Chapter 18: Nuclear Chemistry

1. Chapter 18:NuclearChemistry In this introduction to nuclear science, you will learn why some atoms are unstable (radioactive) and how radioactive atoms interact and change. "According to a study by the National Science Foundation, 70% of Americans do not understand science. Here's the real scary part: 30% don't even know what 70% means.“ -Jay Leno

2. 18.1 Nuclear Stability & radioactive Decay • Recall:nuclei=positively-charged protons and neutral neutrons. p/ncallednucleons • Protons repel each other. • Without neutrons, protons would cause the nucleus to fly apart. • Neutrons act like “nuclear glue”, interacting with a force we call the strong nuclear force. • When the ratio of neutrons to protons is too high or too low, the nucleus becomes unstable and will release particlesto become stable.

3. Unstable Nuclei – General Rules • Nuclide: name for an atom in nuclear chem. • Radioactivity: When a nuclide is unstable and gives off particles. • Nearly all elements have a radioactive isotope. • All isotopes of atoms above atomic number 83 are radioactive. ( Why this is? ) • Very large nucleus = nuclear glue is too weak to hold all the particles together. (Instability!) • Small nuclides = 1:1 n:p ratio = stability • Large nuclides = 1.5:1 n:p ratio = stability. • What if the ratio of protons-neutrons is way off?

4. Band of Stability • A graph of neutrons-protons shows the stable isotopes, (acceptable n:p ratios) • Nuclides outside this range are unstable stable unstable unstable

5. Unstable Nuclei – Radioactive Decay • Nuclear Decay = Atoms release particles to attain a better neutron-proton ratio. • Half-life = Time for ½ of a sample of nuclides to transmutate into another nuclide with a particular type of decay. • Transmutation = conversion of one nuclide into another with different # of protons • Half lives can be as long as millions of years and as short as nanoseconds. • A stability graph shows the band of stability.

6. Types of Decay – Alpha Decay mass • Five basic Decays. either: • particles ejected from nucleus • (or) nucleus capture particles. • Alpha Decay: In alpha decay a helium nucleus is released. • In standard notation, we place the mass above the charge as seen here. • Alpha particles: • move very slowly • because of their size, can be blocked with a few pages of paper or human skin • Causes ionization (damaging!) charge Alpha Decay occurs in all elements with atomic number above 83. Images from ChemZone

7. Types of Decay – Beta Decay • Beta Decay: An electron is ejected from the nucleus and a neutron becomes a proton. We call this high energy electron a beta particle. • The decayed nucleus now has one more proton, the atomic number increases! • Beta particles • move fast • can penetrate thick low-density materials • but can be blocked with concrete and metals Beta Decay occurs when a nucleus has a high neutron-proton ratio. Images from ChemZone

8. Types of Decay – Gamma Decay • Gamma Decay: High energy photons (gamma rays) are given off. • Gamma rays • given off as the spare change during other radioactive decays…. • extremely penetrating and powerful. Several inches of leadis required to slow these particles down to a stop. • Don’t get included in nuclear equations. Summary of three basic particle decays Images from ChemZone

9. Types of Decay – Electron Capture • Electron Capture: An electron is pulled into the nucleus and a proton becomes a neutron. • This is effectually the opposite of beta decay, a beta particle is taken in to the nucleus. • Notice that when electron capture occurs, the atomic number drops by one due to loss of a proton. Electron Capture occurs when a nucleus has a low neutron-proton ratio. Images from ChemZone

10. Types of Decay – Positron Decay • Positron decay: An anti-electron is given off from the nucleus and a proton becomes a neutron. • This is similar to electron capture except an oppositely charged electron is given off. • Notice the charge of +1 on the positron. • Like electron capture, atomic number drops by one with positron decay. “Parent nuclide” “Daughter nuclide” Positron emission occurs when a nucleus has a low neutron-proton ratio. Images from ChemZone

11. Types of Decay – neutron emission • Neutron emission: a high-energy neutron is given of • Neutron emission: • Reduces the n:p ratio • Sometimes accompanies other types of radioactive decay • Makes nuclear weapons possible • Is extremely destructive to living tissue • Is the fuel source reaction for nuclear power reactors

12. 18.2 Kinetics of Radioactive Decay • The decay of Nuclides follows a first order rate law • This can be rewritten in terms of ½ life • N0 = original number (mass) of nuclides • N= number (mass) of nuclides remaining at time t • k=first order rate constant

13. Example Problem 1 The ½ life of 239Pu94 is 2.411x104 years. How many years will elapse before 99.9% of a given sample decomposes? First find the rate constant k Now use k to calculate the time for 99.9% of the sample to decompose. t = 2.4x105 years

14. Example Problem 2 The ½ life of 217Pa91 is 4.9x10-3 seconds. How much of a 3.5mg sample will remain after 1.000 second? First find the number of ½ lives Now use the number of ½ lives to calculate the fraction of mass remaining. The final mass of Pa is 3.5mg x 3.9x10-62 = 1.4x 10-61 mg Essentially none!

