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From the website: "This "online textbook" is designed to provide a thorough, in-depth treatment of this topic at the first-year college or advanced-HS levels, with a special focus on the underlying concepts." Highly recommended in Choice Reviews Online, 2008
A commercial site, Chemicool was the brainchild of David D. Hsu of the Massachusetts Institute of Technology. The data for this site was acquired with help from the following people: Cabrol D.; Moore J. W and Kotz J. C. Additionally, the Perkin-Elmer Periodic Table v2.02g was used for Atomic mass, density, volume, group, heat of fusion, heat of vaporization, specific heat, and shells.