A neutral cloud composed of atoms of this element lies in the orbit of Io. 
A neutral cloud composed of atoms of this element lies in the orbit of Io. Atoms of this element in the mesosphere are excited by lasers tuned to 590 nm, and the Rayleigh backscattering thus formed is treated as an artificial beacon in adaptive optics systems. The broadening of the doublet of this element in the presence of an applied B field caused the broadening of the doublet in the first observation of the (*) Zeeman effect, and the D1 and D2 Fraunhofer lines characterize that doublet. This metal contributes a solvated electron when placed in ammonia, which helps reduce aromatic rings in the Birch reduction. The Castner-Kellner method was the commercial method of producing this element until it was replaced by the Downs process. For 10 points, identify this element which is transported across plasma membranes by a pump that exchanges three of its ions for two potassium ions.
A neutral cloud composed of atoms of this element lies in the orbit of Io.
sodium [or {Na}]