1 Nuclear Chemistry Chapter 22

2 Nucleons  In nucleus of atom  Protons and neutrons

3 Nuclide  An atom  Identified by the number of protons and neutrons in its nucleus  Example: Sulfur-32  Has mass number of 32  Has 16 protons

4 Mass defect  The difference between the mass of an atom and the sum of the masses of its protons, neutrons, and electrons.  Use isotopic mass to calculate, not average atomic mass.

5 Nuclear binding energy  The energy released when a nucleus forms. Mass is converted to energy (E=mc 2 ) when the nucleus is formed.  Also the energy required to break apart the nucleus Measure of stability

6 Binding energy per nucleon  Binding energy divided by number of nucleons  If high, nucleus is held together tightly

7 Band of stability  Neutron-proton ratio Close to 1:1 for smaller atoms Close to 1.5:1 for larger atoms

8 Stability  Protons repel each other through electrostatic forces  They attract each other through nuclear forces – but only over small distances  More neutrons are needed to increase nuclear force without increasing repulsive forces  Beyond bismuth (83), no stable nuclides exist

9 Nuclear shell model  Nucleons exist in different energy levels, or shells, in the nucleus  Magic numbers – the numbers of nucleons that represent completed nuclear energy levels – 2, 8, 20, 28, 50, 82, and 126 Very stable nuclides

10 Nuclear reaction  Affects the nucleus of an atom  Atoms give off large amounts of energy and increase their stability

11 Transmutation  When a nucleus changes identity as a result in the change in its number of protons It becomes a different element

12 Nuclear equations  The total of the atomic numbers and the total of the mass numbers must be equal on both sides of the equation.  Elements have atomic numbers 1 or greater  Neutrons have atomic numbers of 0  Electrons have atomic numbers of -1

13 Examples

14 Example

15 You try

16 You try

17 You try

18 Radioactive decay  The spontaneous disintegration of a nucleus into a slightly lighter nucleus, accompanied by emission of nuclear radiation (particles, electromagnetic radiation, or both).

19 Radioactive nuclide  Unstable nucleus that undergoes radioactive decay.

20 Alpha emission  Alpha particle (  ) – two protons and two neutrons (a helium nucleus)  Emitted from the nucleus during some kinds of radioactive decay.  Restricted to very heavy nuclei – both protons and neutrons need to be reduced for stability

21 Beta emission  Decreases number of neutrons  A neutron is converted into a proton and an electron.  Beta particle (  ) – an electron emitted from the nucleus during some kinds of decay.

22 Positron emission  Decreases number of protons  A proton is converted into a neutron by emitting a positron – a particle that has the same mass as an electron, but a positive charge

23 Electron capture  Increases number of neutrons  A nucleus captures one of its inner orbital electrons  The electron combines with a proton to form a neutron

24 Gamma emission  Gamma rays (  ) – high-energy electromagnetic waves emitted from a nucleus as it changes from an excited state to a ground energy state  Supports the nuclear shell model  Gamma emission usually occurs immediately after other types of decay

25 Decay series  A series of radioactive nuclides produced by successive radioactive decay until a stable nuclide is reached.  Parent nuclide – the heaviest  Daughter nuclides – produced by the decay of the parent nuclide

26 Artificial transmutation  Bombarding stable nuclei with charged and uncharged particles to create artificial radioactive nuclides  Great quantities of energy are needed Particle accelerator  Used to fill in the gaps in the periodic table and extend the table past uranium Transuranium elements – have more than 92 protons

27 Half-life, t 1/2

28 Half-life  We can’t predict when an individual atom will decay, only the rate of decay for a large number of atoms.  There is a table on page 708.

