Video transcript
SAL: Everything we've beendealing with so far in our journey through chemistry hasrevolved around stability of electrons and where electronswould rather be in stable shells. And like all things in life,if you explore the atom a little further you'll realizethat electrons are not the only stuff that's goingon in an atom. That the nucleus itself has someinteractions, or has some instability, that needs tobe relieved in some way. That's what we'll talka little bit about in this video. And actually the mechanics ofit are well out of the scope of a first-year chemistrycourse, but it's good to at least know that it occurs. And one day when we study thestrong nuclear force, and quantum physics, and all thelike, then we can start talking about exactly why theseprotons and neutrons, and their constituent quarksare interacting the way they do. But with that said, let'sat least think about the different types of ways that anucleus can essentially decay. So let's say I have abunch of protons. I'll just draw a couple here. Some protons there, and I'lldraw some neutrons. And I'll draw them ina neutral-ish color. Maybe let me see, like agrayish would be good. So let me just draw someneutrons here. How many protons do I have? I have 1, 2, 3, 4, 5, 6, 7, 8. I'll do 1, 2, 3, 4, 5,6, 7, 8, 9 neutrons. And so let's say this is thenucleus of our atom. And remember-- and this is, youknow, in the very first video I made about the atom--the nucleus, if you actually were to draw an actual atom--and it's actually very hard to drawn an atom because it hasno well-defined boundaries. The electron really could be,you know, at any given moment, it could be anywhere. But if you were to say, OK,where is 90% of the time the electron is going to be in? You'd say, that's the radius,or that's the diameter of our atom. We learned in that very firstvideo that the nucleus is almost an infinitesimal portionof the volume of this sphere where the electronwill be 90% of the time. And the neat takeaway therewas that, well, most of whatever we look at in lifeis just open free space. All of this is justopen space. But I just want to repeatthat because that little infinitesimal spot that wetalked about before, where even though it's a very smallpart of the fraction of the volume of an atom-- it'sactually almost all of its mass-- that's what I'm zoomingout to this point here. These aren't atoms, thesearen't electrons. We're zoomed into the nucleus. And so it turns out thatsometimes the nucleus is a little bit unstable, and itwants to get to a more stable configuration. We're not going to go into themechanics of exactly what defines an unstable nucleusand all that. But in order to get into amore unstable nucleus, sometimes it emits what's calledan alpha particle, or this is called alpha decay. Alpha decay. And it emits an alpha particle, which sounds very fancy. It's just a collection ofneutrons and protons. So an alpha particle is twoneutrons and two protons. So maybe these guys, they justdidn't feel like they'd fit in just right, so they're acollection right here. And they get emitted. They leave the nucleus. So let's just think whathappens to an atom when something like that happens. So let's just say I have somerandom element, I'll just call it element E. Let's say it has p, protons. Actually let me do it in thecolor of my protons. It has p, protons. And then it has its atomic massnumber, is the number of protons plus the numberof neutrons. And do the neutronsin gray, right? So when it experiencesalpha decay, what happens to the element? Well, its protons are goingto decrease by two. So its protons are goingto be p minus 2. And then its neutrons are alsogoing to decrease by two. So its mass number's goingto decrease by four. So up here you'll have p minus2, plus our neutrons minus 2, so we're going tohave minus 4. So your mass is going todecrease by four, and you're actually going to turnto a new element. Remember, your elementswere defined by the number of protons. So in this alpha decay, whenyou're losing two neutrons and two protons, but especially theprotons are going to make you into a different element. So if we call this element 1,I'm just going to call it, we're going to be a differentelement now, element 2. And if you think about what'sgenerated, we're emitting something that has two protons, and it has two neutrons. So that its mass is going to bethe mass of the two protons and two neutrons. So what are we emitting? We're emitting something thathas a mass of four. So if you look at, what is twoprotons and two neutrons? I actually don't have theperiodic table on my [? head. ?] I forgot to cut and pasteit before this video. But it doesn't take you long onthe periodic table to find an element that has two protons,and that's helium. It actually has an atomicmass of four. So this is actually a heliumnucleus that gets emitted with alpha decay. This