Chapter 19 (pg 666) Nuclear Chemistry

C 12 6 A Review of Atomic Terms nucleons – particles found in the nucleus of an atom neutrons protons atomic number (Z) – number of protons in the nucleus mass number (A) – sum of the number of protons and neutrons isotopes – atoms with identical atomic numbers but different mass numbers nuclide – each unique atom type 12 C 6

How many protons, neutrons, and electrons are present in each nuclide below. Nuclide protons neutrons electrons 60Co 31P 27 33 27 27 60 15 16 15 15

23.1

Radioactive Decay radioisotope – nuclide which spontaneously decomposes forming a different nucleus and producing one or more types of radiation nuclear equation – shows the nuclear changes that occur during a nuclear reaction (transmutation). These are balanced to conserve mass number and atomic number. Elements do not need to be conserved like chemical equations.

Radioactive Decay Types of Radioactive Decay Alpha decay Alpha particle – helium nucleus (2 protons and 2 neutrons) Examples

Alpha particles Largest of common radiation types Positive charge (+2) Very dangerous to living tissue Very low penetrating ability – easy to shield

Radioactive Decay Types of Radioactive Decay Beta decay Beta particle – electron the electron is created in the nucleus when a neutron splits into a proton and an electron Examples

Beta particles Smaller and faster than alpha particles Negative charge (-1) Very dangerous to living tissue Higher penetrating ability – harder to shield

Radioactive Decay Types of Radioactive Decay Gamma decay Gamma ray – high energy photon Examples

Gamma rays Not a particle, just a packet of energy (photon) No charge Dangerous to living tissue Very high penetrating ability – difficult to shield

Radioactive Decay Types of Radioactive Decay Positron production (sometimes called beta positve decay) Positron – particle with same mass as an electron but with a positive charge Examples

Radioactive Decay Types of Radioactive Decay Electron capture (sometimes called K capture) Example

Q. Why do some nuclei decay? A. Think about the fundamental forces involved. - Gravity = all matter seems to have an attractive force for all other matter - Electrostatic = opposite charges attract, like charges repel - Nuclear forces = at very, very small distances sub-atomic particles like protons and neutrons exhibit a very strong attraction. In every nucleus there will be opposition between the attractive nuclear forces (plus a little bit of gravity) and the repulsive forces of the like charges of protons.

positron decay or electron capture Y n/p too large beta decay X As the nuclei get larger, a greater proportion of the particles must be neutrons in order for it to be stable. n/p too small positron decay or electron capture Y 23.2

Radioactive Decay Decay series Radioactive particles go through a series of decays until they become a stable isotope.

Nuclear Transformations Nuclear transmutations – change of one element to another Some are spontaneous decays Some can be initiated by highly energetic collisions between particles. – Examples

Nuclear Transformations Transuranium elements – elements with atomic numbers greater than 92 have been synthesized, because they cannot be found in nature.

A positron emission for Pb-206 Write equations for these nuclear reactions: An alpha decay for Ni-60. A beta decay for 251Cf A positron emission for Pb-206 An electron capture by 189Os Ni  60 28 He 4 2 56 26 Fe + e -1 251 99 Es Cf  251 98 + Pb  206 82 e +1 206 81 Tl + Os + 189 76 e -1 189 75 Re 

Nuclear Applications

Geiger-Müller Counter Detecting ionizing radiation: particles and high energy photons will ionize the gas in the tube (knock off an electron). The electron will travel through the counter in order to return to its original atom. Geiger-Müller Counter 23.7

Q. How much radiation does a sample emit? A. It depends on both the amount of radioactive material and the type of nuclides present. amount  the number of decays is proportional to the amount of that nuclide. 40 decays/min 20 decays/min 100 mol sample 50 mol sample type  each nuclide will have a characteristic decay (α,β,γ, etc.) and a characteristic rate. A convenient way to describe the rate is to use a half-life, the time required for half of the sample to decay. For a specific nuclide, this value is the same regardless of the sample size.

