Nuclear Equations, Radioactivity, and Fission/Fusion 1

2 Facts About the Nucleus  Very small volume compared to volume of the whole atom  Essentially entire mass of atom  Very dense  Composed of protons and neutrons that are tightly held together  Nucleons

3 Facts About the Nucleus, Con’t  Every atom of an element has the same number of protons; equal to the atomic number  Atoms of the same elements can have different numbers of neutrons.  Isotopes  Different atomic masses  Isotopes are identified by their mass number  Mass number = number of protons + neutrons

4 Facts About the Nucleus, Con’t  The number of neutrons is calculated by subtracting the atomic number from the mass number.  The nucleus of an isotope is called a nuclide.  Over 90% of isotopes are radioactive. Therefore, their nucleus is called a radionuclide  Each nuclide is identified by a symbol.  Element − mass number.

5 Radioactivity  Radioactive nuclei (radionuclides) spontaneously decompose into smaller nuclei  Is called Radioactive Decay  We say that radioactive nuclei are unstable  The parent nuclide is the nucleus that is undergoing radioactive decay; the daughter nuclide are the new nuclei that are made  Decomposing involves the nuclide emitting a particle ( α, β, etc.) and/or energy  All nuclides with 84 or more protons are radioactive

6 Transmutation  Rutherford discovered that during the radioactive process, atoms of one element are changed into atoms of a different element— transmutation.  In order for one element to change into another, the number of protons in the nucleus must change.

7 Chemical Processes vs. Nuclear Processes  Chemical reactions involve changes in the electronic structure of the atom.  Atoms gain, lose, or share electrons.  No change in the nuclei occurs.  Nuclear reactions involve changes in the structure of the nucleus.  When the number of protons in the nucleus changes, the atom becomes a different element.

8 Nuclear Equations  We describe nuclear processes using nuclear equations.  Use the symbol of the nuclide to represent the nucleus.  Atomic numbers and mass numbers are conserved.  Use this fact to predict the daughter nuclide if you know parent and emitted particle.

U  Th + He Th  Pa + e Alpha decay: Beta decay: SAME ON BOTH SIDES Mass numbers: 238 Atomic numbers: 92 SAME ON BOTH SIDES Mass numbers: 234 Atomic numbers: 90

10 What Kind of Decay and How Many Protons and Neutrons Are in the Daughter? Alpha emission giving a daughter nuclide with 9 protons and 7 neutrons. 11 p + 9 n 0

11 What Kind of Decay and How Many Protons and Neutrons Are in the Daughter?, Continued Beta emission giving a daughter nuclide with 10 protons and 11 neutrons. 9 p + 12 n 0

12 What Kind of Decay and How Many Protons and Neutrons Are in the Daughter?, Continued Positron emission giving a daughter nuclide with 4 protons and 5 neutrons. 5 p + 4 n 0

13 Nuclear Equations  In the nuclear equation, mass numbers and atomic numbers are conserved.  We can use this fact to determine the identity of a daughter nuclide if we know the parent and mode of decay.

14 Practice—Write a Nuclear Equation for Each of the Following:  Alpha emission from Th-238.  Beta emission from Ne-24.  Positron emission from N-13.

15  Alpha emission from Th-238  Beta emission from Ne-24  Positron emission from N-13 Practice—Write a Nuclear Equation for Each of the Following, Continued:

16 Detecting Radioactivity  To detect when a phenomenon is present, you need to identify what it does: 1. Radioactive rays can expose light-protected photographic film.  Use photographic film to detect the presence of radioactive rays — film badges.

17 Detecting Radioactivity, Con’t 2. Radioactive rays cause air to become ionized.  An electroscope detects radiation by its ability to penetrate the flask and ionize the air inside.  Geiger-Müller counter works by counting electrons generated when Ar gas atoms are ionized by radioactive rays.

18 Detecting Radioactivity, Con’t 3. Radioactive rays cause certain chemicals to give off a flash of light when they strike the chemical.  A scintillation counter is able to count the number of flashes per minute  Able to measure alpha and beta particles only

19 Natural Radioactivity  There are small amounts of radioactive minerals in the air, ground, and water.  It’s even in the food you eat!  The radiation you are exposed to from natural sources is called background radiation.

20 Half-Life  Each radioactive isotope decays at a unique rate.  Some fast, some slow.  Not all the atoms of an isotope change simultaneously.  Rate is a measure of how many of them change in a given period of time.  Measured in counts per minute, or grams per time.  The length of time it takes for half of the parent nuclides in a sample to undergo radioactive decay is called the half-life.

21 Half-Lives of Various Nuclides NuclideHalf-lifeType of decay Th x yrAlpha U x 10 9 yrAlpha C yrBeta Rn secAlpha Th x 10 –6 secAlpha

22 Half-Life  Half of the radioactive atoms decay each half-life.

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24 How Long Is the Half-Life of this Radionuclide?

25 Practice—Radon-222 Is a Gas that Is Suspected of Causing Lung Cancer as It Leaks into Houses. It Is Produced by Uranium Decay. Assuming No Loss or Gain from Leakage, if There Is 1024 g of Rn-222 in the House Today, How Much Will There be in 5.4 Weeks? (Rn-222 Half-Life Is 3.8 Days.)

26 Practice—Radon-222 Is a Gas that Is Suspected of Causing Lung Cancer as It Leaks into Houses. It Is Produced by Uranium Decay. Assuming No Loss or Gain from Leakage, if There Is 1024 g of Rn-222 in the House Today, How Much Will There be in 5.4 Weeks? ( Rn-222 Half-Life Is 3.8 Days.), Continued Amount of Rn-222 Number of Half-lives Time (days) 1024 g g g g g g weeks x 7 days/wk = 37.8  38 days Amount of Rn-222 Number of Half-lives Time (days) 16 g g g g g1038

27 Practice — How Much of a Radioactive Isotope, Rn-222 (with Half-Life of 10 Minutes) Did You Start with if, After One Hour if You Have 2 g?

28 Practice—How Much of a Radioactive Isotope, Rn- 222(with Half-Life of 10 Minutes) Did You Start with if, After One Hour if You Have 2 g?, Continued Amount of Rn-222 Number of half-lives Time (min) 128 g00 64 g g g330 8 g440 4 g550 2 g660 Fill in the “Number of half-lives” and “Time…” columns first, then work backwards up the “Amount…” column.

29 Nonradioactive Nuclear Changes  A few nuclei are so unstable, that if their nuclei are hit just right by a neutron, the large nucleus splits into two smaller nuclei. This is called fission.  Small nuclei can be accelerated to such a degree that they overcome their charge repulsion and smash together to make a larger nucleus. This is called fusion.  Both fission and fusion release enormous amounts of energy.  Fusion releases more energy per gram than fission.

30 Fission + energy!!

31 Fission Chain Reaction  A Fission Chain Reaction is the process by which neutrons from one reaction cause the fission process to keep continuing  Only small number of neutrons needed  Many of the neutrons produced in the fission are either ejected from the uranium before they hit another U-235 or are absorbed by the surrounding U-238.  Minimum amount of fissionable isotope needed to sustain the chain reaction is called the critical mass.

32 Fission Chain Reaction, Con’t

33 Fusion H1H 3 1H1H 4 2 He 1 0n0n deuterium + tritiumhelium-4 + neutron