This site has a definition and properties of Leptons, the "Basic building blocks of matter." There are links to table of properties and more information.

This site from Chemicool provides a set of tables summarizing a great deal of information on helium and its properties.

DENSITY is a physical property of matter, as each element and compound has a unique density associated with it. Density defined in a qualitative manner as the measure of the relative "heaviness" of objects with a constant volume. For...

This passage tests your knowledge on the kinetic molecular theory of gases using the five-question quiz.