Nuclear Reactions

Nuclear Reactions Consider the reaction: N2 + O2 NO2 2 2 Balancing this reaction simply involves placing a “2” in front of the NO2… And then a “2” in front of the O2… But what about the reaction: Po  Pb ? (Q1) Clearly this reaction could never be balanced in the traditional way. The first reaction was a chemical reaction, and all the nitrogen atoms were remaining nitrogen atoms and the oxygen atoms were remaining oxygen atoms; all these atoms were doing was changing the way they were bonded to one another. This is true of all chemical reactions. But the second reaction involves a polonium atom transforming itself into a lead atom. The only way this can happen is if there are changes occurring within the nucleus. This sort of reaction is known as a nuclear reaction, and it is “balanced” in a very different way.

Nuclear Reactions Po  Pb 207 211 82 84 82 p 125 n 84 p 127 n Recall that the nucleus is comprised of two types of subatomic particles: (Q2) protons and neutrons, and that their quantities are indicated by two numbers (Q3) The atomic numbers which are placed here indicate the number of protons. (Q4) And the mass numbers which are written here indicate the combined number of protons and neutrons. (Q5) Thus the reaction involves a Po nucleus containing 84 protons and (211 - 84 =) 127 neutrons changing itself into a lead nucleus which contains 82 protons and (207 - 82 =) 125 neutrons.

Nuclear Reactions Po  Pb + He 4 207 211 2 82 84 82 p 125 n 84 p 127 n 2 p 2 n So we can see that the polonium nucleus must have some-how gotten rid of two protons and two neutrons as it turned itself into the lead nucleus. These were given off as a little 2-p-2-n bundle known as an “alpha particle.” (Q6) Since this alpha particle contains two protons and two neutrons, it is essentially a He nucleus. Thus the nuclear equation is “balanced” by adding an alpha particle symbol on the right side (products side) of the reaction. 4 2 When a atomic nucleus spontaneously spits out a small particle, this is referred to as radioactive “decay.”

Nuclear Reactions Po  Pb + He 4 207 211 2 82 84 Po Pb He The animation for this decay reaction would look like this: Note that decay events like this are totally unpredictable. They happen in order to make the nucleus more stable. The polonium nucleus above was unstable because it was just too large. Recall that protons are positively charged and thus tend to repel one another. Once you have more than about 83 protons crowded together in a tightly-packed nucleus, there is just too much overall repulsion going on, and the nucleus will tend to spontaneously undergo alpha decay to make itself smaller and thus more stable. That the polonium nucleus will decay is inevitable, but when exactly it will decay is completely up to chance. It’s all a matter of probability. (We’ll return to this idea when we get to the concept of half-life.)

Nuclear Reactions = Po  Pb + He 4 207 211 = 2 82 84 The important thing to keep in mind when balancing nuclear reactions is that top numbers add up (211 = 207 + 4), And that the bottom numbers add up as well (84 = 82 + 2). Keep in mind the equal signs go where the arrow is. The top numbers represent the mass of the particles, and we have already learned that mass is conserved during a chemical reaction. Mass is also conserved (more or less) during a nuclear reaction. The bottom numbers represent the number of protons, but it is better to think of these numbers as the charge of the particles. Charges are also conserved during all chemical and nuclear reactions. Thus, making sure the top and bottom numbers “add up” is really just assuring that mass and charge are conserved.

Nuclear Reactions = U  Th + He 4 231 235 = 2 90 92 Uranium-235 is another nucleus that is unstable because it contains too many protons, so it undergoes alpha decay just as the polonium nucleus did. (Q7)Can you figure out what U-235 decays into – in other words, what goes in the ? Let’s put in our equal signs. Making sure that mass is conserved, Clearly m must equal 231. gives us 235 = m + 4. gives us 92 = q + 2. And making sure that charge is also conserved, Clearly q must equal 90. And a quick glance at the periodic table would inform us that a charge of 90+ on the nucleus (atomic number = 90) corresponds with the element thorium, so we add a “Th” to complete the answer.

