1. Unit 15
Nuclear Chemistry

2. Overview Nuclear Chemistry Radioactive decay Nuclear Stability
Isotopes
Nuclear force
Radioactive decay
Alpha, beta, gamma decay
Positron emission
Electron capture
Nuclear Stability
Radiometric Dating
Half-life
Nuclear fusion
Nuclear fission
Nuclear energy
Mass Defect
Nuclear binding energy

3. Nuclear Chemistry Involves the change in the nucleus of an atom
Nuclear reactions are everywhere
Produce sunlight
Create elements (synthetic and natural in stars)
Radiation therapy (cancer treatment)
Generate electricity
Nuclear weapons

4. World Energy Use

5. The Nucleus
Remember – the nucleus is comprised of the two nucleons (protons and neutrons)
Atomic Number – number of protons
Mass Number – number of protons and neutrons together
It is effectively the mass of the atom

6. C Nuclear Symbols 12 6 Mass number (p+ + no) Element symbol
Atomic number
(number of p+)

7. Isotopes
Not all atoms of the same element have the same mass due to different numbers of neutrons in those atoms
Example: There are three naturally occurring isotopes of uranium:
Uranium-234
Uranium-235
Uranium-238

8. Nuclear Force Strong nuclear force
Holds protons and neutrons in nucleus very close together
Strongest force known

9. Nuclear Force
Nucleus is not stable when atoms experience certain ratios of protons to neutrons
Unstable atoms decay and emit radiation
Radioactive decay
Elements with more than 83 protons (bismuth) are naturally radioactive

10. Radioactive Decay Radionuclides: Radioactive elements
During radioactive decay
The makeup of the nucleus changes
The number of protons may change
Means that the element has changed

11. Natural Radioactive Isotopes
Radon-222
Comes from decomposition of Uranium rocks
2nd leading cause of lung cancer
Comes up through cracks in basements
Radium-226
Some radium salts glow in the dark
Early 1900s used to be used as paint for watches and clocks (workers licked paint brushes and got cancer – “radium girls”)
Uranium-238
Rocks create radon gas
Used in radioactive dating
Potassium-40
One of few light radioactive elements
Produces argon that is found in atmosphere

12. Other Common Radioisotopes
Use
14C
Archaeological dating
24Na
Circulatory system testing for obstruction
32P
Cancer detection
51Cr
Determination of blood volume
59Fe
Measurements of red blood cell formation and lifetimes
60Co
Cancer treatment
131I
Measurement of thyroid activity
153Gd
Measurement of bone density
226Ra 3H
235U
Nuclear reactors and weapons
238U
241Am
Smoke detectors

13. Measuring Radioactivity
One can use a device like this Geiger counter 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.

14. Radioactive Decay (3 Most Common Types)
Alpha (a, He)
2 protons, 2 neutrons
Beta (b, e)
High energy electron
Gamma (g)
Electromagnetic radiation
High energy photons

15. Alpha, Beta, Gamma Radiation

16. He or  U Th He Alpha Decay: +
Loss of an -particle (a helium nucleus)
He or  4 2 4 2 U
238 92
 Th
234 90 He 4 2 +

17.  e I Xe e Beta Emission: +
Loss of a -particle (a high energy electron)  −1 e or I
131 53 Xe 54
 + e −1

18.  Gamma Emission: Loss of a -ray
High-energy radiation that almost always accompanies the loss of a nuclear particle
Not usually written in nuclear equation 

19.  e C B e Positron Emission: +
Loss of a positron (a particle that has the same mass as but opposite charge of an electron)  1 e or C 11 6
 B 5 + e 1
Has a very short life because it is destroyed when it collides with an electron, producing gamma rays:
e + e   1 -1

20. p n e Positron Emission + A positron can convert a proton to a neutron1
 n + e

21. Electron Capture
Capture by the nucleus of an electron from the electron cloud surrounding the nucleus
Addition of an electron to a proton in the nucleus
As a result, a proton is transformed into a neutron p 1 + e −1
 n

22. Nuclear Stability
Several factors predict whether a particular nucleus is radioactive
Neutron-to-proton ratio
Radioactive series
Magic Numbers
Evens and Odds

23. Neutron-Proton Ratios
The strong nuclear force helps keep the nucleus from flying apart
Protons repel each other
Neutrons help the strength of the nuclear force
As protons increase, neutrons have to counter-act increasing proton-proton repulsions
In low atomic number elements (1-20) protons and neutrons are approximately equal
In high atomic number elements number of neutrons much larger than protons
Neutron-proton ratio helps stabilize nucleus

24. Neutron-Proton Ratios
For smaller nuclei (Atomic Number  20) stable nuclei have a neutron-to-proton ratio close to 1:1.