15. 18.3 Nuclear Transformations Artificial Transmutation = conversion of one nuclide into another with different # of protons by bombardment with another particle. This produces heavier elements.

16. 18.4 Detection and Uses of Radioactivity • Geiger counter detects the number of disintegrations (particles emitted) per unit time. • Two key assumptions of radiocarbon dating • Ratio of C14 to C12 has been constant over time • Living systems have this same constant ratio till they die. Sample Problem A bone dated at 12000 years with carbon dating will give off how many disintegrations per gram per minute (d/g·min)? The atmospheric constant is 13.6 d/g·min and the ½ life is 5730 years

17. Sample Problems .A rock contains .141 g 206Pb82 for every 1.000 g of 238U92. How old is the rock? t 1/2 238U92 = 4.5x109 yr and you must assume all intermediates decay instantly.

18. 18.5 Thermodynamic Stability of the Nucleus • A nucleus is more than just a bundle of protons and neutrons. • It is more stable than an individual array of protons and neutrons. • The mass of the nucleus of any atom is less than the sum of the • masses of the individual protons and neutrons that make it up. • This difference in mass is called the mass defect. • This mass has been converted into energy according to E=mc2 • or ΔE=Δmc2 • ΔE is called the binding energy which holds the nucleus together.

19. Sample Problem Calculate the binding energy per nucleon in MeV for 187Ir77 First calculate binding energy in joules/mol Now convert to MeV per nucleon

20. 18.6 Nuclear Fission & Fusion • Nuclear fission = splitting of large, unstable atoms, releases large amounts of energy • Critical mass (or critical density) = amount of fissionable fuel before reaction will begin. • Uncontrolled, nuclear fission proceeds to completion with great speed. • In a nuclear weapon, two half-spheres of fissionable material are compressed together with conventional explosives, creating the critical mass. • In order to harness nuclear fission to create useable electricity, we slow down the process with control rods… Once fission begins, it is difficult to stop.

21. Fission Reactor Power Plants • In each reactor, heat is generated when high-energy neutrons slow down in collisions with the moderator. • Pressurized Steam Reactor: Steam is produced and drives turbines, creating electricity. • Reactor fuel must be fissionable, usually weapons-grade uranium or plutonium. • Moderators become highly radioactivity (dirty). Popular Moderators: B(s), Cd(s), Na(l), Li(l) 238U – natural – not fissionable 235U – enriched – fissionable

22. Nuclear Fusion • Nuclear Fusion: • Joining of smaller nuclei to form larger nuclei. • Releases far more energy that nuclear fission. • The sun’s (stars) energy comes from the fusion of hydrogen atoms into helium atoms. • The hydrogen bomb is a fusion weapon. • Extensive research is being carried out to find ways of creating and harnessing nuclear fusion for industrial power production. Unlike fission, fusion reactions can be easily controlled, by controlling the fuel flow.

23. E=mc2 • Einstein’s famous equation relates mass an energy. Energy and mass are interchangeable. • E=Energy, m=mass, c=speed of light (3x108m/s) • When a high-energy gamma ray is given off, the mass of the nucleus drops a measurable value. • Similarly, we can calculate how much energy will be given off in particular nuclear reactions. • We use the formula ΔE= Δmc2 to build new, artificial elements in supercolliders (particle accelerators.) Fermilab cyclotron, Argonne National Laboratory, Chicago

24. 18.7 Effects of Radiation Units of Radiation 1.curie- # of disentegrations per second from 1 g of radium. Doesn’t consider type of radiation. 2.rad – absorbed dose of radiation doesn’t consider type or effect. 3.rem – roentgen equivalent man – measures the energy transferred by radiation and the bodies sensitivity to that radiation. 4.dosimeter – instrument used to detect radiation exposure. Some use photographic film. 5. SI Units 1. gray (Gy) - one joule of energy per kilogram of body tissue (100 rad) 2.sievert (Sv) – exposure equal to one gray of gamma () radiation (100 rem) Effects of radiation 1. Ionization – strips electrons from atoms. Creating free radicals that disrupt living cells

25. Effects of radiation cont…. A.Types of damage • Somatic damage – damage to the organism that received the dose. (cancer) • Genetic damage – damage to reproductive organs or offspring.(mutation) • Linear Model- increasing damage with increasing exposure • Threshhold Model- no damage until a critical threshhold is reached B.Beneficial uses of radiation 1.radiotracers (barium): used to follow substances as they move through a system. 2.Cancer treatment – radiation therapy 3.Food preservation- read handout for discussion