Half Life Examples Br-84 has a half-life of 32 min. If you start with 12 moles of 84Br, after 32 minutes you will only have 6.0 moles left. After another 32 minutes you will have 3.0 moles left. Every 32 minutes half of the 84Br nuclides will decay. How much will be left after another 32 minutes? 1.5 moles

Half-Life Problems When you are setting-up a half life problem, ask yourself which of these pieces of information you know: The starting amount of the nuclide. The amount of nuclide that remains. The amount of time that has passed. The half-life of the nuclide.

Half Life Problems 1. If a sample begins with 640 nuclides, how many will remain after 3 half-lives? 640 320 160 80 2. If you begin with 16 moles of a radioactive nuclide, how many half-lives must pass before only 1 mole remains? 1 2 3 4 16 8 4 2 1

Half Life Problems 3. A nuclide has a half-life of 30 minutes. What percentage of the sample will be left after 1 hour? 1 hour = 60 minutes 60 min ÷ 30 min = 2 half lives will pass 100% 50% 25% 4. A radioisotope has a half-life of 15 minutes. How long will it take for 97% of the sample to decay? 97% decayed means 3% is left 100% 50% 25% 12.5% 6.25% 3.1% 5 half lives x 15 min = 75 min

N = N0(1/2) Half Life Problems What if the problem doesn’t involve a whole number of half lives? total time this is also the # of half lives N = N0(1/2) T t half-life amt left amt start 1) If you start with 100. moles of C-14, what amount is left after 3.4 half lives? N = 100(1/2)3.4 N = 9.5 moles left

2) If you start with 25. 0 g of Ca-47 which has a half life of 4 2) If you start with 25.0 g of Ca-47 which has a half life of 4.53 days, how much is left after one week? 7 days 1 week = 7 days = 1.55 half lives 4.53 days N = 25.0 g(1/2)1.55 N = 8.54 g left

N = N0(1/2) 6.25 = 10.0(1/2) 0.625 = (1/2)12.0/T 0.678 = T = 17.7 hrs 3) What is the half-life of a nuclide if a sample initially contains 10.0 mg and only 6.25 mg remains after 12.0 hrs? N = N0(1/2) T t 6.25 = 10.0(1/2) T 12.0 hrs 10.0 10.0 0.625 = (1/2)12.0/T log log log 0.625 = (12.0/T) log(1/2) 0.678 = 12.0 hrs T T = 17.7 hrs

Radiocarbon dating Suggested by Willard Libby in 1949 at U. of Chicago Based on the radioactivity of carbon-14 Used to date plants or artifacts made from living components Plants absorb a small amount of radiocarbon from the atmosphere during photosynthesis. The level of 14C begins to diminish when the organism dies The half-life of 14C is 5730 years

4) You find a woolly mammoth in the permafrost layer of your backyard when vacationing at your summer cabin in the Yukon. Radiocarbon dating shows that it has only 25% of the C-14 expected in a living mammoth. How old is your mammoth? 100%  50%  25% it is 2 half lives old 2 x 5730 = 11460  about 11,000 years old

Nuclear binding energy (BE) is the energy required to break up a nucleus into its component protons and neutrons. BE + 19F 91p + 101n 9 1 Mass defect (∆m) = 9 x (p mass) + 10 x (n mass) – 19F mass ∆m (amu) = 9 x 1.007825 + 10 x 1.008665 – 18.9984 ∆m = 0.1587 amu = 2.636x10-28 kg When this atom forms, a tiny amount of matter is converted to energy E = mc2 Energy = mass(kg) x [speed of light ]2 Speed of light = c = 300,000,000 m/sec BE = 2.636x10-28 kg x (3x108 m/sec)2 = 2.37 x 10-11J This may look small, but it means that when 19 g of flourine forms there is 14,000,000,000 kJ of energy released. That’s the same as burning 285,000 tons of coal! 23.2

Nuclear binding energy per nucleon vs Mass number large nuclides decay or fission to form more stable nuclides small nuclides can fuse to form more stable nuclides nuclear binding energy nucleon nuclear stability

Nuclear Fission 235U + 1n 90Sr + 143Xe + 31n + Energy 92 54 38 Energy = [mass 235U + mass n – (mass 90Sr + mass 143Xe + 3 x mass n )] x c2 Energy = 3.3 x 10-11J per 235U = 2.0 x 1013 J per mole 235U 235 g of uranium can produce as much heat as 400,000 tons of coal! 23.5

