1 Nuclear Changes Physical Science Chapter 10

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

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

4 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

5 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

6 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.

7 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

8 Examples

9 Example

10 You try

11 You try

12 You try

13 Half-life  The amount of time it takes for half of a radioactive sample to decay  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 333.

14 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

15 Stability  Protons repel each other through electrostatic forces  They attract each other (and also neutrons) through nuclear forces – but only over small distances  More neutrons can increase attractive force without increasing repulsive force  Too many protons or neutrons makes the nucleus too big and unstable  Beyond bismuth (83), no stable nuclei exist

16 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 E=mc 2

17 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

18

19 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

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

Discuss  Section 2 review question #2 on page

Background radiation  Natural radiation everyone is exposed to  We are adapted to survive these low levels of radiation 22

23 rem  How radiation exposure is measured  Up to 0.5 rem per year is considered safe  In the US, average yearly exposure is 0.1 rem.

24 Radiation exposure damage  DNA mutations Cancer Genetic effects

Smoke detectors  Produce current by releasing alpha particles  Smoke particles reduce the current and set off the alarm 25

26 Radioactive medicine  Used to treat cancer  Used to detect cancer and other diseases CT scans MRIs PET scans Radioactive tracers used to locate tumors

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

28 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  Best sites for storage have low populations, little water at the surface or underground, and no earthquakes.

29 Nuclear reactors  Use controlled fission chain reactions to produce energy.  Research is being done to try to build controlled fusion reactors.