Science Struck
Science Struck: How to Find Protons, Neutrons and Electrons
Brief explanations of how to determine how many protons, neutrons, and electrons are in an element.
California State University
California State University: Proton, Electron, Neutron
An interesting and useful tool to practice recognition/calculation of atomic number, mass, and number of neutrons, electrons, etc. Can be used with all elements.
Annenberg Foundation
Annenberg Learner: Isotopes, a Weighty Matter: Isotope Quiz
You will be presented with three different isotopes of the same element. See if you can calculate the atom's average relative mass.