Before we try to explain the hydrogen emission spectrum it's useful to think a little bit about light. So let's take a light bulb and let's pass that light through a prism and as you can see in this diagram as my white light passes through the prism the different wavelengths or colours or frequencies of light are refracted and end up being split into a nice rainbow coloured spectrum. And we're focusing here just on the visible part of the spectrum because that's the bit that we can see with our eyes. Now, this spectrum is considered a continuous spectrum because it shows all of the frequencies of light in that visible part of the spectrum.
Let's now try the same thing. But instead of a light bulb, we're going to use a hydrogen gas lamp. So in my gas lamp, I've got lots of hydrogen atoms floating around.
And what I'm going to do is pass. lots of energy through my gas lamp using electricity or heat. And again we're going to pass the light that's emitted from this gas lamp through a prism and it's going to look something like this. So in this diagram you can see that the emitted light only contains very specific frequencies of light which is why we end up seeing a line spectrum with just single lines of colour and not a continuous spectrum.
So a line spectrum is a spectrum that shows only specific frequencies of light. And it's important to use the word frequency and not colour, otherwise you won't get the mark in an exam. The spectrum we can see here is called the hydrogen emission spectrum because it is the line spectrum produced when a gas lamp of hydrogen is excited by lots of energy. Before we try and explain how those lines relate to the structure of an atom, let's first think about what actually happens in a hydrogen atom when I pass lots of energy through it. So here is a simple ball model of a hydrogen atom where the nucleus would be a very small dot somewhere right in the middle and the rings are representing different energy levels.
We know that hydrogen has one electron so let's put that in the lowest energy level, and this would be known as the ground state, or the most stable state of my atom. As I pass lots of energy through my hydrogen atom, an electron can absorb a very specific frequency of energy and jump up to a higher energy level, where the electron is now in an excited state. It's important to note for the electron to jump up to that next energy level, it has to absorb exactly the energy difference from the first energy level to the second energy level, which is why in this process only a specific frequency of energy is absorbed. Conversely, when that electron drops back down to the first energy level, according to the conservation of energy, it must release that exact same amount of energy to do so.
Now of course because there are lots of different energy levels, my electron could jump up to the second energy level or the third or fourth or fifth and so on and also can drop back down to any of the lower levels, which means there's a number of possible transitions that can occur. Let's now see if we can identify the exact transitions that are occurring to produce our hydrogen emission spectrum that we saw earlier. So here's the emission spectrum we saw from our hydrogen gas lamp earlier.
It's worth noting on here that the low energy end of the visible light spectrum is at the red side and the high energy end is at the violet side. Let's now draw the Bohr model of a hydrogen atom as we saw before, and you can see my blue dot representing an electron in the ground state, and you'll often see the energy levels labelled. as n equals 1 for the first energy level, n equals 2 for the second energy level, and so on. Now instead of having to draw hundreds of circles in an exam, we can actually simplify this diagram into an energy level diagram, which is effectively taking a little cross section like this and representing it as the following.
So what Bohr suggested, is that each of the frequencies or colours of light that I can see on my line spectrum represent an electron dropping from a higher energy level down to the second energy level. Now, given that there are four lines on my hydrogen emission spectrum, let's start with the red line, which is the lowest energy or lowest frequency transition. So I'm looking for the smallest gap or the smallest drop down to the second energy level.
And that's going to be a transition from the third energy level dropping down to the second. If I go back to my hydrogen emission spectrum. The next highest energy emission is the green line, so that's going to represent the next biggest drop to the second energy level, which would be from the fourth energy level.
The next highest energy line on my emission spectrum is the blue one, which must be representing a drop from the fifth energy level to the second. And finally the violet line must be representing a drop from the sixth energy level down to the second. The brilliant thing about the Bohr model of the atom is that he looked at the emission spectrum and noticed that as I move from low energy to high energy emissions the lines begin to converge or get closer together and this suggested that as I move out in my energy levels in the atom the energy levels must also get closer together and converge so at some point when the energy levels are effectively in the same place we reach the edge of our atom The key point in our explanation of the hydrogen emission spectrum is that these lines are all representing emissions or drops from higher energy levels down to the second energy level.
Because it so happens that the energy released in those drops corresponds to light on the visible part of the spectrum that we can see with our eyes. However of course there are many other emissions that could occur. For example, emissions caused by electrons dropping back down to the first energy level.
Now because the amount of energy released when electrons drop to the first energy level is much greater, these emissions would actually be seen on the UV part of the spectrum that we can't see with our eyes. And conversely, if I think about electrons dropping down to the third energy level, The amount of energy being released here is very low, so we would expect to see it on the infrared part of the spectrum, which again we can't see with our eyes. And just like with my hydrogen emission spectrum on the visible part, we would expect these lines to converge as well, indicating that energy levels in an atom converge as we move out.
Let's now try to summarise the key points from this video. Firstly, emission spectra are produced when energy or photons are emitted by excited electrons dropping to lower energy levels. Secondly, the hydrogen emission spectrum suggests that electrons are found in discrete energy levels that converge at higher energies.
And thirdly, lines in the visible part of the spectrum are caused by electrons dropping to the n equals 2 or second energy level. Any drops to the first energy level would be seen in the UV part of the spectrum and drops to the third energy level would be seen in the infrared part of the spectrum. Hopefully this video is of some help.