There are many great resources available on the Internet...  here are a few...

http://www.azom.com/

http://www.aip.org/history/electron/

http://www.cs.uh.edu/~clifton/macro.l.html

http://www.howstuffworks.com/

http://www.libraryspot.com/encyclopedias.htm

Making of America:  http://cdl.library.cornell.edu/moa/

We the People: http://www.eduplace.com/links/wtp/grade6.html

http://www.allmeasures.com/Formulae/static/materials/31/density.htm