Nuclear Fission Nuclear chain reaction is a self-sustaining sequence of nuclear fission reactions. The minimum mass of fissionable material required to generate a self-sustaining nuclear chain reaction is the critical mass. Non-critical Critical 23.5

No radiation or waste products are released during operation. Schematic Diagram of a Nuclear Reactor A nuclear reactor uses a controlled fission chain reaction to boil water. The steam then turns a turbine which generates the electricity. No radiation or waste products are released during operation. Control rods absorb neutrons between the fuel rods to control the rate of the reaction. 23.5

Annual Waste Production Which source of electricity is better for the environment? 35,000 tons SO2 4.5 x 106 tons CO2 1,000 MW coal-fired power plant 3.5 x 106 ft3 ash Nuclear Power Coal Power 1,000 MW nuclear power plant only about one ton of vitrified solid waste

Nuclear Fission Unfortunately, the small amount of nuclear waste is very hazardous It takes millions of years for this waste to decay back to the radiation levels of the original ore from which the fuel was isolated. Where is it safe to store this waste? 23.5 From “Science, Society and America’s Nuclear Waste,” DOE/RW-0361 TG

Tokamak magnetic plasma confinement Nuclear Fusion The sun fuses small elements into larger elements. The binding energy changes dictate that fusion is only favorable up to the size of iron atoms. We do not currently have a way to create fusion reactors that are practical for generating electricity. Tokamak magnetic plasma confinement 23.6

Radioisotopes in Medicine 1 out of every 3 hospital patients will undergo a nuclear medicine procedure 24Na, t½ = 14.8 hr, b emitter, blood-flow tracer 131I, t½ = 14.8 hr, b emitter, thyroid gland activity 123I, t½ = 13.3 hr, g-ray emitter, brain imaging 18F, t½ = 1.8 hr, b+ emitter, positron emission tomography 99mTc, t½ = 6 hr, g-ray emitter, imaging agent Brain images with 123I-labeled compound 23.7

Radioisotopes in Medicine 99mTc 99Tc + g-ray 43 t½ = 6 hours Bone Scan with 99mTc Small amounts of radio nuclides are taken up by tissues and the radiation they emit can greatly enhance our resolution of the tissue. risk – there is a small risk because the radiation can damage tissue benefit – we can gather diagnostic data without cutting people open 23.7

Biological Damage from Radiation Exposure Several factors affect the amount of damage a person experiences when they get exposed to radiation. 1) damage varies based on the type of radiation ex. alpha is 20x more damaging than gamma 2) damage varies based on the type of tissue ex. rapidly dividing cells (like skin, gut, etc) are more sensitive than slower growing cells (brain) 3) time makes a difference ex. one large dose is much more harmful than an equal amt split into many small exposures (think ‘sunburn’)

Biological Damage from Radiation Exposure The SI unit for radiation exposure is the gray (Gy). It is equal to one joule of ionizing radiation absorbed by one kg of tissue. A single whole body exposure above 5 Gy is usually fatal. When radiation is used as treatment, it is targeted and a single dose is usually below 2 Gy. An abdominal X-ray is only about 1.4 mGy (or 0.0014 Gy)

Chemistry In Action: Food Irradiation Gamma radiation can be used to help preserve food and prevent food poisoning. The food does not become radioactive. Low dose applications (up to 1 kGy) Sprout inhibition in bulbs and tubers 0.03-0.15 kGy Delay in fruit ripening 0.25-0.75 kGy Insect disinfestation including quarantine treatment and elimination of food borne parasites 0.07-1.00 kGy Medium dose applications (1 kGy to 10 kGy) Reduces spoilage microbes to prolong shelf-life of meat, poultry and seafoods under refrigeration 1.50–3.00 kGy Reduces pathogenic microbes in fresh and frozen meat, poultry and seafoods 3.00–7.00 kGy Reduces microorganisms in spices to improve hygienic quality 10.00 kGy High dose applications (above 10 kGy) Sterilization of packaged meat, poultry, and their products that stable without refrigeration 25.00-70.00 kGy Sterilization of Hospital diets 25.00-70.00 kGy Product improvement as increased juice yield or improved re-hydration