isotopes, and their weighted average gives rise to the quoted atomic mass. Capisce?