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Chemistry 106 University of Nebraska SPRING 2009 17 April 2009.

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Presentation on theme: "Chemistry 106 University of Nebraska SPRING 2009 17 April 2009."— Presentation transcript:

1 Chemistry 106 University of Nebraska SPRING April 2009

2 What is the mass defect for iodine 131? Given: 1 H atom mass = amu, neutron mass = amu, 131 I mass = amu **have to determine the number of protons and neutrons (from the periodic table) Iodine 131 has 53 protons so it must have 78 neutrons (131-53=78) Δm=[(53x amu)+(78 x amu)] amu Δm = amu (this is mass defecet) What is the binding energy per nucleon for iodine 131? 1 amu=931.5 x 10 6 eV MeV Binding energy =( amu x MeV/amu)/131 nucleons =8.42 MeV/nucleon

3 KINETICS OF RADIOACTIVE DECAY Chemical and nuclear systems both tend toward maximum stability Radioactive nuclei decay at a characteristic rate as they move toward stability Decay rate (or activity) of a radioactive sample = change in number of ______ divided by the change in time Rate = (ΔN/ Δt)=kt **k=constant t=time Used to find decay rate from nuclear decay measurements Used to find activity remaining at time t. kt= ln(A 0 /A t )

4 HALF-LIFE Decay rates are commonly expressed in terms of the fraction of nuclei that decay over a given time interval. Half-life (t 1/2 ): the time it takes for half of the nuclei present to decay. The number of nuclei remaining is halved after each half-life t 1/2 = (ln2/k) Fraction decayed = A 0 -A t A 0

5 Decrease in the number of 14 C nuclei over time HALF-LIFE

6 The charred bones of a sloth in a cave in Chile represent the earliest evidence of human presence in the southern tip of South America. A sample of the bone has a specific activity of 5.22 disintegrations per minute per gram of carbon (d/min. g). If the ratio of 12 C: 14 C in living organisms results in a specific activity of 15.3 d/min. g, how old are the bones? (t 1/2 of 14 C = 5730 yr) t 1/2 = (ln2/k) **solving for k K =ln2/ t 1/2 = ln2/5730 yrs = 1.21x10 -4 /yr Calculate age of bones – solving for t -kt= (ln(A 0 /A t )) t= (lnA 0 /lnA t )/k = [Ln(15.3 d/minxg)/5.22 d/minxg)]/ 1.21x /yr) = yrs.

7 Strontium-90 is a radioactive by-product of nuclear reactors that behaves biologically like calcium, the element above it in Group 2A(2). When 90 Sr is ingested by mammals, it is found in their milk and eventually in the bones of those drinking the milk. If a sample of 90 Sr has an activity of 1.2x10 12 d/s, what is the activity of the sample after 59 yr (t 1/2 of 90 Sr = 29 yr) and what fraction has decayed (that is how much is left over)? Fraction decayed = A 0 -A t A 0 A 0 = 1.2x10 12 d/s, A t =2.9x10 11 d/s FD= (1.2x10 12 d/s - 2.9x10 11 d/s)/ 1.2x10 12 d/s =.76 or 76% decayed and 24% remaining

8 Radioactivity Units Measurement of activity: Becquerel (Bq): 1 disintegration per second (dps) Curie (Ci): 3.7x10 10 disintegrations per second (dps) **know these number for exam (won’t have to work any problems with them just know the number and what it relates to) Measurement of dose: Radiation absorbed dose (Rad): measures amount of absorbed radiation in terms of energy (J/kg) Radiation equivalent man (Rem): measures actual biological tissue damage from radiation

9 Sources of background radiation Inhaled Radon Medical

10 RADON Element 86 Radon is a radioactive gas Product of Uranium decay (decay series) Radon decays to polonium-218 by alpha emission Polonium is a radioactive solid This is a problem if radon is inhaled and decays in the lungs to form polonium!! Causes lung cancer

11 Applications of Radioactivity 1.Food irradiation -Irradiation using gamma rays from Co-60 or Cs Retards growth of bacteria, molds and yeasts -Fear is that radiation may change the food in some way to cause cancer or other abnormal growths

12 2. Radiocarbon Dating - Plants use carbon dioxide to form sugars via photosynthesis - C-12 and C-14 are used in the same way, chemically speaking - they exist in nature in relative constant percentages or ratios - C-14 is radioactive and undergoes decay - by comparing the ratio in the air to that in a dead plant or fossil we can determine the approximate time the plant lived Applications of Radioactivity

13 2. Radiocarbon Dating - the half life of C-14 is about 5730 years - an object less than 100 years old or more than 40,000 years old (about 7 half lives) cannot be carbon dated with much accuracy Applications of Radioactivity

14 Radiocarbon Dating


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