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CHAPTER 20 Nuclear Chemistry and Radioactivity 20.3 Rate of Radioactive Decay.

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Presentation on theme: "CHAPTER 20 Nuclear Chemistry and Radioactivity 20.3 Rate of Radioactive Decay."— Presentation transcript:

1 CHAPTER 20 Nuclear Chemistry and Radioactivity 20.3 Rate of Radioactive Decay

2 2 What is carbon dating? How can we tell how old fossils are?

3 Rate of Radioactive Decay What is carbon dating? How can we tell how old fossils are? We introduce the time variable Reaction rates

4 Rate of Radioactive Decay What is carbon dating? How can we tell how old fossils are? We introduce the time variable In Chapter 12 we studied reaction rates for chemical reactions Nuclear reactions also involve rates! Reaction rates

5 Rate of Radioactive Decay Some reactions take place very quickly; they have a short half-life, t 1/2. half-life: the time it takes for half of the atoms in a sample to decay. Decay

6 Rate of Radioactive Decay Half-life

7 Rate of Radioactive Decay Half-life Every radioactive isotope has a different half-life. Isotopes with short half-lives do not occur in nature, but must be generated in the laboratory.

8 Rate of Radioactive Decay Carbon dating Carbon dating revolves around carbon-14, a radioactive isotope. Carbon-14 is generated in the upper atmosphere through a bombardment reaction: Neutrons generated by cosmic rays becomes 14 CO 2 in the atmosphere

9 Rate of Radioactive Decay Carbon-14 goes through the same cycle as carbon-12 Carbon dating 14

10 Rate of Radioactive Decay In living organisms: Carbon dating This ratio stays constant while the organism is alive

11 Rate of Radioactive Decay In living organisms: Carbon dating Over time, carbon-14 decays by emission: This ratio stays constant while the organism is alive

12 Rate of Radioactive Decay Over time, carbon-14 decays by emission: Carbon dating When the organism dies, it no longer consumes carbon from the environment. The number of carbon-14 atoms in the dead organism will decrease over time.

13 Rate of Radioactive Decay Carbon dating Ratio not to scale An archeologist looks at the ratio of carbon-14 to carbon-12. Carbon dating works reliably up to about 10 times the half-life, or 57,300 years (beyond that time, there is not enough carbon-14 left to detect accurately). Carbon dating only works on material that has once been living: tissue, bone, or wood.

14 Rate of Radioactive Decay About 18% of the mass of a live animal is carbon. If 1 g of live bone contains about 90 billion carbon-14 atoms (t 1/2 = 5,730 years), how many C-14 atoms remain in 1 g of bone 17,190 years after the animal dies?

15 Rate of Radioactive Decay About 18% of the mass of a live animal is carbon. If 1 g of live bone contains about 90 billion carbon-14 atoms (t 1/2 = 5,730 years), how many C-14 atoms remain in 1 g of bone 17,190 years after the animal dies? Given:The half-life and the number of carbon-14 atoms

16 Rate of Radioactive Decay About 18% of the mass of a live animal is carbon. If 1 g of live bone contains about 90 billion carbon-14 atoms (t 1/2 = 5,730 years), how many C-14 atoms remain in 1 g of bone 17,190 years after the animal dies? Given:The half-life and the number of carbon-14 atoms Solve:Since 17,190 years is three half-lives, the initial amount must be reduced by a factor of 2 x 2 x 2 = billion / 8 = billion

17 Rate of Radioactive Decay About 18% of the mass of a live animal is carbon. If 1 g of live bone contains about 90 billion carbon-14 atoms (t 1/2 = 5,730 years), how many C-14 atoms remain in 1 g of bone 17,190 years after the animal dies? Given:The half-life and the number of carbon-14 atoms Solve:Since 17,190 years is three half-lives, the initial amount must be reduced by a factor of 2 x 2 x 2 = billion / 8 = billion Answer:After three half-lives the amount of carbon-14 atoms is reduced by a factor of 8, from 90 billion to billion.

18 Rate of Radioactive Decay Every radioactive isotope has a different half-life, t 1/2 Carbon dating is based on the knowledge that t 1/2 for carbon-14 is 5,730 years Ratio not to scale

19 Rate of Radioactive Decay Rate of decay The number of nuclei in the sample (N) is constant A short half-life implies a large rate constant, k.

20 Rate of Radioactive Decay Rate of decay Carbon-14 and radium-220 have half-lives of 5,730 years and 1 minute, respectively. Calculate the rate constants for their decay, in units of 1/s.

21 Rate of Radioactive Decay Rate of decay Carbon-14 and radium-220 have half-lives of 5,730 years and 1 minute, respectively. Calculate the rate constants for their decay, in units of 1/s. Asked:The rate constant k Given:The half-life t 1/2 for each radioactive decay process. Relationships:The equation that relates t 1/2 to k: k = / t 1/2

22 Rate of Radioactive Decay Rate of decay Carbon-14 and radium-220 have half-lives of 5,730 years and 1 minute, respectively. Calculate the rate constants for their decay, in units of 1/s. Asked:The rate constant k Given:The half-life t 1/2 for each radioactive decay process. Relationships:The equation that relates t 1/2 to k: k = / t 1/2 Solve:For C-14, t 1/2 = 5,730 years, and.

