Xenon gas
From Kerbal Space Program Wiki
Xenon gas is a resource that fuels ion engines. It's density is 0.1 kg per 1 xenon.
Uses
There are only two xenon gas related parts available at the moment.
No other uses aren't currently planned.
The PB-X150 Xenon Container can't be placed radially. A larger radial xenon tank and a larger ion engine are included in the KSPX mod.