http://www.msm.cam.ac.uk/doitpoms/tlplib/index.php
http://www.engsc.ac.uk/er/index.asp
http://chemistry.about.com/od/atomicmolecularstructure/Atomic_Molecular_Structure.htm
Thorough overview of atomic and molecular structure, organised into sub-topics with quizzes on each area
http://www.ndt-ed.org/EducationResources/CommunityCollege/Materials/Structure/introduction.htm
Comprehensive learning source for structure of materials
http://www.ami.ac.uk/courses/topics/0124_seom/index.html
Explains different types of material stresses and fatigues with real photos to show the cracks, etc in the element