Possible Clues:

• H, O and N, in short
• Symbolically N and O, for instance?
• Subject of Avogadro's law
• Neon and butane
• Eg, O, H, Ne, CO, etc.
• Amorphous space fillers
• Terrific times, slangily
• Argon and xenon, e.g.
• Aerostatics subject
• Eg, oxygen, helium