Nuclear Reactions = C  N + e 0 14 14 = -1 7 6 C-14 is another nucleus that is unstable but not because it contains too many protons. It’s unstable because its neutron to proton ratio is too high. It decays into N-14 by giving off some other kind of particle: (Q8)What particle would this be? Let’s put in our equal signs. Making sure that mass is conserved, Clearly m must equal 0. gives us 14 = 14 + m. gives us 6 = 7 + q. And making sure that charge is also conserved, Clearly q must equal -1. So what small particle has a mass of 0 and a charge of -1? An electron (e) does. When a nucleus has too high a neutron to proton ratio, it has something interesting take place: a neutron transforms itself into a proton plus an electron. The electron that is shot out is known as a “beta particle,” and this transformation is called a “beta decay.”

Nuclear Reactions = proton = neutron = beta particle (electron) C  N + e 0 14 14 -1 7 6 If we let red = protons, yellow = neutron and blue = beta particle, then the animation would look something like this: Replay this animation and watch very carefully this time. Could you see the neutron that transformed into a proton as it shot out a beta particle? It was in the upper right corner. (Q9) And do you see that whereas the original nucleus had a neutron:proton ratio of 8:6 (4:3), the new nucleus has a n:p ratio of 7:7 (1:1) which is a bit more stable. So the two things that make a nucleus unstable are: 1) too many protons total – which leads to alpha decay – and 2) a n:p ratio that is too high – which leads to beta decay.

Nuclear Reactions By the way, below is a list of some of the small particles/emissions that are commonly involved in nuclear reaction. You may want to familiarize yourself with them and their names, though you will not need to memorize them. = proton = neutron = electron Particle: Symbol: Description: alpha a He-4 nucleus beta an electron gamma high energy light positron a positively charged electron! neutron mass but no charge proton a H-1 nucleus deuteron a H-2 nucleus ora He 4 2 orb e 0 -1 g 0 0 e 0 +1 n 1 0 H 1 1 H 2 1

Nuclear Reactions = + Na n  Na 1 24 23 n 1 = 0 11 11 0 Here is a nuclear reaction that would not be categorized as a decay. (Q10) In a way, it is the opposite of decay. A sodium-23 nucleus takes in a neutron. (Q11) Notice how a neutron has a mass of 1 but no charge, so its symbol is simply: . (Q12) Let’s put the equal signs here. Making sure that mass is conserved, Clearly m must equal 24. gives us 23 + 1 = m. gives us 11 + 0 = q. And making sure that charge is also conserved, Clearly q must equal 11. Atomic number 11 is Na. This makes sense: if an atom absorbs a neutron, that will make its mass increase by 1, but since its number of protons didn’t change, it will remain the same element. This type of nuclear reaction is called “absorption” or “capture.” Specifically the reaction above is an example of a “neutron capture.”

Nuclear Reactions = proton = neutron = electron + Na n  Na 1 24 23 0 11 11 The animation for this neutron capture would look like this: Here’s another example of a capture event: this time an electron capture: B + e  Be 0 8 8 -1 4 5 (Q13) Can you figure out what particle it produces? Is this what you were thinking? The animation for this electron capture would look like this: Notice how electron capture turns a proton into a neutron, so electron capture helps to stabilize a nucleus with a n:p ratio that is toolow.

Nuclear Reactions = proton = neutron = electron = + H + O n  N 1 1 16 16 = 1 0 7 8 If a slow moving particle collides with a nucleus, it is likely to get absorbed, but if a faster moving particle hits, it might cause a reaction like this. (Q14, Q15) Put in the equal signs. gives us 16 + 1 = 16 + m. Making sure that mass is conserved, And making sure that charge is also conserved, Clearly m must equal 1. Clearly q must equal 1. gives us 8 + 0 = 7 + q. Atomic number 1 is H. But, if you think about it, this H nucleus with a mass of 1 and a charge of 1+ is really just a proton. This kind of nuclear reaction is called a “bombardment” reaction and it would look like this:

Nuclear Reactions = Y + Pu n  Cs + + 3 n 99 1 141 242 1 = 39 0 55 94 0 So far, we have discussed decay, capture and bombardment nuclear reactions. A fourth type of nuclear reaction occurs when a neutron hits a large unstable nucleus. It can cause enough of a distortion in the nucleus, that the repulsive forces between the protons can cause the large nucleus to split into two smaller nuclei.(Q16) This is called “fission.” (Q17) Let’s put the equal signs where the reaction arrow is. Making sure mass is conserved (note: this 3 distributes) This gives us: 242 + 1 = 141 + m + 3(1). m = 243 – 144 = 99 Making sure charge is conserved (again the 3 distributes) This gives us: 94 + 0 = 55 + q + 3(0). q = 94 – 55 = 39 Element #39 is yttrium (Y), so we place a Y in the box.

