The characteristic colors are dominated by one spectral line, for example, the yellow sodium D line at 589-590 nanometers (it's actually a close
doublet). This is the 3p->3s transition of the valence electron, which isn't present in the sodium Na+ ion. It's a transition in the sodium
neutral atom, not the Na+ ion. Similar story with lithium.
Potassium is different since the analogous transition, 4p->4s, is in the infrared so you can't see it. The lilac color you see comes from other
transitions, but even there I'll bet they are transitions in the neutral potassium atom, not the ion.
I'm not saying how the ions in a methanol solution (for example in the standard way of doing the flame test) get turned into neutral atoms in the
flame, or if you vaporize a small amount of salt on the end of a platinum wire. What I'm saying is that the main colors are caused by transitions in
the neutral atom. |