1. Chapter 19 (pg 666)
Nuclear Chemistry

2. 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

3. 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

4. 23.1

5. 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.

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

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

8. 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

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

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

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

12. 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

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

15. 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.

16. 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

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

18. 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

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

20. 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 

21. Nuclear Applications

22. 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

23. 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.

24. 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

25. 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.

26. 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

27. 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

28. 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

29. 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

30. 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 = (12.0/T) log(1/2)
0.678 =
12.0 hrs T
T = 17.7 hrs

31. 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

32. 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 =  about 11,000 years old

33. Nuclear binding energy (BE) is the energy required to break up a nucleus into its component protons and neutrons.
BE + 19F p + 101n 9 1
Mass defect (∆m) = 9 x (p mass) + 10 x (n mass) – 19F mass
∆m (amu) = 9 x x –
∆m = 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

34. 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

35. Nuclear Fission 235U + 1n 90Sr + 143Xe + 31n + Energy92 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

36. 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

37. 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

38. 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

39. 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

40. 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

41. 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

42. Radioisotopes in Medicine
99mTc Tc + 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

43. 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’)

44. 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 Gy)

45. 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 kGy
Delay in fruit ripening kGy
Insect disinfestation including quarantine treatment and elimination of food borne parasites 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 kGy
High dose applications (above 10 kGy)
Sterilization of packaged meat, poultry, and their products that stable without refrigeration kGy
Sterilization of Hospital diets kGy
Product improvement as increased juice yield or improved re-hydration