is actually ahelium nucleus. And because it's a heliumnucleus and it has no electrons to bounce off its twoprotons, this would be a helium ion. So essentially it hasno electrons. It has two protons so ithas a plus 2 charge. So an alpha particle is reallyjust a helium ion, a plus 2 charged helium ion that isspontaneously emitted by a nucleus just to get toa more stable state. Now that's one type of decay. Let's explore the other ones. So let me draw anothernucleus here. I'll draw some neutrons. I'll just draw some protons. So it turns out sometimes thata neutron doesn't feel comfortable with itself. It looks at what the protons doon a daily basis and says, you know what? For some reason when I look intomy heart, I feel like I really should be a proton. If I were a proton, the entirenucleus would be a little bit more stable. And so what it does is, tobecome a proton-- remember, a neutron has neutral charge. So what it does is, itemits an electron. And I know you're saying, Sal,you know, that's crazy, I didn't even know neutronshad electrons in them, and all of that. And I agree with you. It is crazy. And one day we'll studyall of what exists inside of the nucleus. But let's just say that itcan emit an electron. So this emits an electron. And we signify that with its--roughly its mass is zero. We know an electron reallydoesn't have a zero mass, but we're talking aboutatomic mass units. If the proton is one, anelectron is 1/1,836 of that. So we just round it. We say it has a mass of zero. Its mass really isn't zero. And its charge is minus 1. It's atomic, you can kindof say its atomic number's minus 1. So it emits an electron. And by emitting an electron,instead of being neutral, now it turns into a proton. And so this is calledbeta decay. And a beta particle is reallyjust that emitted electron. So let's go back to our littlecase of an element. It has some number of protons,and then it has some number of neutrons. So you have the protons andthe neutrons, then you get your mass number. When it experiences betadecay, what happens? Well, are the protons changed? Sure, we have one more protonthan we had before. Because our neutronchanged into one. So now our protons are plus 1. Has our mass number changed? Well let's see. The neutrons goes downby one but your protons go up to by one. So your mass numberwill not change. So it's still goingto be p plus N. so your mass stays the same,unlike the situation with alpha decay, but yourelement changes. Your number of protonschanges. So now, once again, you'redealing with a new element in beta decay. Now, let's say we havethe other situation. Let's say we have a situationwhere one of these protons looks at the neutrons andsays, you know what? I see how they live. It's very appealing to me. I think I would fit in better,and our community of particles within the nucleus wouldbe happier if I too were a neutron. We'd all be in a morestable condition. So what they do is, that littleuncomfortable proton has some probability ofemitting-- and now this is a new idea to you-- a positron,not a proton. It emits a positron. And what's a positron? It's something thathas the exact same mass as an electron. So it's 1/1836 of themass of a proton. But we just write a zero therebecause in atomic mass units it's pretty close to zero. But it has a positive charge. And it's a little confusing,because they'll still write e there. Whenever I see an e, Ithink an electron. But no, they say e because it'skind of like the same type of particle, but insteadof having a negative charge, it has a positive charge. This is a positron. And now we're starting to getkind of exotic with the types of particles and stuffwe're dealing with. But this does happen. And if you have a proton thatemits this particle, that pretty much had all of itspositive charge going with it, this proton turnsinto a neutron. And that is called positronemission. Positron emission is usuallypretty easy to figure out what it is, because they callit positron emission. So if we start with the same E,it has a certain number of protons, and a certainnumber of neutrons. What's the new elementgoing to be? Well it's going to losea proton. p minus 1. And that's going to be turnedinto a neutron. So p is going togo down by one. N is going to go up by one. So that the mass of the wholeatom isn't going to change. So it's going to be p plus N. But we're still going to havea different element, right? When we had beta decay,we increased the number of protons. So we went, kind of, to theright in the periodic table or we increased our, well,you get the idea. When we do positron emission,we decreased our number of protons. And actually I shouldwrite that here in both of these reactions. So this is the positronemission, and I'm left over with one positron. And in our beta decay, I'm leftover with one electron. They're written theexact same way. You know this is an