Glass tempering can be accomplished by both thermal and chemical means. Each puts the near surface region of the glass piece into compression, thereby suppressing growth of surface cracks. The thermal process involves cooling the surface of a molten piece of glass more rapidly than the interior. The chemical process is accomplished by diffusing impurity cations into the near surface region. For soda glass, an impurity cation replaces a Na+ cation. Based on ionic radii and ionic charge, which of the following cations are good candidates for chemical tempering? Select as many as appropriate. Note: For Na+, rionic = 0.102 nm. 1. Al3+ (rionic = 0.053 nm) 2. Ba2+ (rionic = 0.136 nm) 3. Ca2+ (rionic = 0.100 nm) 4. Cs+ (rionic = 0.170 nm) 5. Fe2+ (rionic = 0.077 nm) 6. K+ (rionic = 0.138 nm) 7. Ti4+ (rionic = 0.061 nm)