23 Rate of Radioactive Decay Rate of decay Carbon-14 and radium-220 have half-lives of 5,730 years and 1 minute, respectively. Calculate the rate constants for their decay, in units of 1/s. Asked:The rate constant k Given:The half-life t 1/2 for each radioactive decay process. Relationships:The equation that relates t 1/2 to k: k = / t 1/2 Solve:For C-14, t 1/2 = 5,730 years, and For Ra-220, t 1/2 = 1 min, and

24 Rate of Radioactive Decay Rate of decay Carbon-14 and radium-220 have half-lives of 5,730 years and 1 minute, respectively. Calculate the rate constants for their decay, in units of 1/s. Asked:The rate constant k Given:The half-life t 1/2 for each radioactive decay process. Relationships:The equation that relates t 1/2 to k: k = / t 1/2 Solve:For C-14, t 1/2 = 5,730 years, and For Ra-220, t 1/2 = 1 min, and Discussion:Note that a small t 1/2 gives a large k. The rate constant k gives us an indication of the number of decays over a certain period of time.

25 Rate of Radioactive Decay Decay rate law The rate of decay of a radioactive sample is also called the activity of the sample

26 Rate of Radioactive Decay Decay rate law Plutonium-236 decays by emitting an alpha particle and has a half-life of 2.86 years. If we start with 10 mg of Pu-236, how much remains after 4 years?

27 Rate of Radioactive Decay Decay rate law Plutonium-236 decays by emitting an alpha particle and has a half-life of 2.86 years. If we start with 10 mg of Pu-236, how much remains after 4 years? Asked:N, the amount left after 4 years Given:The half-life t 1/2, the initial amount N 0, and the elapsed time t Relationships:The equation that relates t 1/2 and N 0 to N is

28 Rate of Radioactive Decay Decay rate law Plutonium-236 decays by emitting an alpha particle and has a half-life of 2.86 years. If we start with 10 mg of Pu-236, how much remains after 4 years? Asked:N, the amount left after 4 years Given:The half-life t 1/2, the initial amount N 0, and the elapsed time t Relationships:The equation that relates t 1/2 and N 0 to N is Solve:

29 Rate of Radioactive Decay Decay rate law Plutonium-236 decays by emitting an alpha particle and has a half-life of 2.86 years. If we start with 10 mg of Pu-236, how much remains after 4 years? Asked:N, the amount left after 4 years Given:The half-life t 1/2, the initial amount N 0, and the elapsed time t Relationships:The equation that relates t 1/2 and N 0 to N is Solve: Discussion:After 4 years, the initial 10 mg is reduced to 3.79 mg, which is 37.9% of the initial amount of Pu-236.

30 Rate of Radioactive Decay Radioactive dating the composition of the atmosphere over time the age of rocks that are billions of years old the age of a once-living organism Information can be extracted from the ratio of specific isotopes Carbon-14 and carbon-12 Oxygen-18 and oxygen-16 Uranium-238 and plutonium-239

31 Rate of Radioactive Decay Radioactive dating The amount of sample remaining, compared to the initial amount of sample, can be used to determine the age of the sample.

32 Rate of Radioactive Decay An ancient Greek scroll written on an animal skin was discovered by archeologists in They isolated 10 g of it and measured the carbon-14 decay rate to be 111 disintegrations/minute. Calculate the age of the scroll. (Assume that living organisms have a carbon-14 decay rate of 15 disintegrations per minute per gram of C.)

33 Rate of Radioactive Decay An ancient Greek scroll written on an animal skin was discovered by archeologists in They isolated 10 g of it and measured the carbon-14 decay rate to be 111 disintegrations/minute. Calculate the age of the scroll. (Assume that living organisms have a carbon-14 decay rate of 15 disintegrations per minute per gram of C.) Given:The initial decay rate of C-14: N 0 = 15 disintegrations/(min·g). The present decay rate of C-14 is 11 disintegrations/(min·g). The half-life of C-14 is 5,730 years.

34 Rate of Radioactive Decay An ancient Greek scroll written on an animal skin was discovered by archeologists in They isolated 10 g of it and measured the carbon-14 decay rate to be 111 disintegrations/minute. Calculate the age of the scroll. (Assume that living organisms have a carbon-14 decay rate of 15 disintegrations per minute per gram of C.) Given:The initial decay rate of C-14: N 0 = 15 disintegrations/(min·g). The present decay rate of C-14 is 11 disintegrations/(min·g). The half-life of C-14 is 5,730 years. Solve:For the rate constant k:

35 Rate of Radioactive Decay An ancient Greek scroll written on an animal skin was discovered by archeologists in They isolated 10 g of it and measured the carbon-14 decay rate to be 111 disintegrations/minute. Calculate the age of the scroll. (Assume that living organisms have a carbon-14 decay rate of 15 disintegrations per minute per gram of C.) Given:The initial decay rate of C-14: N 0 = 15 disintegrations/(min·g). The present decay rate of C-14 is 11 disintegrations/(min·g). The half-life of C-14 is 5,730 years. Solve:For the rate constant k: And the time is:

36 Rate of Radioactive Decay An ancient Greek scroll written on an animal skin was discovered by archeologists in They isolated 10 g of it and measured the carbon-14 decay rate to be 111 disintegrations/minute. Calculate the age of the scroll. (Assume that living organisms have a carbon-14 decay rate of 15 disintegrations per minute per gram of C.) Given:The initial decay rate of C-14: N 0 = 15 disintegrations/(min·g). The present decay rate of C-14 is 11 disintegrations/(min·g). The half-life of C-14 is 5,730 years. Solve:For the rate constant k: And the time is: Discussion:The animal skin on which the scroll was written was 2,491 years old. It was written in about 483 BC.

37 Rate of Radioactive Decay Mathematics of radioactive decay


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