Wikimedia
Wikipedia: Atomic Number
Wikipedia provides the definition of the term, "Atomic number," a term used in chemistry and physics to represent the number of protons in the nucleus of an atom.
CK-12 Foundation
Ck 12: Chemistry Simulation: Atom Builder Challenge
[Free Registration/Login Required] Learn what makes each element unique by building the mystery atoms in the periodic table.
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.