 Greetings and welcome to the Introduction to Astronomy. In this video, we are going to discuss a couple of things. Overall, we're going to look, first of all, at the atomic structure and how the atom is structured and how that applies to cause the formation of spectral lines so that we can see how those spectral lines are formed and those are important because they do help us to determine what things are made up of in the universe. So let's start off looking at what the structure of the atom is and when we look at the structure, we see that there are two parts to an atom. There is a nucleus, which is the central portion in here and there are the electrons that orbit around it. So in many ways, you look at it kind of as a mini-solar system. There are some significant differences as well, but you do have objects here at the center and those are the protons and the neutrons, which are both at the nucleus, the protons having a positive charge, the neutrons having no charge. So those are condensed to the nucleus. The electrons orbit around the nucleus and they have a negative charge. So overall, when you look at an atom like this one of lithium, you have one, two, three positive charges and one, two, three negative charges so that the overall atom is electrically neutral. However, there is a concentration of positive charge at the center and of negative charge far around it. Now this does not even come close to being to scale. At this scale with the electrons, the nucleus would be microscopic. So the material is condensed in a very, very tiny portion at the middle and the electrons take up all the space of the atom. So let's look at a few definitions that we need for atomic structure and we'll start off here with a couple of them, which we have, which are the terms isotope, which are two atoms that have the same number of protons, so same number of protons, but different numbers of neutrons in the nucleus. So some examples of this would be here helium. Here we can have helium three, helium four, helium five and helium six. Those are all helium because the atom is defined by the number of protons in the nucleus and each of these has two protons. So they all have the same number of protons. The difference is the number of, the difference is the number of neutrons that they have. So helium three has one neutron, helium four has two neutrons, helium five three and helium six four. So what has changed is the mass of the atom. So helium three has the least mass and helium six would have the most mass. So we can then tell which atom they are depending on the number of neutrons in the nucleus. That can change. If you change the number of protons, you change actually what the atom is. Now an ion on the other hand is when you change the electrons. So electrons are added or removed from the atom. So when you change those, then we change the charge. So the charge changes in this case. The charge has changed, but the mass is still exactly the same. So this is the example we're going to look at here. This is helium has an atomic number of two, two protons in the nucleus, and it has an atomic weight or mass of four units because it has also has two neutrons. So the overall mass is four because most of the mass is in the nucleus. So you have two protons and two neutrons in the nucleus and you have two electrons around it making it electrically neutral. However, if we remove one of these neutrons such as this one, then all we have done is to change. Now it's gone. And now we have two positive charges in the nucleus, but only one positive charge orbiting. So now this is what we would call an ion. It has been ionized as one electron has been removed and it now has a positive charge. So sometimes this can be written as helium plus. It now is helium, but it has a positive charge. And this can be important because it does change the energies that are allowed so it can change the spectrum that we would see. And we would see a different spectrum for ionized helium for a different ion or for a different isotope than we see for a regular atom. Now let's look at a model of the atom here. This is called the Bohr model named after the scientist who came up with this. And the Bohr model is based on the electrons have very specific energy levels. So there are distinct energy levels where they are allowed. The electrons can move between these energy levels but cannot go in between them. So you can have an electron orbiting at this energy level here or at this energy level, level one or level two but you can't have any electron out here. The electron cannot be in between those two. So all the electron can do is jump from one energy level to the next or jump back down. When that happens, it takes or gives off energy. When you move the electron from a lower state to a higher state, that takes energy to do it. When you move energy from a higher state to a lower state that gives off energy. So we can see the amount of energy changes depending on those various energy levels. This is where we begin to see the specific wavelengths that are allowed. So these wavelengths written here in nanometers are the exact wavelengths that the hydrogen atom is allowed to emit. And we often look at pictures. This is a very prominent one right here. This 656 nanometers is a very prominent line of hydrogen that happens to fall in the visible part of the spectrum. This is in the red portion of the spectrum and that gives us a very prominent red color to anything that is glowing in hydrogen. So lots of nebulae we see are primarily glowing in this hydrogen gas because of that specific transition. But the key is that you cannot move in between them. So you can move from this level to this level but you can't just go halfway. You can't stop there. That is not permitted. There's no way to do that based on the studies of quantum mechanics. So let's look at how this leads to a spectral line formation. So when we do this, we can only form very specific lines. There are only specific wavelengths that can be admitted or absorbed. This gives us a fingerprint for each element. Hydrogen, for example, in the visible portion of the spectrum has this set of lines that we can see. We cannot see other lines. So it's not possible in hydrogen to get a wavelength of, say, 520 nanometers. Hydrogen cannot do that because there is no energy level that corresponds to that. We can go at 656 or we can go at 486 but nothing in between. So we are able to get those and that gives us a very specific fingerprint of what we are able to see for these lines. Now, here if we look at some of those when we actually do the transitions, we can then see that we get various different elements here. We can have hydrogen that is very specific lines that we've looked at before and that's all we see. So these are the only lines that can be emitted or absorbed. That is it. No other possibilities and each element is different. So here you get one element. Here you get another element and if you see this pattern of lines, you know it's made up of one thing. If you see this pattern of lines, you know it's made up of another. So the two are completely different and the problem is, of course, that this will become much more complicated. We've made it simple here looking at one element at a time. However, when we look at an actual object like the Sun, it becomes significantly more complicated because the Sun is made up of dozens of different elements. Hydrogen and helium may be the two most prominent ones but we see all of those. We also have to note that different ionizations give different spectra. So things like helium and helium that's been ionized do not give you the same spectrum. They give you a completely different one. We can look at carbon. We can look at carbon that's been ionized and we can look at carbon that's been ionized two times, two electrons removed. They give us different spectra. So that is a difficulty in terms of finding. If you look at the image here, all these different lines, we have to match up each of those and look for the patterns that are associated with different atoms, different ionization states and different molecules. Molecules give even more complicated spectra than atoms because of the way they are put together. So the process of figuring this out is not quite as simple as we've gone through at the beginning. It takes a lot to go through and be able to understand, but this is the way we learn what things are made up of in the universe. So let's finish up here as we always do with our summary. So what have we gone over in this section? We've talked about the basic composition of atoms, protons, neutrons and electrons. Recall that these are in the nucleus and the electrons orbit. We found that electrons can only have very specific energy levels. They cannot have just any energy level. They can only go at specific levels and they can jump between those. And that means that each atom has its very own fingerprint. The pattern of lines tells us exactly what that atom or ion is. Remember that the ionization states will also have different patterns. So it gives us a fingerprint and that allows us to determine what the composition is. What is a star? A galaxy? What are they made up of? Well, this is one way we can find out because we can't just go get samples of those. We can use, but we can use their spectra and the light that comes from them to be able to determine what things are made up of in space. So that concludes our lecture on atomic structures and energy levels. So we'll be back again next time for another topic in astronomy. So until then, have a great day everyone and I will see you in class.