Learn about Advanced Placement Chemistry, Atomic Theory 2, in this comprehensive video by bannanaiscool.
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Rob Lederer: Okay, what you see here is hydrogen gas being electrically stimulated by a current in a sealed gas tube. So H2 molecules are gaining energy and they are breaking into H atoms. Now they H atoms are gaining energy and the electron in the hydrogen atom is being elevated to higher energy levels after absorbing certain quanta or packages or energy. When those electrons have the ability to drop back down to lower levels or their original level, they emit certain packages of energy. There is mercury; that's pretty blue, don't you think? There is neon. Now don't you think that, that is impressive? So Bohr, he looks at that emission spectrum for hydrogen and he looks through something like this spectrometer. He breaks it down that light into prism by prism and prismatically he sees that there are four distinct color bands only that are coming off of that hydrogen. So his postulation was that electrons are actually going from high energy levels to lower ones and releasing certain packages of energy, that translate into wavelengths in the visible spectrum and only four. Other EMR like gamma rays and X-rays were coming off of that machine too. That's why I was standing kind of far back from it, you know what I mean. So Bohr says, okay but visibly there are six transitions that we can see. See the model of the atom had developed up to a certain point where Bohr understood that electrons were outside of a nucleus that have protons in it somewhere maybe in the middle of the atom. That was developed by Rutherford and before that J. J. Thomson, and then before that John Dalton. So now Bohr is saying, we know that protons are there in a nucleus but what are the electrons doing? Where are they? Here is what I think they are. I think they are found at distinct levels outside the nucleus. We're going to call those energy levels. Here is what he was able to figure out. If we got one proton in the nucleus here and we've got one electron on the first energy level or by n equals one. If we want to take that electron and excited by giving it a shock just like we saw in the emission spectrometer lab test, then that electron can go from n equals one to n equals two, but it has to absorb a certain package of energy or quanta in order to do that. See, this electron can absorb half of that package and go up to 'n equals one and half'; there is no such thing. All of the energy levels go up in whole numbers and you can't have n equals zero, it's not possible. So the electrons at n equals one, if they wants to go to n equals two, you got to add a certain quanta of energy. How much? Here is how he did the calculation. He said the energy was going to equal, now look at the number, negative 2.178 times 10 at negative 18 joules times Z-square. Z is just the number of protons in the nucleus called the nuclear chart, so it's easy for hydrogen is always one; 1-squared over n-squared. So if you want to know how much energy is at this level, plug in n, which is one. 1 n here and then of course the answer is going to be negative 2.178 times 10 at negative 18. It's a negative quantity of energy; sounds kind of confusing, but it really works nicely when you want to calculate how much energy is involved if the electron is going from n equals one to n equals three. What you can do is, you just say, okay, there is a difference between z-square to n-square final, where n is up to z-square over n-square initial, where it came from. If you do that calculation right there, that will give you the amount of energy it takes to make the electron go from n equals one to n equals three or n-square final here is going to be 3-square, that's going to be 9, so it's going to be one-ninth minus one. Notice that, that's going to give you negative number. Negative times negative equals positive, because it requires energy to go from n equals one to n equals three. But in reverse, if you want to go the electron molecule from n equals three to n equals one, then you would put th