For 10 points, identify this element which produces a bright yellow color in a flame test.
An amalgam of mercury and this element is used in the Emde modification of the Hofmann degradation to reduce quaternary ammonium salts. The splitting of its doublet lines at 588 and 589 nanometers upon the application of a magnetic field was the first observation of the Zeeman effect. An electride complex formed by ammonia, an alcohol, and this element attacks a benzene ring to produce a cyclohexadiene in the Birch reduction. Aldosterone promotes the uptake the of ions of this element in the distal tubules of the kidneys, and 3 atoms of this element are exchanged for 2 atoms of potassium by a namesake "pump." For 10 points, identify this element which produces a bright yellow color in a flame test. 
An amalgam of mercury and this element is used in the Emde modification of the Hofmann degradation to reduce quaternary ammonium salts.
{Sodium} [or {Na}; or {natrium}]