4 IsotopesAtoms of a single element have the same number of protons but may differ in neutrons.Example 1: Carbon-12 vs Carbon-14Example 2: Uranium-238 vs Uranium-235Some isotopes are stable while others are unstable and radioactive.The STRONG NUCLEAR FORCE acts between protons & neutrons to hold them together. However protons will repel each other with their mutual positive charge.
5 Carbon Isotopes Isotope Half – life Carbon – 9 0.1265 s Carbon – 10 20.38 minCarbon – 12StableCarbon – 13Carbon – 145715 yCarbon – 152.449 sCarbon – 160.75 sHow long does it take 400 g of each isotope to decay to less than 1 mg?
6 Geiger Counter Demos Sample Counts per Minute Reason Humans NaCl vs KClSmoke DetectorOld Fashioned Lantern MantleOld Glow in the Dark ClockUranium Ore
7 Becquerel/CurriesBecquerel – First observed photographic evidence of radioactivityCurie – Discovered radioactive elements of radium and poloniumCurie – made gift of 1 gram of Radium to President Truman to help fight cancer.
8 Rutherford’s Discovery of Alpha, Beta & Gamma Radiation
9 Rutherford & Radiation Discovered alpha, beta & gamma radiation using a magnetic field.
10 Blocked by 1ft of concrete or few inches of lead RadioactivityTypes ofRadiationAlphaBetaGammaSymbola (He)b (e-)gMass4 amu1/1837 amu0 amuCharge+2-10 (movie)Composition2 protons,2 neutrons1 electronHigh energy photonPenetrationBlocked by paperSheet metalBlocked by 1ft of concrete or few inches of lead
11 Radioactivity Alpha – large Beta – much smaller Gamma – no mass Relatively slowBeta – much smallerRelative fastGamma – no massPure energyTravels at theSpeed of light
12 Alpha RadiationThe unstable nucleus simultaneously ejects two neutrons and two protons, which correspond to a helium nucleus. The emission of gamma photons is a secondary reaction that occurs a few thousandths of a second after the disintegration.
14 Beta Emission The unstable nucleus ejects an electron. The emission of gamma photons is a secondary reaction that occurs a few thousandths of a second after the disintegration
15 Beta EmissionA neutron disintegrates, ejects an electron and another proton is left over. A gamma ray is also released.14C6e-114N7g++
16 Gamma Radiation involves high energy waves that don’t change the identity of the nucleus
17 Nuclear Reactions Radioactivity results from changes in atomic nuclei. Fission – splitting of a large nucleus into smaller pieces releases energy.Fusion – small nuclei join to make a larger nucleus and release energy.Energy is released when a small amount of mass converts to energy as E = mc2.The atomic masses of isotopes deviate from whole numbers due to loss of mass from energy released as they fused.
18 Fusion of Hydrogen Isotopes At high temperatures and pressures, 2 nuclei may collide and form a bigger nucleus.This example produces helium and a stray neutron.Stars are fueled by the energy released by fusion which also builds atoms of increasing sizes in their cores.
19 Fission of Uranium A neutron splits the nucleus. The fragments include:2 different smaller atoms,3 more neutrons.The 3 neutrons can split more atoms.If every fission splits 3 more atoms, the reaction will multiply out of control!
27 Nuclear Equations Uranium 238 does alpha decay: Mass numbers balance on both sides.Atomic numbers balance on both sidesThorium 234 then does beta decay:238U924He2234Th90a+234Th90e-1234Pa91b+
28 Nuclear Equations Problems U–238 does alpha decay in nuclear reactors.Am-241 does alpha decay in smoke alarms.Tc-98 does beta decay in medical exams.C–14 does beta decay in carbon dating.The Curies used Ra-226 which does alpha decay.Co–60 does beta decay in food irradiation.Th-232 does alpha decay in camp lanterns.P-35 does beta decay in DNA studies(Place isotopes activity in Outbox)
29 Nuclear Equations Quiz Write the nuclear equation for the alpha decay of Iodine 131.Write the nuclear equation for the beta decay of cobalt 60.
