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The Nonmetals Group 18: the Noble Gases Properties of Noble Gases Page [1 of 2] So Let’S Talk About the Noble Or Inert Gases

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The Nonmetals Group 18: the Noble Gases Properties of Noble Gases Page [1 of 2] So Let’S Talk About the Noble Or Inert Gases The Nonmetals Group 18: The Noble Gases Properties of Noble Gases Page [1 of 2] So let’s talk about the noble or inert gases. Nothing to say right? End of conversation? Well not quite, there is a little bit of something to say. Good. First of all let’s make sure we know what we are talking about. The elements in the far most right column of the periodic table, helium, neon, argon, krypton, xenon and radon. I left off helium here for a reason that will become clear. I think I will put the helium back to just to make sure that you don’t forget. And the elements that are in green here are isolated from air, so if you take air and you cool it down until it is a liquid and then you just fractionally distill it, the stuff that boils off, there will be some neon, argon, krypton and xenon and you can collect useful amounts that way. If you look at the graphic and you will see the distribution of these noble gases in air. Skip way ahead for right now. Let’s talk about helium. Helium obviously is less dense than air and it so it is just going to float away and you know this if you make a helium balloon, you let it go, it floats away. So if we had any helium, it would be gone, right? There is not going to be any helium hanging around in air because it is going to float away. So where is the helium? Where do we get helium to make our helium balloons? And the answer is, that it comes from radioactive decay and there is not an unlimited supply, this is an important point. We cannot make helium balloons forever. We might be able to make them for longer than anyone here if we blow up the planet or something, but the point is that is a finite supply. Radioactive elements like uranium 238 will alpha decay, which means they will go thorium 234 plus a helium nucleus. And that helium nucleus will grab to electrons and become a helium atom and there are other elements that will also alpha decay. If are not familiar with this, study some nuclear chemistry. Anyway the point is, that if this happens on the surface of the planet, it would all just go away as well. If you have a uranium nucleus sitting right here and it alpha decays, that helium is just going to rise and go away. But if it happens in the ground, it is possible that this gas will actually collect. It will collect in natural gas wells, which are just big open spaces that gases collect in. There are some natural gas wells in the United States where there is so much helium that it is more economical to get the helium than it is to get the natural gas. Texas is one of the places that the helium comes from, as there is obviously oil wells and gas wells and things like that. Now helium is interesting, because its boiling point is 4.2 Kelvin. In other words, you will have to cool it down really cold before it becomes a liquid. Up until then it is a gas. By the way these are all gases at room temperature. But at atmospheric pressure it boils at 4.2 Kelvin; that makes it a really good refrigerant if you can make it into a liquid. So once you cool it down to a liquid, it boils at 4.2 Kelvin and a lot of low temperature physics goes on in liquid helium. I myself use liquid helium because I study magnetic materials at low temperatures. Let’s talk about some of the compounds of noble gases and you might think noble gases are inert gases, how can it make a compound? Isn’t the whole point that they are inert? And the answer is yeah mostly that is true, but if you were going to make something. And Neil Bartlett in the early 1960’s said, “If we were going to make something, what could we do”? And the reality is that you are very, very unlikely to get one of these atoms to pick up another electron, because if you add another electron to them that electron is going to go into the next empty electron shell. So these have relatively low electron affinity, they don’t really want another electron. But it is not so impossible to ionize them. It is almost impossible to ionize helium, it is very difficult to ionize helium, but xenon is not so difficult to ionize. We are not going to talk about the compound radon, because it is radioactive and so people haven’t focused on that. But ionization energies get smaller and smaller as you go down the column in the periodic table. Take a look at the graphic and you will see that that is true. So you were to make a compound of xenon, you would make something where xenon is formally donating an electron. The thing that could accept an electron would be something like fluorine, something that is very electronegative, that really wants another electron and that is exactly what happens. So we discovered, again in the early 1960’s, that you can make compounds of xenon and the homolytic fluorine compounds, which we talked about in the context of octet rule, xenon fluoride, xenon tetra fluoride, xenon hexo fluoride, they are all actually stable compounds. They are solids, and they have melting points, these are melting points by the way, that are pretty modest. And one important point is that these all have negative enthalpies formation, which means that they are stable with respect to the elements. So you can make these and they are perfectly happy, just like water is stable with respect to dissociation with hydrogen and oxygen. In contrast the oxifluorides, of which there are four known, have positive enthalpies formation, standard enthalpies formation. Assume that these are unstable with respect of going back to the elements. And they melt at relatively The Nonmetals Group 18: The Noble Gases Properties of Noble Gases Page [2 of 2] lower temperatures. This is minus 41 to minus 28 degrees C. The trioxide just decomposes, it is a solid, but it just decomposes. Anyway, so let’s talk about some of the specific applications for the noble gases. Being noble they do things that sort of prevent reactivity. In other words they stand in when you want something that isn’t going to do anything. And so we have already talked about helium as being good for balloons and things like that, things that kids play with and again it is a good refrigerant, a good coolant if you turn it into a liquid, it boils at the lowest temperature of any liquid and so it is useful for cooling things. Neon you are probably familiar with in the context of neon signs. I think when you fill a tube with neon and you apply an electric field across it, it glows red. The different colors that you see in neon signs are the result of different gases, so neon doesn’t make every color in the spectrum. Different gases give rise to the different colors. Argon is used in light bulbs and standard light bulbs as a gas that just fills the glass envelope. And it is also used as the inert gas between double pane or triple pane windows. So if you have two windows, what you are trying to do it you are trying to make an insulator in exactly the same way a thermos bottle is an insulator, by having two layers with something inert in the middle or a vacuum. It is hard to maintain a vacuum between two pieces of glass so they just put an inert gas in there. Something like argon, for reasons that I don’t want to get into, argon is better than gases that don’t react very much, like nitrogen. You know nitrogen is difficult to get to react, but argon is just ever so slightly better, so that’s what is typically used. Xenon and krypton are used in flash lamps. You make a tube, you put some of the gas in, you apply an electric field and you can get a flash out of it. So for cinematography, if you want to get a flash, you put xenon into the tubes and you can make xenon flash lamps. As an aside, xenon flash lamps were used to charge the first ruby laser, but I don’t want to get into that right now. The last noble gas that I want to talk about is the one that I have been ignoring up until now, which is radon the last element in that column. And it is radioactive so people haven’t really studied its chemistry, but it does present somewhat of a health hazard. It is very dense so it tends to collect in people’s basements some places that have a lot of radioactive elements in the soil. Colorado is one of those places. There is a lot of uranium. And as a result there is a lot of radon. And so radon collects in basements and it, in itself is radioactive. The danger is that you go down in your basement and you inhale some air and there is some distribution of radon in there. Radon has a relatively short half-life, take a look at the graphic and you will see the half-life of radon.
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