Interactive periodic table with dynamic layouts showing electrons, oxidation, trend visualization, orbitals, and isotopes; provides photos, videos and podcasts for each element. Allows searching for compounds, formulas, etc.
An extremely large and comprehensive collection of physical and chemical properties data, including units and fundamental constants. Produced and offered free online by the U.K. National Physical Laboratory.
National Institute of Standards and Technology's online access to elemental, atomic, molecular, spectroscopic, dosimetry, & x-ray/gamma-ray data; physical constants; & data for condensed matter & nuclear physics.