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CHAPTER 28 Nuclear Chemistry Radioactive Decay Radioactive Decay.

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Presentation on theme: "CHAPTER 28 Nuclear Chemistry Radioactive Decay Radioactive Decay."— Presentation transcript:

1 CHAPTER 28 Nuclear Chemistry Radioactive Decay Radioactive Decay

2 A. Nuclear Stability Nuclide = atom of an isotope

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

4 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:

5 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:

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

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

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

9 B. Nuclear Decay Beta Emission electron Positron Emission positron

10 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.

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

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

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

14 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

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

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

17 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

18 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

19 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!)

20 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.

21 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.

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

23 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?

24 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

25 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.)

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

27 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)

28 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.

29 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).

30 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.

31 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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