Explains the basics of atomic structure, from simple definitions to information about quantum theory. Accurate and helpful basics whether or not you need the more advanced information.
Provides a definition for both mole and molar mass and gives a step-by-step example problem to work through that shows how to compute molar mass. Also has a a molar mass calculator in which you can practice problems on.
An exceptionally thorough resource with an amazing set of data on krypton, it chemical and physical properties, its electron configuration and its isotopes, among other things.