Related Clues:

• Grades K-6
• Periodic table entry (Abbr.)
• 21-Down, e.g.
• C or Al
• It's hard to get a reaction out of it
• It's before middle sch.
• A first principle, found in a kettle?
• Constituent part
• Passive element in the helium group
• Constituent part (7)
• Constituent component
• Chemically and currently it may be glowing
• One might find one in the bulb in the laboratory
• Chemically, one may get heated
• The chemist might get a glow from it