Simple categories matching monatomic:

• solid,
• crystal,
• ion,
• gas,
• liquid,
• element,
• substance,
• system,
• semiconductor,
• fluid,
• material,
• atom,

Members with property monatomic:

• sodium,
• ge_si,
• copper,
• helium,
• argon,
• kr,
• nitrogen,
• ge,
• he,
• krypton,
• molecular_hydrogen,
• ar,
• diamond,
• cube,
• germanium,
• simple_metal,
• carbon,
• rare_gas,
• noble_gas,
• silicon,
• xe,
• neon,
• metal,
• li,
• crystalline_germanium,
• liquid_helium,
• ne,
• hydrogen,
• inert_gas,
• silicon_crystal,
• xenon,
• si,
• rn,
• halide_ion,