30 ½ Lives Activity Roll (minutes) Remaining Atoms Class Average Obtain a set of “radioactive” blocks. Notice that each one has a mark on one side – either a, b or g.Roll the collection of blocks onto your table. Each time you roll, remove any blocks that come up a, b or g.Count and record the remaining blocks. Roll the remaining blocks repeatedly 20 times and complete the chart below.Enter your group data into the excel file.Make graphs of Time(minutes) Remaining Atoms for both individual & class averages. **Use “exponential” rather than “linear” trendlines.Roll(minutes)Remaining AtomsClass Average
31 ½ Lives Activity Questions How do your lab pair results compare with the class average results?Use the class average results and compute the 1st ½ life, 2nd ½ life, average ½ life.What importance do ½ lives have to society?(dating, medical uses, wastes)
32 ½ Lives Each radio-isotope decays at a characteristic rate. The decay rate is determined by the time that it takes for ½ of the radio-isotope nuclei to break down by fission.Each ½ life reduces the remaining number of radioactive atoms by ½.The number remaining approaches but never reaches zero.Example: Iodine 131 has a ½ life of 8 days. How much of 1.00 gram sample would remain after 24 days?
33 ½ Lives of Isotopes Isotope Half – life Radiation emitted Carbon – 14 5715 yBetaPotassium – 401.3e9 yBeta, gammaRadon – 2223.8 dAlpha, gammaRadium – 2261600 yIodine – 1318.07 dThorium – 23424 dUranium – 2357.0e8 yUranium – 2384.5e9 y
34 Solving ½ Life Problems Masses:STARTING MASSDivided in ½ the # of half lives.ending massTimes:Time for 1 half life (HL)Total time elapsed (T)T = HL*(#)HL = T/## = T/HL# of halflives
35 ½ Life Example 1 – Iodine 131 has a ½ life of 8 days. How much of 1 ½ Life Example 1 – Iodine 131 has a ½ life of 8 days. How much of 1.00 gram sample would remain after 24 days?Times: ½ life = 8 days Total = 24 days 24days / 8days = 3 half livesMasses: Start = 1.00 g End = ?unknown If 3 half lives occur, divide start by 2 3-times 1.00 g / 2 / 2 / 2 = .125 g
36 ½ Life Example 2: A 8.8mg sample of chromium-55 is analyzed after 15 min and found to contain 1.1mg remaining. What is the ½ life of Cr55?Masses: Start = 8.8 mg End = 1.1mg Divide 4.4 by 2 until reaching /2 = /2 = /2 = – ½ lives occurredTimes: Total = 15 min ½ life = ? Unknown Divide 15 min by 3 – ½ lives 15min/3 HL = 5 min/1HL 5 min = 1 – ½ life
37 ½ Life ProblemsIf you have $1 million dollars and every 2 seconds it decreases by 1/2, how long will it take until you are penniless?If a sample of a fossil mammoth has 1/8th the amount of carbon 14 as it would today, how old must the fossil be? (1/2L C14 = 5715 years.If a rock contained 1.2 g of potassium 40 when it formed, how many grams remain after 4 billion years. (1/2L K40 = 1.3e9 y)Asmt: P780 #1&2, P803 #24&25
38 More ½ Life Problems4. If a sample of radioactive isotope has a half-life of 1 year, how much of the original sample will be left at the end of the second year? The third year? The fourth year?5. The isotope cesium-137, which has a half-life of 30 years, is a product of nuclear power plants. How long will it take for this isotope to decay to about one-sixteenth its original amount?Iodine-131 has a half-life of 8 days. What fraction of the original sample would remain at the end of 32 days?The half-life of chromium-51 is 28 days. If the sample contained 510 grams, how much chromium would remain after 56 days? How much would remain after 1 year?
39 ½ Lives Quiz1. A sample of a radioactive isotope with an original mass of 8.00g is observed for 30 days. After that time, 0.25g of the isotope remains. What is its half-life?2. The starting mass of a radioactive isotope is 20.0g. The half-life period of this isotope is 2 days. The sample is observed for 14 days. What PERCENTAGE of the original amount remains after 14 days?
40 Health Physics Society Q:What are some health effects of the element uranium?A:The toxicity of uranium has been under study for over 50 years, including life-span studies in small animals. Depleted uranium and natural uranium both consist primarily of the uranium isotope 238U. They are only very weakly radioactive and are not hazardous radioactive toxicants, but uranium is a weak chemical poison that can seriously damage the kidneys at high blood concentrations. Virtually all of the observed or expected effects are from nephrotoxicity associated with deposition in the kidney tubules and glomeruli damage at high blood concentrations of uranium. The ionizing radiation doses from depleted and natural uranium are very small compared to potential toxic effects from uranium ions in the body (primarily damage to kidney tubules).