Nuclear Reactions Y + Pu n  Cs + + 3 n 99 1 141 242 1 39 0 55 94 0 The animation for this fission reaction would perhaps look something like this: Replay it a few more times and note how the high speed neutron gets absorbed, and this causes an elongation of the nucleus. Without this distortion, the high number of protons was already causing the nucleus to be unstable, but it would have just undergone a gradual series of alpha (and beta) decays to eventually end up small enough to be stable. With this distortion, however, the repulsive forces now have a unified direction and they cause the nucleus to split into two smaller, more stable pieces.

Nuclear Reactions Y + Pu n  Cs + + 3 n 99 1 141 242 1 39 0 55 94 0 Since the nucleus becomes more stable all at once instead of gradually, a great deal of energy is released by fission reactions: (Q18) Some people mistakenly think that splitting one nucleus like this releases enough energy to destroy entire cities. The energy released by one nucleus being split would be enough to raise the temperature of a drop of water by about one-billionth of a degree Celsius. That may not seem like much energy, but when you multiply it by 1 x 1026 nuclei that get split in a typical fission bomb, then it adds up to the amazing power that everyone associates with an atom bomb. (Q19)

Nuclear Reactions Y + Pu n  Cs + + 3 n 99 1 141 242 1 39 0 55 94 0 Also note that as the nucleus is split, it produces a few high speed neutrons. (Q20) The significance of these neutrons is that they might end up hitting other Pu-242 nuclei around them and cause them to split. This could lead to the sort of run-away chain reaction that occurs in an atom bomb with all 1 x 1026 nuclei getting split in a fraction of a second. But that only happens if there are enough fissionable nuclei nearby. (Q21). The minimum amount of fissionable material necessary to allow a chain reaction is referred to as “critical mass.”

Nuclear Reactions Y + Pu n  Cs + + 3 n 99 1 141 242 1 39 0 55 94 0 Finally, what about the bi-products of the fission reaction. They are often referred to as “daughter isotopes;” in this case they are Cs-141 and Y-99, but they can be any of several different combinations. (Q22) The problem they present is that they generally have way too many neutrons and therefore are unstable and will undergo beta decay. This causes them to remain radioactive sometimes for thousands of years. This same fission-type reaction is used in nuclear power plants -- but in a far more controlled system. Still, one of the big issues facing these nuclear plants is what to do with the radioactive bi-products.

Nuclear Reactions = + Li He  B 4 11 7 = 2 5 3 So, we have discussed decay, capture, bombardment and fission. The fifth and final type of nuclear reaction occurs when two small nuclei fuse together into one, as is shown in the reaction above. This is the type of nuclear reaction that occurs in our sun – and in all stars in the universe. (Q23) It is called “fusion.” (Q24) What do you think is produced? Again we put the equal signs where the reaction arrow is. we get 7 + 4 = m. m = 11 Making sure mass is conserved, we get 3 + 2 = q. q = 5 Making sure charge is conserved, Element #5 is boron (B), so we place a B in the box.

Nuclear Reactions + Li He  B 4 11 7 2 5 3 The animation for this fusion reaction is quite simple; it just involves a Li-7 nucleus and a He-4 nucleus colliding with one another like this: Hmmm, let’s try that again a little faster. The reason that fusion requires such high energy particles is that the two nuclei, both being positively charge, naturally repel one another. OK, once more – this time let’s really crank it up! (Q25) The atoms have to be at very high temperatures to get the nuclei to come close enough that strong nuclear forces can take over and allow them to fuse into one nucleus.

Nuclear Reactions + Li He  B 4 11 7 2 5 3 Gram for gram, fusion reactions release several times more energy that fission reactions. And although fusion reactions have been used in making nuclear weapons that are even more powerful than atom bombs (fusion weapons are known as “hydrogen bombs”), fusion has yet to be successfully harnessed in nuclear power plants. (Q26) This is because scientists have yet to figure out how to contain the reaction… It’s as hot as the sun, and no known material is able to withstand temperatures that high! If and when fusion is harnessed to generate power, many believe that it would solve all the world’s energy problems!