electronbecause it's a minus 1 charge. You know this is a positronbecause it has a plus 1 charge. Now there's one lasttype of decay that you should know about. But it doesn't change the numberof protons or neutrons in a nucleus. But it just releases a ton ofenergy, or sometimes, you know, a high-energy proton. And that's called gamma decay. And gamma decay means that theseguys just reconfigure themselves. Maybe they get a littlebit closer. And by doing that they releaseenergy in the form of a very high wavelength electromagneticwave. Which is essentially a gamma, you couldeither call it a gamma particle or gamma ray. And it's very high energy. Gamma rays are something youdon't want to be around. They're very likelyto maybe kill you. Everything we did, I've saidis a little theoretical. Let's do some actual problems,and figure out what type of decay we're dealing with. So here I have 7-berylliumwhere seven is its atomic mass. And I have it being convertedto 7-lithium So what's going on here? My beryllium, my nuclear massis staying the same, but I'm going from four protonsto three protons. So I'm reducing my numberof protons. My overall mass hasn'tchanged. So it's definitelynot alpha decay. Alpha decay was, you know,you're releasing a whole helium from the nucleus. So what am I releasing? I'm kind of releasing onepositive charge, or I'm releasing a positron. And actually I have thishere in this equation. This is a positron. So this type of decay of7-beryllium to 7-lithium is positron emission. Fair enough. Now let's look atthe next one. We have uranium-238 decayingto thorium-234. And we see that the atomic massis decreasing by 4, minus 4, and you see that your atomicnumbers decrease, or your protons are decreasing,by 2. So you must be releasing,essentially, something that has an atomic mass of four,and a atomic number of two, or a helium. So this is alpha decay. So this right here isan alpha particle. And this is an exampleof alpha decay. Now you're probably saying, heySal, wait, something weird is happening here. Because if I just go from 92protons to 90 protons, I still have my 92 electrons out here. So wouldn't I now havea minus 2 charge? And even better, this helium I'mreleasing, it doesn't have any electrons with it. It's just a helium nucleus. So doesn't that havea plus 2 charge? And if you said that, you wouldbe absolutely correct. But the reality is that rightwhen this decay happens, this thorium, it has no reasonto hold on to those two electrons, so those twoelectrons disappear and thorium becomes neutral again. And this helium, likewise,it is very quick. It really wants two electronsto get stable, so it's very quick to grab two electrons outof wherever it's bumping into, and so thatbecomes stable. So you could writeit either way. Now let's do another one. So here I have iodine. Let's see what's happening. My mass is not changing. So I must just have protonsturning into neutrons or neutrons turning into protons. And I see here that Ihave 53 protons, and now I have 54 protons. So a neutron must haveturned into a proton. A neutron must havegone to a proton. And the way that a neutrongoes to a proton is by releasing an electron. And we see that in thisreaction right here. An electron has been released. And so this is beta decay. This is a beta particle. And that same logic holds. You're like, hey wait, I justwent from 53 to 54 protons. Now that I have this extraproton, won't I have a positive charge here? Well you would. But very quickly this might--probably won't get these exact electrons, there's so manyelectrons running around-- but it'll grab some electrons fromsome place to get stable, and then it'll be stable again. But you're completely right inthinking, hey, wouldn't it be an ion for some smallamount of time? Now let's do one more. So we have to 222-radon-- ithas atomic number of 86-- going to 218-polonium, withatomic number of 84. And this actually is aninteresting aside. Polonium is named after Poland,because Marie Curie, she-- At the time Poland, thiswas at the turn of the last century, around the end of the1800's, Poland didn't exist as a separate country. It was split between Prussia,Russia, and Austria. And they really wanted letpeople know that, hey, you know, we think we'reone people. So they discovered that when,you know, radon decayed it formed this element. And they named it after theirmotherland, after Poland. It's the privileges ofdiscovering new elements. But anyway, backto the problem. So what happened? Our atomic mass wentdown by four. Our atomic number wentdown by two. Once again, we must havereleased a helium particle. A helium nucleus, somethingthat has an atomic mass of four, and an atomicnumber of two. And so there we are. So this is alpha decay. We could write this asa helium nucleus. So it has no electrons. We could even say immediatelythat this would have a negative charge, butthen it loses
