Credo, Gale, and Oxford offer reference collections that let you cross-search many dictionaries and encyclopedias at once!
Online reference works from a variety of publishers.
Reference books by Oxford University Press, including the Oxford Digital Reference Shelf.
In addition to the resources below, you may want to browse the chemistry textbooks in the Clemens Library, QD 31 - QD 33
Not sure if it's okay to use Wikipedia or a web search as a starting point for your research? Check with your instructor or a librarian!
For an overview of the issue, check out Wikipedia's Citing Wikipedia article!