25. Neutron-Proton Ratios
As nuclei get larger, it takes a greater number of neutrons to stabilize the nucleus.

26. Stable Nuclei
The shaded region in the figure shows what nuclides would be stable, the so-called belt of stability.

27. Stable Nuclei Nuclei above this belt have too many neutrons.
They tend to decay by emitting beta particles.
(If an isotopes mass number is greater than its atomic weight, the same trend will happen example C) 16 6

28. Stable Nuclei Nuclei below the belt have too many protons.
They tend to become more stable by positron emission or electron capture.
(If an isotopes mass number is less than its atomic weight, the same trend will happen example C) 11 6

29. Stable Nuclei
There are no stable nuclei with an atomic number greater than 83.
These nuclei tend to decay by alpha emission.
Decreases both protons and neutrons

30. Radioactive 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).
Often occur in nature

31. Magic Numbers
Nuclei with 2, 8, 20, 28, 50, or 82 protons or 2, 8, 20, 28, 50, 82, or 126 neutrons tend to be more stable than nuclides with a different number of nucleons.
These are called the “Magic Numbers”

32. Evens and Odds
Nuclei with an even number of protons and neutrons tend to be more stable than nuclides that have odd numbers of these nucleons.

33. Kinetics of Radioactive Decay
Radioactive decay is a 1st order process
Remember this equation:
= t1/2
0.693 k

34. Radiometric Dating
Half life can help determine the age of different objects
Carbon-14
Half life of 5,715 years
Can determine age of organic materials up to about 50,000 years old

35. Radiometric Dating Uranium-238 Half life of 4.5×109 years
Used to determine age of Earth (measured rocks)
Oldest rock found is almost 4.5 billion years old

36. Nuclear Fusion
Elements can be man-made by bombarding nuclei with particles
Alpha particles accelerated and collided with nucleus
Neutrons bombard nucleus
Bombard nuclei to create transuranium elements
Heavy elements beyond uranium on periodic table

37. Particle Accelerators
Nuclear transformations can be induced by accelerating a particle and colliding it with the nuclide
These particle accelerators are enormous, having circular tracks with radii that are miles long

38. Nuclear Fission The splitting of heavy nuclei
(Fusion is the combination of light nuclei)
Process begins by bombarding heavy nucleus with a neutron
2 main commercial uses
Nuclear Weaponry
Nuclear Energy

39. Nuclear Fission This is called a chain reaction
About 2 neutrons are produced for each fission
These 2 neutrons cause 2 additional fissions
Which cause 2 more fissions each
Which cause 2 more fissions each…
This is called a chain reaction

40. Nuclear Fission Chain reactions can escalate quickly
If the reaction is not controlled, it results in a violent explosion because of the release of too much energy too quickly

41. Nuclear Energy
We can control fission reactions and use it to create energy

42. Nuclear Energy Fission reactions are carried out in nuclear reactors
The reaction is kept in check by the use of control rods
These block the paths of some neutrons, keeping the system from escalating out of control
The heat generated by the reaction is used to produce steam that turns a turbine connected to a generator
Video:

43. Debates on Nuclear Energy
Pros…
Cleaner energy than coal and fossil fuel plants
Doesn’t add to global warming
High amount of electricity can be generated in one plant
Cheaper to run a nuclear facility than a fossil fuel plant
Cons…
Nonrenewable source of energy
Produces nuclear waste that must be stored for thousands of years
Accidents (Chernobyl, Three Mile Island, Fukushima)
Very expensive to build a nuclear facility (about $10 billion per reactor)

44. Nuclear Energy E = mc2 We can measure the energy
associated with nuclear reactions
E = mc2
E = energy (J)
m = change in mass (kg) during reaction (mass of products-mass of reactants)
c = speed of light (3.0×108 m/s)
When a system loses mass, it is exothermic (-E)
When a system gains mass, it is endothermic (+E)

45. Nuclear Energy
The mass change in chemical reactions is so small that we treat them as though mass is conserved
Ex: Mass change for exothermic process of combustion of 1 mol of CH4 is -9.9×10-9 grams
Mass change in nuclear reactions is measureable
Ex: Mass change accompanying decay of 1 mol of uranium-238 is 50,000 times greater than combustion of CH4

46. Nuclear Energy (example)
For example, the mass change for the decay of 1 mol of uranium-238 is − g.
The change in energy, E, is then
E = (m) c2
E = (−4.6  10−6 kg)(3.00  108 m/s)2
E = −4.1  1011 J

47. Mass Defect
When protons and neutrons form a nucleus, the mass of the nucleus is less than the sum of the masses of its constituent protons and neutrons
Example: Helium (He) – 2 protons, 2 neutrons
Protons and Neutrons Mass of Nucleus
Mass of 2 protons (2× = ) amu
Mass of 2 neutrons (2× = )
Total mass = amu
Difference = – = amu (mass defect)

48. Mass Defect E = mc2 E = (5.1×10-29 kg)(3.0×108 m/s)2 E = 4.6×10-12 J
To measure the energy associated with the mass defect use
E = mc2
Example: Helium (He) – 2 protons, 2 neutrons
E = (5.1×10-29 kg)(3.0×108 m/s)2
E = 4.6×10-12 J
NOTE: 1 gram = 6.022×1023 amu

49. Nuclear Binding Energy
Energy required to separate a nucleus into its individual nucleons (protons and neutrons)
Also use E = mc2
The larger the binding energy, the more stable the nucleus toward decomposition