Gallium induced structural failure of an aluminum baseball bat – Watch as the structure of aluminum is destroyed by gallium atoms that dissolve the aluminum bonds. This is a pretty good indication that matter is atomic; gallium would be unable to diffuse inside of the aluminum if it were a continuum.
What and where is antimatter? – This short article from CERN explains some basics of antimatter and brings up some of the big questions about antimatter that physicists are trying to answer.
Ptable.com – This website has a great interactive periodic table of the elements.
PhET Build an Atom Simulation
Play around with the number of protons, neutrons and electrons to see what happens to an atom.