This video explains what isotopes are, and it features tons of examples of the main type of problem that teachers give you for isotopes: giving you a big table that asks you to figure out the number of protons, neutrons, electrons, and the mass of an atom.
Mass Number Notation
This is what you call it when -- usually in the isotopes chapter -- atomic symbols have little numbers on the left which are the atomic number and mass of the atom. This video covers the common types of problems your teacher might ask, and helps you avoid the most common mistake students make with mass notation.
Most every element comes in a few different isotopes, which have different numbers of neutrons and therefore different masses. But the periodic table only tells you the average mass. What's a chemistry student to do?!
If you do not have an account, you should get one, because it is awesome! You can save a playlist for each test or each chapter, and save your "greatest hits" into a "watch right before the final" list (not that we recommend cramming, but when in Rome...)