Nuclear Reactions Jespersen Chapter 21 (VERY loosely) (only need to know what is presented here) Dr. C. Yau Spring 2013 1

Review: The nuclear symbol What do you need to know? You are responsible only for the topics on nuclear reactions I present here. Review: The nuclear symbol Mass number = # protons + # neutrons (due to 6 p and 8 n) Atomic number (also, nuclear charge) = # protons (C has charge of +6 in nucleus, due to 6 protons) In nuclear chemistry, this number does not always give us # protons, but it does give us the nuclear charge. (Not all nuclear particles have protons.) 2

Isotopes 1p 1 p 1 p 0 n 1 n 2 n protium deuterium tritium* Give the number of p and n for each of the following: 1p 1 p 1 p 0 n 1 n 2 n protium deuterium tritium* 99.985% 0.015% zero% These are isotopes of hydrogen. What exactly are isotopes? -Atoms of the same element with different masses. -Atoms with the same #p but different # n *Tritium is the only one that is radioactive. D2O = heavy water (MW=20 amu instead of 18 amu) 3

Radioactivity is the spontaneous disintegration of a nucleus accompanied by an emission of rays. Radioisotopes are the isotopes that are radioactive. Note that not all isotopes are radioactive. (e.g. Tritium is, but not protium or deuterium.) Alpha Decay (-decay) is the disintegration of a nucleus accompanied by an emission of  particles. Beta Decay (-decay) is the disintegration of a nucleus accompanied by an emission of  particles. 4

Gamma Decay (-decay) is emission of -rays. -rays are high energy rays (no particles involved). Why alpha, beta, gamma? That’s Greek for A, B, C. It turns out that the  particle is the nucleus of the He atom:  particle = particle is the electron = neutron has the symbol = proton has the symbol = positron is a "positive electron" = ?  is not a particle but is given the symbol You do not need to memorize the symbols of these particles. Explain the numbers! 5

Example 1 Uranium-238 undergoes -decay. Write the nuclear equation for its decay. "Uranium-238" means the isotope of uranium with a mass number of 238. We do not need to specify the atomic number as we can easily find it on the periodic table. Example 2 The product of U-238 decay undergoes -decay. Write the nuclear equation for it. Note that U changed into another element! This is not possible in chemical reactions! We are doing it here because we are now dealing with nuclear reactions. 6

Example 3 If we begin with polonium-210 and end with lead-206, what nuclear particle is emitted during the decay? Example 4 The U-238 decay series involve 14 steps. Somewhere along the series there is a step that involves emission of alpha rays and it produces radioactive radon-222. Write the nuclear equation. What do you know about radon? Where have you heard about it? U-238 decay series produces many other radioactive products. Why do we single out Rn to talk about? Practice with p.1012 #21.58 A, C, D; #20.59 7

Review of Half-life and Decay Rate Radioactive decay is 1st order. For 1st order, t½ = Note that the half-life is not affected by the initial concentration of the reactant. What does half-life (t½) mean? It is the amount of time it takes for the sample to decrease by ½ (by mass or by number of particles). Example 5: U-238 has a half-life of 4.5x109 yr. Will it be all gone after two half-lives? Before you do any calculations, first check to see whether it can be done easily.

Review of Half-life and Decay Rate (cont’d.) Example 6: P-32 has a half-life of 14 days. Starting with 10.0 g of P-32, how much is left after 42 days? Example 7: P-32 has a half-life of 14 days. Starting with 10.0 g of P-32, how much is left after 45 days?

You can determine the half-life graphically by plotting concentration vs. time. What is the half-life of I-131? What is its decay constant? What is the unit of the constant?

There is another isotope of uranium of interest: U-235. It is also radioactive but more important it is fissionable (or fissile). Nuclear fission is splitting the nucleus into two large particles by bombardment of a high energy particle (such as a neutron). Nuclear fusion is fusing together two small nuclei into a larger one. Both are accompanied by an enormous amount of E! ΔE = - 1010 kJ ΔE = - 108 kJ 11

Nuclear Fission U-235 does not always split into the same products. However, the products are always radioactive and given the term “radioactive daughters”. These radioactive products are in the fallout from a nuclear bomb. Sr-90 has half-life of 28.1 yr. Rule of thumb: It takes 10 half-lives for a radioisotope to be considered "gone." Note also the formation of 3 n. Why is this significant? 12

Nuclear Chain Reaction Fig. 21.14 (p.1001) Fig. 20.14 p.843

Nuclear Fission Nuclear fission is what we use in the nuclear reactor at the power plants and in atomic bombs. Not all isotopes are “fissile” (not fissionable). U-238 is radioactive but not fissile. In nature, only 0.7% of naturally occurring uranium is U-235. The rest (99.3%) is U-238 which is not fissile. Nuclear power plants only require about 3% U-235. Atomic bombs require about 97.3% U-235. To take 0.7% U-235 and convert to weapon-grade 97.3% U-235 we make use of the different rate of effusion and applying Graham’s Law: Heavier atoms effuse slower. Allowing the two isotopes to effuse (a long process as the difference in mass is only 3 amu) eventually we can separate U-235 from U-238. This process is called URANIUM ENRICHMENT. 14

Nuclear Fusion Another example is Complete the equation: Nuclear fission is what takes place in the sun and the stars. … and used in the H-bomb. Advantage over nuclear fission? H isotopes are much easier to obtain than uranium, and much more abundant (from water). Products have much shorter half-lives. H-3 has t ½ = 12. 3 yrs compared to U-235 has t ½ = 7.04x108 yrs Nuclear wastes of nuclear fusion also have much shorter half-lives. Disadvantage is the difficulty in harnessing it for use: requires millions of degrees to initiate. 15

Fe has the lowest “binding energy.” The lower the binding energy, the more stable the nucleus is. Very large nuclei (as in uranium) break down to smaller ones. Very small nuclei (as in H) fuse to larger ones.

Anything good about nuclear reactions? Nuclear energy does not produce “greenhouse” gas (CO2) or acid rain (SO2). We do not rely on the Middle East to supply us with uranium. Nuclear medicine: PET scan: Positron Emission Tomography (Nuclear medicine imaging to produce a 3-D image of functional processes in the body) It uses nuclides of short half-lives, such as C-11 (20 mins); N-13 (10 mins); O-15 (2 mins). Why is a short half-life desirable? Write the nuclear eqn for C-11 emitting a positron. Write the nuclear eqn for the positron meeting with an electron.