Related Clues:

• About 21% of the atmosphere
• A noble gas
• Element in water
• Chemical element
• The 'O' of H2O
• You can't live without it
• Gas used in high-intensity headlights
• Space shuttle supply
• Gas used in tubes
• Flashbulb element
• It's 54, periodically speaking
• Rare gas
• It's under krypton on the periodic table
• Arc-lamp gas
• About 21% of air