Xenon is a member of the zero-valence elements that are called noble or inert gases, however, "inert" is not a completely accurate description of this chemical series since some noble gas compounds have been synthesized. In a gas filled tube, xenon emits a blue glow when the gas is excited by electrical discharge. Using tens of gigapascals of pressure, xenon has been forced into a metallic phase.[3] Xenon can also form clathrates with water when atoms of it are trapped in a lattice of the water molecules.