Chapter 2 Nuclear Chemistry: the study of nuclear reactions Learning goals and key skills: Write balanced nuclear equations Know the difference between fission and fusion Predict nuclear stability in terms of neutron-to-proton ratio Calculate ages of objects or amounts of materials from data on nuclear abundances using the half-life of a radioactive material Convert between nuclear activity units Calculate mass and energy changes for nuclear reactions Understand the meaning of radiation dosage terms Understand the biological effects of different kinds of radiation The nucleus The nucleus is comprised of the two nucleons: protons, p + neutrons, n Radionuclides are nuclei that are radioactive i.e., they will spontaneously emit radiation. Atoms containing these nuclei are called radioisotopes.
Atomic and Mass Numbers Mass number Atomic number (optional) 2 C 6 Symbol Atomic number: equal to the number of protons in the nucleus. All atoms of the same element have the same number of protons. Mass number: equal to the sum of the number of protons and neutrons for an atom. Isotopes atoms with identical atomic numbers but different mass numbers 2 C 6 4 C 6 carbon-2 carbon-4 2 3 ydrogen or protium deuterium, D tritium, T 238 92 U abundance: 99.27% Isotopes half-life 4.47 billion years 235 92 U.72% 7 million years 234 92 U.55% 246 years It is not uncommon for some nuclides (radionuclides) of an element to be unstable, or radioactive. There are several ways radionuclides can decay into a different nuclide. 2
a, b and g Radiation b-ray igh speed electron: charge= -, mass= 9. -28 g a-ray e core: charge= +2, mass= 7295 mass of electron g-ray Electromagnetic Radiation: no charge, no mass a, b and g Radiation a-ray b-ray g-ray 4 4 symbol a or e b or e g or 2 2 - - g charge 2+ - mass 6.65-24 g 9. -28 g g 4.5 amu.549 amu penetrating paper.5 cm Pb cm Pb ability Types of Radioactive Emissions 3
Other common nuclear particles neutron proton positron symbol n p + b or or + + b e charge + + mass.675-24 g.673-24 g 9. -28 g.867 amu.728 amu.549 amu Alpha decay Nuclear reactions Balance using conservation principles, baryon number conservation. Radium-226 emits an alpha particle. Beta decay 226 88 Ra An unstable neutron in the nucleus will emit a beta particle. n b + p - Carbon-4 undergoes beta decay. 4 6 C 4 2 - b a + + 4 7 222 86 N Rn Positron emission Nuclear reactions A neutron-poor nuclei will undergo positron emissions. p b + n + Carbon- undergoes positron emission. 6 C + e + 5 B 4
Nuclear reactions Electron capture (K-capture) A neutron-poor nucleus can decay by positron emission or electron capture. p + - e n Iron-55 decays by electron capture. Nuclear reactions Gamma emission An excited nuclear state emits gamma radiation. (Generally not shown when writing nuclear equations.) Uranium-238 undergoes alpha decay and then gamma emission. Technetium-99 is metastable and undergoes gamma emission. (alf-life is 6 hours.) 5
Belt of stability shown. Only and 3 e have more p + than n. Up to Z = 2 (Ca), n /p +. Above Z = 2, n /p + >. After Z = 83 (Bi), all isotopes are unstable. For unstable nuclei: the heavier, the shorter the half-life. the further from the line, the shorter the half-life. Nuclear stability: EVEN more stable than ODD Nuclei with 2, 8, 2, 28, 5, 82, 26, or 84 p + or 2, 8, 2, 28, 5, 82, 26, or 84 n are generally more stable than other nuclei. Predicting nuclear decay too high N: b emission too high N and Z ( 84): a emission too high Z : electron capture too high Z: b + emission 6
Radioactive/nuclear disintegration series Large radioactive nuclei cannot stabilize by undergoing only one nuclear transformation. They undergo a series of decays until they form a stable nuclide (often a nuclide of lead). Example The radioactive series beginning with uranium-238 (Z = 92) terminates with lead-26 (Z = 82). ow many alpha decays occur, and how many beta decays occur? Measuring Radioactivity Geiger counters are used to measure the amount of activity present in a radioactive sample. The ionizing radiation creates ions, which conduct a current that is detected by the instrument. 7
Nuclear kinetics: st order N ln t = - kt.693 N = t k /2 Kinetics of Radioactive Decay: Radiometric Dating A wooden object from an archeological site is subjected to radiocarbon dating. The activity of the sample that is due to 4 C is measured to be.6 disintegrations per second. The activity of a carbon sample of equal mass from fresh wood is 5.2 disintegrations per second. The halflife of 4 C is 575 yr. What is the age of the archeological sample? Measuring Radioactivity activity: rate at which a sample decays, disintegrations per unit time, typically measured in dps (disintegrations per second) becquerel (Bq): SI unit for activity. Bq = dps curie (Ci): rate of decay of g of radium. Ci = 3.7 dps 8
Example A. mg sample of uranium-238 decays at a rate of 2 alpha emissions per second = 2 dps. Find the half-life of uranium-238. Nuclear transformations A nucleus can be transformed when it is struck by a neutron or another nucleus. This type of reaction is called a nuclear transmutation. 4 7 N 4 7 + + 2 e target bombarding ejected product nucleus particle particle nucleus 8 O Nuclear Transformations Nuclear transformations can be induced by accelerating a particle and colliding it with the nuclide. 9
Nuclear binding energy The mass difference between a nucleus and its constituent nucleons is called the mass defect. We can use Einstein s equation to find the nuclear binding energy: the energy required to separate a nucleus into its individual nucleons. E = mc 2 ev =.62-9 J c = 2.99792458 8 m s - Example Calculate the nuclear binding energy for deuterium. + n 2.7825 amu.8665 amu 2.4 amu The mass defect is.239 amu. Note: Avogadro Constant = 6.2242 23 mol - Example Calculate the nuclear binding energy per mole for deuterium. + n 2.7825 amu.8665 amu 2.4 amu The mass defect is.239 amu.
