Here is a good introduction to the electromagnetic spectrum:
imagine.gsfc.nasa.gov/docs/science/know_l2/emspectrum.html

For a broad overview of ionizing radiation, this link to a World Health Organization site should be helpful:
http://www.who.int/ionizing_radiation/about/what_is_ir/en/index.html

For a sense of just how small atomic particles are, see this animated video:
www.ted.com/talks/just_how_small_is_an_atom.html