What is the difference between an isotope and an element?

Elements are simply what you see on the periodic table. E.g. carbon, oxygen, hydrogen etc.

Isotopes are elements with different masses. E.g. carbon have two stable isotopes found in nature, one is #C^12# and one is #C^13# (#C^14# does also exist by not stable form of carbon therefore not found in nature - produced under nuclear reaction). The difference between #C^12# and #C^13# is the amount of neutrons, both have 6 protons where #C^12# have 6 neutrons whereas #C^13# have 7 neutrons. So isotopes have the same amount of protons but with different amounts of neutrons.

And #Z# represents the number of CHARGED, MASSIVE nuclear particles, i.e. #"nuclear protons"#.... The nucleus also contains UNCHARGED, MASSIVE nuclear particles, i.e. #"neutrons"#. Interactions between protons and neutrons result in the strong nuclear force, the which at nuclear ranges is attractive, and strong enuff to overcome the force of electrostatic repulsion.

Isotopes have the same atomic number, #Z#, and are thus the same elements, but different numbers of neutrons.... We represent the mass of the isotope by a superscript on the left hand side of the elemental symbol to give the mass number, which to a first approx. is the combined number of nuclear particles...

Let us consider the isotopes of hydrogen....of which there are commonly 3...

#""^1H# #"protium isotope"#....how many neutrons does this isotope contain...?

#""^2H# #"deuterium isotope"#....how many neutrons does this isotope contain...?

#""^3H# #"tritium isotope"#....how many neutrons does this isotope contain...?

See this old answer for more of the same....