Chem Tutor
Chem Tutor: Atomic Structure: Atomic Weights and Atomic Numbers
This lesson explains many basic concepts of atomic structure. It starts with atomic mass and number and then works up to more sophisticated information on electron configuration and how each element fits into the Periodic Table.
CK-12 Foundation
Ck 12: Earth Science: Atoms to Molecules Study Guide
Understand the basic structure of atoms and molecules in the Earth's crust.
Corrosion Source
Corrosion Source: Erbium
This site, which is provided for by Corrosion Source, gives a summary of basic physical data on erbium. This is a great site to check out for information on the subject.