Wikimedia
Wikipedia: Atomic Radius
This site from the encyclopedia Wikipedia provides a basic definition for what the atomic radius is, and an explanation of what covalent and metallic radii are.
Ministerio de Educación (Spain)
Ministerio De Educacion: El Nucleo Atomico
Read about the nucleus and its properties. These pages have a lot of illustrations.
Upper Canada District School Board
Tom Stretton's Chemistry Pages: The Nucleus
This online slide show presents the basic ideas behind the structure of the atomic nucleus.
Other
Walter Fendt: Ley De La Desintegracion Radiactiva
Learn how you can find the probability of survival but cannot predict the time of disintegration of an atomic nucleus with this simulation. This site is best viewed with Internet Explorer.
Science Struck
Science Struck: What Makes Up an Atom?
Describes the structure of an atom and the characteristics of the electrons, neutrons, and protons inside it. Includes some interesting facts about atoms.
Other
Nuclear Twin: The Discovery of the Proton and Neutron
Trace the history of the discovery of protons and neutrons in this informative site.
Royal Society of Chemistry
Periodic Landscapes: Atomic Radii
This website provides a short but detailed definition of atomic radii. Also shows the atomic radius plotted against atomic number.