29 Example  Uranium-238 decays through alpha decay with a half-life of 4.46 x 10 9 years. How long would it take for 7/8 of a sample of uranium-238 to decay?  3 half-lives, or 1.34 x years

30 Example  The half-life of polonium-210 is days. How many milligrams of polonium-210 remain after days if you start with 2.0 mg of the isotope?  0.25 mg

31 Example  The half-life of iodine-131 is days. What percentage of an iodine-131 sample will remain after 40.2 days?  3.12 %

32 Nuclear Radiation  Alpha particles, beta particles (positive or negative), and gamma rays.  Have different penetrating powers

33 Alpha particles  Large mass (4 amu) and charge (+2).  Can’t travel far in air  Low penetrating power Cannot penetrate skin Can be stopped by a sheet of paper  Harmful if ingested or inhaled

34 Beta particles  Travel close to the speed of light  Penetrate about 100 times as much as alphas  Can travel a few meters in air  Can be stopped by lead or glass

35 Gamma rays  Travel at the speed of light  Greatest penetrating ability  Can travel indefinitely through air or empty space  Can only be stopped by thick layers of lead or concrete.

36 Roentgen  Unit used to measure nuclear radiation  The amount of radiation that produces 2 x 10 9 ion pairs when it passes through 1 cm 3 of dry air.

37 rem  Roentgen equivalent man  The quantity of ionizing radiation that does as much damage to human tissue as is done by 1 roentgen of high-voltage X-rays.

38 Radiation exposure damage  DNA mutations Cancer Genetic effects  Can come from direct radiation exposure or by interaction with previously ionized molecules  In the US, average yearly exposure is 0.1 rem.  Up to 0.5 rem is permissible.

39 Radiation detection  Film badges Used by people working with radiation Film is exposed by radiation  Geiger-Müller counters Count electric pulses carried by ionized gas Best for beta particles  Scintillation counters Used when radiation causes materials to emit visible light

40 Radioactive dating  Determining the age of a substance based on the amount of radioactive nuclides present  Carbon-14 is used for organic materials up to years old  Others used for older materials and minerals up to 4 billion years old

41 Radioactive medicine  Used to destroy cancer  Used to detect cancer and other diseases Radioactive tracers

42 Radioactive agriculture  Tracers can be used to determine fertilizer effectiveness  Radiation can be used to extend shelf life by killing bacteria and insects

43 Nuclear waste containment  Waste can have a half life from a few months to thousands of years.  It must be contained to protect living organisms  Can be on-site storage or off-site disposal

44 Nuclear Waste storage  Usually for rods from power plants.  Can be stored in pools of water or dry casks (concrete and steel).  Usually a temporary solution

45 Nuclear waste disposal  Materials are never meant to be retrieved.  Careful planning is needed.  There are currently 77 disposal sites in the US.

46 Nuclear fission  A very heavy nucleus splits into more- stable nuclei  Mass of products is less than mass of reactants Releases enormous amounts of energy

47 Chain reaction  The material that starts the reaction is one of the products and can start another reaction.  Critical mass – minimum amount of nuclide that is needed to sustain a chain reaction

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49 Nuclear reactors  Use controlled fission chain reactions to produce energy or radioactive nuclides.  Uncontrolled fission chain reactions – atomic bombs

50 Nuclear power plants  Use heat from nuclear reactors to produce electrical energy  Main components Shielding Fuel Control rods Moderator Coolant

51 Shielding  Radiation-absorbing materials that decreases amount of gamma rays leaving the reactor.

52 Fuel  What powers the chain reaction  Usually Uranium-235

53 Control rods  Neutron absorbing rods  Control reaction by limiting number of free neutrons

54 Moderator  Used to slow down fast-moving neutrons Fission of uranium is more efficient with slower neutrons

55 coolant  Absorbs heat from the reaction to produce electricity

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57 Nuclear fusion  Light-mass nuclei combine to form a heavier, more stable nucleus  Releases more energy per gram of fuel than fission  Takes place in stars (including the sun) Hydrogen to helium  Uncontrolled reactions – hydrogen bombs

58 Fusion requirements  High heat and pressure needed  Right now, no known material can withstand the initial temperatures (100 million K) needed for controllable fusion.