|Energy-level diagram for sodium.
The ground state for the sodium's
valence electron is designated
3s. Some of the allowed transitions to other energy levels are shown
as red lines; the transitions which principally feature in the sodium
emission spectrum are labeled
in nanometers. The transition 3p to 3s is responsible
for the yellow sodium D lines of the
solar spectrum and the bright yellow color of the sodium flame and
of sodium vapor lamps. Electrons raised above the series limit (SL)
value are lost from the atom, which is thus ionized.