Here's a selection of useful online and print resources for finding organic and inorganic chemical properties & reference data.
Remember also that the 'SciFinder,' 'ScienceDirect' & 'Web of Knowledge' databases are additional huge sources of chemical data:
A freely accessible collection of data relating to biologically-active small molecules, including compound structure, synonyms, descriptions, and bio-activity. Provided by the National Center for Biotechnology Information (NCBI).
Access note: Freely available to the public. Government resources may be unexpectedly unavailable or altered in alignment with current federal administration priorities.
Database of substance properties, structures & toxicity from the National Library of Medicine.
Access note: Freely available to the public. Government resources may be unexpectedly unavailable or altered in alignment with current federal administration priorities.
In addition to using the handbooks listed elsewhere on this page, here are some additional recommended sources of chemical and physical constants and laws.