Nuclear Chemistry Unit 15

I. Nuclear Reactions  A. Involve a change in the nucleus of the atom  1. made of protons and neutrons (called nucleons together)  2. atoms referred to as nuclides, identified by number of protons and neutrons carbon – 14 or nuclear symbol carbon – 14 or nuclear symbol (on board)  

B. Stability  1. For low atomic numbers the ratio of protons:neutrons should be 1:1, for high atomic numbers, the ratio should be 1:1.5 (neutrons help hold it together)  2. Even number of nucleons usually stable.  3. Others unstable - can undergo transmutations - change in nucleus identity because protons change (converts one element to another)

C. Radiation produced in nuclear change  Capture: taken in, Emission/decay: given off)  1. alpha particle – helium nucleus, α, 4 2 He  2. beta particle – electron, β (on right), 0 –1 e  3. positron – like electron but positive, 0 +1 e  4. electron capture (K capture) – electron, on left, 0 –1 e on left, 0 –1 e  5. neutron n  6. gamma rays – energy, 0 0 

D. Writing and Balancing Nuclear Equations  Mass must be equal on both sides  Therefore, the top numbers must be add to the same number on both sides  Charge must be equal on both sides  Therefore, the bottom numbers must add to the same number on both sides  Find the missing numbers  Identify the particle or element (look on the periodic table for the bottom number)

Nuclear Equations Examples: Q. Write the nuclear equation for C-14(  ) C 14 6  e 0 N Po  He 4 2 Pb  ) Q. Write the nuclear equation for 209 Po(  ) Q. Complete this fission reaction: Kr U  n 1 0 Ba n

Half-Life Half-life is the time is takes a sample to decay to half its original amount. Half-Life Demonstration from GCSE Examples: (on board) 1.What fraction will remain after 15 days if the half life is 36 hours? 2.How many g of a 100 g sample will remain after 4 half-lives?

Radioactive Dating  N-14 converts to C-14 in the upper atmosphere. There has been a constant rate of production in the past 75 years.  C-14 is part of the CO 2 ingested by plants and animals. When something dies, it stops ingesting and the C-14 starts to decay. The half-life is 5715 years.  Organic materials can be dated back to 50,000 years. Carbon dating agrees with tree ring data within 10%.  Minerals and rocks can be dated back to as many as 4 billion years.

Other Applications  2. Radioactive tracers: help doctors follow the transport of materials through the body. Help diagnose cancer and other diseases.  3. Agriculture: determine how much fertilizer is absorbed by plants, increase shelf-life of food (irradiation kills bacteria and insects)  4. Cancer treatment 

III. Biological Effects of Radiation  A. Everyone is subject to background radiation. About.1 rem per year. Other sources include cosmic rays, medical/dental x-rays/other tests.  B. Damage to nucleic acids (DNA/RNA) is worst type.  C. Radiation affects cells that are undergoing cell division the most. Most at risk:  1. Children (especially developing fetuses)  2. Cancer cells (cancer is just uncontrolled cell division)

D. Penetrating abilities  1. Alpha: least penetrating - only travels a few cm, stopped by paper or skin, most damaging - can cause damage if ingested or inhaled  2. Beta: medium penetration and damage - travel close to speed of light, 100 times more penetrating than alpha, stopped by thin sheet of metal (like aluminum)  3. Gamma: most penetrating, least damaging - stopped by thick sheet of lead &/or concrete

Damage can be affected by:  Time – how long you are exposed  Shielding – radiation absorbing material material  Distance – how far away from source  Amount of radiation (usually can’t control this, except with medical imaging like x-rays)

Nuclear Fission  Chemical reactions involve outer electrons  Nuclear reactions involve changes in nuclei  Nuclear Fission – breaking apart of heavy nuclei into two parts A.Some happens spontaneously - radioactive decay series like uranium-235 B.Others need bombardment by neutrons 1.Can start a chain reaction - produces neutrons which can also start reactions 2.Must reach critical mass before it can be sustained a.uncontrolled - explosion b.controlled - nuclear reactor to create energy or radioactive nuclides for medical or agricultural use

Nuclear Reactor

Nuclear Reactors (Fission only)  All are fission reactors - fusion reactors require more energy than they give back (future??)  Shielding: radiation-absorbing material  Fuel rods contain enriched uranium pellets  Control Rods (boron, cadmium, gadolinium) – absorbs neutrons to slow or shut down reaction (alternate with fuel rods )  Moderators – slow down neutrons to sustain reaction  Coolant – water is most common, energy gained can power generator can power generator  Small amount of waste produced  Compare nuclear vs. coal power plants

Nuclear Fusion  Joining of two light nuclei into a heavier, more stable nucleus  A. Powers the sun and stars - hydrogen joins to form helium  B. Hydrogen Bomb  1. Uncontrolled fusion  2. Fission gives heat and pressure to cause fusion to happen

Fusion Reactors (Don’t exist yet)  FUTURE?? – none now  1. Advantages: a. fuel is cheap and available (use seawater) b. little or no radioactive waste  2. Major Problem: very high activation energy (high temps needed for sustained output) not energy efficient yet