Nuclear Reactions • So, you have now been shown five types of nuclear reaction: • Decay (which involves unstable nuclei spontaneously spitting out tiny particles) • Absorption (AKA capture, which involves the taking in of tiny particles) • Bombardment (a nucleus is struck by a particle so hard it knocks a different particle out the other side) • Fission (a large unstable nucleus is struck by a neutron so hard that it causes it to elongate and then split into two smaller nuclei and a few neutrons) • Fusion (which simply involves two small nuclei fusing together into one)

Nuclear Reactions • So, you have now been shown five types of nuclear reaction: • Decay (which involves unstable nuclei spontaneously spitting out tiny particles) • Absorption (AKA capture, which involves the taking in of tiny particles) • Bombardment (a nucleus is struck by a particle so hard it knocks a different particle out the other side) • Fission (a large unstable nucleus is struck by a neutron so hard that it causes it to elongate and then split into two smaller nuclei and a few neutrons) • Fusion (which simply involves two small nuclei fusing together into one) + + Ir H He H   Hg He 4 2 197 4 193 2 2 1 79 2 77 1 See if you can identify which type of reaction applies to each of the following: (Q27) You may or may not have noticed, but there are three mistakes (intentional) in the numbers and symbols below. See if you can find them! (Q28) Fusion: you can tell because there are two small nuclei on the left side, and they fuse into just one on the right. Decay: easy to recognize since it has just one nucleus on the left side. All the others have two. Mistake #1: these masses do not add up; changing the 135 to a 134 for example would fix the mistake. Fission: one big nucleus hit by a neutron and split into two. Co Fe + e 0 56 56 Mistake #2: element #79 is gold, not mercury (Ay-You! You should have caught that mistake!) +1 26 27 Absorption: looks like fusion (2 becoming 1), but it has one small particle and one big nucleus coming together. Sn + Th n  Zr + + 4 n 135 1 95 232 1 Mistake #3: no such combination exists! If the atomic number is 2, the mass number must be at least 2! Changing the 1 to a 4 and the 40 to a 37 would fix this mistake. 50 0 40 90 0 + He + Ar H  Cl 1 1 40 40 Bombardment: easy to recognize – two particles on the left & two particles on the right. 2 1 17 18

Nuclear Reactions So far we have mentioned three conditions that cause the nucleus to be unstable: 1. Too many protons (>83). This leads to a-decay. 2. Too high a neutron to proton ratio. This leads to b-decay. 3. Too low a n:p ratio. This leads to electron capture. A fourth cause of nuclear instability has to do with the arrangement of neutrons and protons in the nucleus. Usually when a nucleus undergoes decay and its composition changes, the neutrons and protons in the new nucleus immediately rearrange themselves into a new more stable configuration: this gives off energy in the form of light – specifically gamma radiation. This gamma ray is usually given off at the same time as the decay particle, as shown in the following animation:

Nuclear Reactions + g Ac  Fr + He 4 0 216 220 2 0 87 89 Go back and replay the animation and watch specifically how the gap (created as the alpha particle left) immediately closes into a more stable neutron-proton arrangement and how this closing of the gap gives off the gamma ray. But… sometimes the nuclear particles do not immediately rearrange themselves and a nucleus is produced that is unstable just because of its arrangement of neutrons and protons.

Nuclear Reactions Th Ra + He + g 4 0 215m 219 2 0 88 90 Radium-215m is one such isotope. It is created when a Th-219 nucleus undergoes alpha decay. Notice how the gap does not immediately close into a more stable arrangement. This closing of the gap happens later on as a second sort of decay event: The “m” stands for “metastable” – a term used to describe a nucleus that has an unstable n-p arrangement. The equation for this event is written like this: Ra  Ra 215m 215 88 88 Notice how this decay does not involve any particles entering or leaving the nucleus. The nucleus simply goes from one arrangement to another.

Nuclear Reactions + g Ra  Ra 0 215m 215 0 88 88 This sort of decay is often referred to as an IT: an isomeric transition. “Isomer” is a term generally used to describe two compounds with the same atomic composition, just with the atoms arranged differently. Here the term refers to nuclear isomers: that is, two nuclei with the same neutron-proton composition, but different arrangements. C2H6O, ethanol C2H6O, dimethyl ether 88p,127n Ra-215m 88p,127n Ra-215

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Nuclear Reactions