[Heisenberg97]

Why does the lowest energy transition in the absorption spectrum of sodium appear as a single line unless viewed at high resolution when splitting into two lines can be resolved?

Okay so the lowest energy transition would be when a sodium atom absorbs one electron into its 3s orbital, right? Does it appear as a double line because the electron could have 2 spins, but this is dependent on one with the opposite spin to the electron already being present in the sodium atom. And they have similar absorptions, so wouldn't be seen unless in high resolution.

I honestly have no idea and any help would be greatly appreciated

[mjc123]

Okay so the lowest energy transition would be when a sodium atom absorbs one electron into its 3s orbital, right?

Absorption spectrum is not about absorbing electrons. It is about absorbing light to move an electron from one energy level to another. The valence electron of sodium is in the 3s orbital, and the next lowest level is 3p. The sodium D line is due to the excitation of the electron from 3s to 3p with the absorption of a photon.
The splitting of the line is due to spin-orbit coupling. Electrons have angular momentum due to their orbiting the nucleus, and also angular momentum due to their spin. To describe an electronic state fully we must specify the total angular momentum that arises from the combination of orbital and spin angular momenta.
Electrons in an s orbital have orbital angular momentum quantum number L = 0, and in a p orbital L = 1.
An electron has a spin quantum number S = 1/2.
The total angular momentum quantum number J can have all the allowed positive values from L + S, L + S - 1,....L - S.
So for a 3s electron J = 0 + 1/2 =1/2. There is only one J state for 3s.
For a 3p electron J = 1 + 1/2 = 3/2 or 1 - 1/2 = 1/2. There are two J states for the 3p electron. Transitions are allowed between both of these states and 3s, so the D line actually consists of two transitions, one from 3s to 3p (J = 1/2) and one from 3s to 3p (J = 3/2). The energies of the two 3p states are very close together, so you need high resolution to separate the two lines, which have wavelengths of 589.0 and 589.6 nm.

[mjc123]

Woohoo, a thousand posts! Makes me feel old. I hope at least some of them have made sense!

[Heisenberg97]

Thank you so much, that makes a lot of sense! And wow 1000 posts - keep going!