Why does every element have unique spectral lines?

I have studied that spectral lines help us to identify the properties of an element. This is because after excitation when the excited electron(s) fall back to their original energy level(s), photons of certain frequency corresponding to the difference between the energy levels are emitted.

Now I have a question, every substance consists of atoms, and each atom consists of electrons, electrons are fuzzy clouds and are described by wave-functions given by the solutions of the Schrodinger equation, so what makes the spectral lines different although every substance has the same thing?

I searched on the site and discussed with various members of the community and some of the conclusions I drew are mentioned below:

Electron-nucleus attraction and the electron-electron repulsion. Hydrogen is a special case because it has only a single electron so there is no electron-electron repulsion. Helium has two electrons so now we have some e-e repulsion, Lithium has three electrons so there is even more e-e repulsion and so on... Every atom has a different number of electrons and a different nuclear charge, so the balance of nuclear attraction and e-e repulsion is different. So every atom has its own set of atomic orbitals that are specific to it and are different from every other atom. The spectrum comes from transitions between orbitals, and since the orbitals are different for every atom the spectrum is different for every atom.

Are my conclusions reasonable or is there something else I missed to note?

But this answer creates one more question in my mind,

Do all noble gases or alkaline earth metals have similar spectral lines considering the above points I mentioned?

  • 3
    $\begingroup$ Well, in the question I'd replace atom with element. I'd just use Rydberg equation and point out the effect that Z has on the series of lines. Shielding is another factor, electron-electron repulsion a smaller one, and nuclear mass (different isotopes) an even smaller one. $\endgroup$
    – MaxW
    Jan 23, 2020 at 19:03
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    $\begingroup$ No offence, but can you try to put some more effort into the grammar and logical structure of your text? As is, this is very hard to read. $\endgroup$
    – Karl
    Jan 23, 2020 at 22:10
  • $\begingroup$ @Yuvraj Singh, Did you notice that Raman spectrum has nothing to do with atomic spectrum. And there is nothing such as a fuzzy cloud of electrons. It is just a mathematical construct so do not take it in a literal sense. $\endgroup$
    – AChem
    Jan 24, 2020 at 1:05
  • $\begingroup$ @M.Farooq I will edit will you open my answer again! $\endgroup$ Jan 24, 2020 at 3:03
  • 4
    $\begingroup$ @M.Farooq Please do not give the advice of deleting and reposting a question. Question on Chemistry should always be edited to be improved; this one doesn't even have a negative score. You can always vote to reopen a closed question, as long as you have not already done it (this might include previous attempts). $\endgroup$ Jan 24, 2020 at 15:24

1 Answer 1


Do all noble gases or alkaline earth metals have similar spectral lines considering the above points?

The question is interesting after you modified it. The basic set of reasoning you provided is the main story. Each element has a different nuclear charge and the outermost electron(s) is responsible for the atomic emission spectrum. Since the nuclear charge is different, those outermost electrons experience a different potential energy. Their kinetic energy is also different from element to element.

The key question is what is meant by similarity? The atomic spectra of all the elements is visualized as bright lines on a dark background. The reason they appear as lines is just because of the instrument used to observe the atomic spectrum. There is nothing fundamental in the "line"spectrum. The atomic emission appears as lines because the slit in the monochromator is shaped like a very narrow rectangle. This is the image of the slit. If I made a very narrow circular opening, the images will appear as bright points rather than lines. Consider that line spectrum is the conventional way of looking at the atomic spectrum.

For the visible region, let us look at the visible spectrum of alkali and alkaline earth metals.

spectrum of alkali metals in the visible range

or look at the visible spectrum of noble gases

enter image description here

Are they similar in any way? Visually there is no similarity at all in the spectrum. Look at the spectrum. The reason we cannot see any similarity is because the dispersion of the lines is on the "wavelength" scale. However, very bright, I mean really genius spectroscopists of the 18th and the 19th century were able to find out a patterns (or call it as mathematical series). They figured out that each set of lines forms a pattern as Sharp series, Principal series, Diffuse Series and Fundamental series and there they found similarities in patterns-in terms of mathematical series. In short, our eyes cannot interpret those series. Another complication arises because we are just looking at the visible region. There is a ultraviolet region and then there is an infrared region. We cannot see it without using more sophisticated instruments.

  • 1
    $\begingroup$ I should like to clarify a couple of points for the OP. When Fraunhofer first observed to Sun's spectrum it was in absorption so that the lines appear black, the ones illustrated are in emission so are bright. Either method, absorption or emission can be used. Secondly, if the spectrograph slits are narrow enough the spectral profile of the atomic transition is observed, and not the width of the slit, because a transition has an inherent width that is not determined by the slit width of the spectrograph. This spectral line-shape is observed if the experiment is done properly. $\endgroup$
    – porphyrin
    Jan 26, 2020 at 13:54
  • $\begingroup$ Thanks sir, I am not too much known to this topic . and learning is good that was my topic. $\endgroup$ Jan 26, 2020 at 14:01
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    $\begingroup$ @porphyrin, I think you misunderstood my point. The wanted the OP to know the origin of the word line spectrum is because the slit image is projected on a photographic plate. If we replace slit with a pin hole, while avoiding the diffraction limit,, we will see "points" rather than typical lines. Then historically, atomic spectrum would be a point spectrum rather than line spectrum. $\endgroup$
    – AChem
    Jan 26, 2020 at 15:17

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