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CHAPTER 28 Nuclear Chemistry Radioactive Decay Radioactive Decay

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A. Nuclear Stability Nuclide = atom of an isotope

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A. Nuclear Stability Nuclear stability – stable nuclei always have at least as many neutrons as protons.

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A. Nuclear Stabiity For an odd/even or even/odd nucleus, if the mass number is different by more than 1 amu from the rounded atomic mass, the nuclide is unstable. Ex:

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A. Nuclear Stability For an even/even nucleus, if the mass number is different by more than 3 amu from the rounded atomic mass, the nuclide is unstable. Ex:

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A. Nuclear Stability For odd/odd nuclei, only four stable isotopes are found in nature:

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B. Nuclear Decay Alpha particle ( ) helium nucleus paper 2+ Beta particle ( -) electron 1- lead Positron ( +) positron 1+ Gamma ( ) high-energy photon 0 concrete

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B. Nuclear Decay Alpha Emission parent nuclide daughter nuclide alpha particle Top and bottom numbers must balance!!

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B. Nuclear Decay Beta Emission electron Positron Emission positron

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B. Nuclear Decay Electron Capture electron Gamma Emission Usually follows other types of decay. Transmutation Atom of one element changes into an atom of another element.

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B. Nuclear Decay Why nuclides decay… need stable ratio of neutrons to protons DECAY SERIES TRANSPARENCY

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C. Half-life Half-life (t ½ ) Time required for half the atoms of a radioactive nuclide to decay. Shorter half-life = less stable.

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C. Half-life m f :final mass m i :initial mass n:# of half-lives

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C. Half-life Fluorine-21 has a half-life of 5.0 seconds. If you start with 25 g of fluorine-21, how many grams would remain after 60.0 s? GIVEN: t ½ = 5.0 s m i = 25 g m f = ? total time = 60.0 s n = 60.0s ÷ 5.0s =12 WORK : m f = m i (1/2) n m f = (25 g)(0.5) 12 m f = g

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C. Half-life N t :final mass N 0 :initial mass t:elapsed time

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C. Half-life k:rate constant t 1/2 :half-life

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C. Half-life A sample of radium-223 has a half-life of 11.4 days. What is the rate constant for this isotope? GIVEN: t ½ = 11.4 days WORK : k = / t 1/2 k = / 11.4 days k = days -1

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C. Half-life The rate constant for gold-200 is /year. What is the half-life of gold-200? GIVEN: k = yr -1 WORK : k = / t 1/2 t ½ = / k t ½ = / yr -1 t ½ = 19.8 yr 20. years

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C. Half-life The half-life of iodine 129 is 1.7 x 10 7 years. If a nuclear bomb explosion resulted in 3.75 g of iodine-129, how much time would have to elapse for the amount of iodine-129 to be 0.75 g? GIVEN: t ½ = 1.7 x 10 7 yr N 0 = 3.75 g N t = 0.75 g t = ? WORK : ln (N 0 /N t ) = kt ln (3.75/0.75) = 0.693/1.7 x 10 7 t t = 3.9 x 10 7 years (39, 000, 000 years!)

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D. Radiocarbon Dating Carbon-14 is in all living things through the carbon cycle. Amount of carbon-14 stays constant until organism dies, then it begins to decay.

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D. Radiocarbon Dating Amount of carbon-14 can be expressed as either a percentage or as a decimal number. Example: amount of carbon-14 in a dead tree could be expressed as 38% or 0.38 of the original amount.

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D. Radiocarbon Dating Half-life of carbon-14 : 5730 years

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D. Radiocarbon Dating The remnants of an ancient canoe are found in a cave in northern Australia. The amount of carbon- 14 is 6.28 counts per minute, and the amount of carbon-14 in a tree today is 13.6 counts per minute. What is the approximate age of the canoe?

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D. Radiocarbon Dating GIVEN: t ½ = 5730 yr N 0 = 13.6 cts/min N t = 6.28 cts/min t = ? WORK : k = / 5730 yr k = x yr -1 ln (N 0 /N t ) = kt ln (13.6/6.28) = x t t = 6390 years 2011 – 6390 = 4379 BC

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E. Fission Occurs when isotopes are bombarded with neutrons and split the nucleus into smaller fragments, accompanied by the release of neutrons and a large amount of energy. (Each atom can capture 1 neutron.)

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E. Fission Chain reaction – occurs when atomic nuclei that have split release energetic neutrons that split more nuclei.

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E. Fission Two steps in controlling fission: Neutron moderation – water or carbon slows down the neutrons Neutron absorption – decreases the number of slow neutrons through the use of control rods made of neutron- absorbing materials (usually cadmium)

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F. Fusion Occurs when two light nuclei combine to produce a nucleus of heavier mass, accompanied by the release of a large amount of energy.

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F. Fusion Occurs in all stars High temperatures are necessary to initiate fusion (no cold fusion yet) Possible future energy source Hydrogen bomb is a fusion reaction (fusion of two deuterium nuclei).

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G. Methods of Detection Geiger Counters (primarily beta) Scintillation counter – coated screen detects radiation particles. Film badge – several layers of photographic film encased in a holder. Detects beta and gamma.

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H. Radioisotopes in Medicine X-rays: Useful in imaging soft-tissue organs. Tracers: Iodine-131 is used to check for thyroid problems Radiation treatment: Some cobalt isotopes are used as radiation sources to treat cancer.

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