Example Calculate the nuclear binding energy per nucleon for deuterium. + n 2.7825 amu.8665 amu 2.4 amu The mass defect is.239 amu. Nuclear binding energies Nuclear Fission Fermi s proposed transuranium synthesis 238 92 U 239 239 + n U X + 92 93 - b Meitner, Strassman, ahn discovered U-235 did something but didn t make a new element... 235 92 U 4 92 + n Ba + Kr + 56 32 3 n + E
Nuclear chain reaction Bombardment of the radioactive nuclide with a neutron starts the process. More neutrons are produced from the transmutation. A critical mass of radioactive nuclides is needed for a self-sustaining chain reaction. Nuclear reactors 2
Nuclear reactors The reaction is kept in check by the use of control rods made of boron carbide, Ag, In, Cd, or f. These block the paths of some neutrons, keeping the reactor core from overheating. Nuclear fusion potentially superior method of generating power products are radioactive several million K needed (~4,, K) A tokamak fusion test reactor has only been able to get to 3-,, K still too low. Origin of the elements 3
yellow stars red giants Origin of the elements fusion e produces 4 e fusion C, O, Ne, Mg red supergiants 4 e + 2 C Na, Si, S, Ar, Ca 2 C + 2 C Fe, Ni 2 C + 6 O supernovae, neutron stars 56 Fe + n Z > 28 Radiation doses gray (Gy): SI unit of absorbed dose Gy = absorption of J/kg tissue rad: radiation absorbed dose rad = absorption of. J/kg tissue Gy = rads RBE: relative biological effectiveness RBE (b and g) = RBE (a) = Radiation doses rem = (# rads) (RBE) roentgen equivalent for man SI unit for dosage is sievert (Sv). Sv = rem msv =. rem 4
Average individual background radiation dose:.34 μsv/h or 3. msv/year for Americans Dental radiography:.5 msv Average dose to people living within 6 km of Three Mile Island accident:.8 msv during the accident Mammogram: 3 msv Brain CT scan:.8 5 msv Chest CT scan: 6 8 msv Gastrointestinal series X-ray investigation: 4 msv Current average limit for nuclear workers: 2 msv per year Dose from smoking 3 cigarettes a day: 3-6 msv/year (.4 picocuries of polonium 2) Criterion for relocation after Chernobyl: 35 msv/lifetime Typical dose near Chernobyl reactor 4 and its fragments, shortly after explosion: 3 msv /hour March 6, 2, a radiation level of 3 msv/h was recorded between the exteriors of the secondary containment buildings of Unit 2 reactor and Unit 3 reactor of Fukushima Daiichi Nuclear Power Station. In Fukushima Prefecture, the level in the town of Namie (2 km), which peaked at 7 μsv/h at 2 p.m. on March 7, has fallen to 25.2 μsv/h as of April. The accumulated radiation for the March 23 to April 9 period was 3.95 msv. Radiation levels have also fallen to.8 μsv/h (24.24 μsv/h at peak time) in Fukushima city. 5
Medical uses PET positron emission tomography can monitor flow of blood and oxygen uses b+ emitting radiotracers: C (t /2 = 2.4 min), 8 F ( min), 5 O (2 min), 3 N ( min). 6
Other radiotracers BNCT Boron Neutron Capture Therapy B can capture slow neutrons Tumor cells preferentially take up boron compounds B + n B * 7 Li + 4 a 5 5 3 2 Food irradiation Food can be irradiated with g rays from 6 Co or 37 Cs. Irradiated milk has a shelf life of 3 months without refrigeration. USDA has approved irradiation of meats and eggs. 7
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