1. The Nucleolus: A Chemist’s View
Chapter 18
The Nucleolus: A Chemist’s View

2. Topics Nuclear stability and radioactive decay
The kinetics of radioactivity
Nuclear transformations
Detection and use of radioactivity
Thermodynamic stability of the nucleus
Nuclear fission and nuclear fusion
Effects of radiation

3. Introduction Nuclear Reactions vs Chemical Reactions
Chemical reactions: Changes in the outer electronic structure of atoms or molecules
Nuclear reactions: study of changes in structure of nuclei and subsequent changes in chemistry.
Radioactive nuclei: spontaneously change structure and emit radiation.
Differences between nuclear and chemical reactions:
Much larger release in energy in nuclear reaction.
Isotopes show identical chemical reactions but different nuclear reactions.
Nuclear reactions not sensitive to chemical environment.
Nuclear reaction produces different elements.
Rate of nuclear reaction not dependent upon temperature.

4. Representation of atomic nuclei
Mass number- A
Atomic number- Z
Isotopes

5. Nucleus components
Nucleon: any nuclear particle, e.g. protons, p, and neutrons, n.
Nuclide
Isotopes: atoms that have identical atomic numbers but different mass numbers
Nuclide: is a term used to identify an individual atom. Each individual atom is called nuclide

6. Radioactivity
Radioactivity is a nuclear reaction in which an unstable nucleus decomposes spontaneously
Natural radioactivity
Natural unstable nuclei decompose more stable nuclei
Artificial radioactivity
Synthetic unstable nuclei decompose more stable nuclei
Decay
Parent
nuclei
Daughter nuclei

7. Radioactive Decay Series

8. Decay of P-32 to S-32

9. 18.1 Nuclear stability and radioactive decay
Thermodynamic stability: the potential energy of a nucleus as compared with sum of the potential energies of its components protons and neutrons
Kinetic stability: it describes the probability that a nucleus will undergo decomposition to form a different nucleus- a process called radioactive decay
Stability depends upon a balance between repulsive forces (between protons) and strong attraction forces between nuclei

10. Nuclear Stability
The stability of a nucleus depends mainly on A, the mass number and Z, the atomic number. Up to the mass number 30 or 40, a nucleus has approximately the same number of neutrons and protons to be stable.
Bigger nuclei must have more neutrons than protons. As Z gets bigger, repulsive forces get bigger.
When nucleus gets big enough, no neutron is enough to keep it stable. After, Z= 82, no nuclei is stable. Such unstable nuclei are radioactive, which means they undergo radiations in order to become stable.

11. To understand this we may look at this graph,
Nuclear Stability
A nucleus having very much protons compared to neutrons will never be stable
This does not mean that a nucleus with many neutrons and little protons will be stable.
To understand this we may look at this graph,

14. Empirical rules for predicting stability of nuclei
Neutron-to-proton ratio varies with atomic number
Light isotopes (small atomic number) have a Neutron-to-proton ratio almost = 1(almost stable)
Nuclei are held together by strong attractive forces; but electrostatic repulsion causes large atoms (>83 protons) to be unstable.

15. Nuclides with even number of nucleons
(p +n) are more stable than those with odd number
Certain number of protons or neutrons appear to be particularly stable. The magic numbers are:
2, 8, 20, 28,50, 82, 126
These numbers are in parallel to those produce chemical stability:
2, 10, 18, 36, 54 and 86
(Noble gas configuration)

16. Types of radiation emitted in natural radioactivity

17. Types of radioactive decay
radiation = attracted towards
negatively charged plate
Þ Positively charged 
radiation = attracted towards positively
charged plate
Þ Negatively charged =1e- 
radiation = not attracted to either plate Þ
Neutral. When emitted it does not change atomic or mass numbers
Very high energy photons; very short wavelength
Positron is a positive electron
Positron emission is equivalent to a fall of e-1 in
nucleus

18. NUCLEAR REACTIONS
Radioactivity: nucleus unstable and spontaneously disintegrates.
Nuclear Bombardment: causes nuclei to disintegrate due to bombardment with very energetic particles.
Particles in nuclear reactions:

19. Balancing nuclear equations
Protactinium
Total Nucleon Number (TOP VALUES) =Total number of protons and neutrons
Total electric charge (BOTTOM VALUES)
Are kept the same.

20.  Nuclear reaction is written maintaining mass and charge balance.
E.g. + 

21. Examples of adioactive decay
Beta emission: Converts neutron into a proton by emission of energetic electron; atomic # increases:
E.g. Determine product for following reaction:
Alpha emission: emits He particle.
E.g. Determine product:

22. Positron emission: Converts proton to neutron:
E.g. Determine product of
Gamma emission: no change in mass or charge but usually part of some other decay process.
E.g.
Electron capture: electron from electron orbitals captured
to convert proton to neutron.
E.g. Determine product

23. More examples of radioactive decay
Alpha production (): helium nucleus,
Beta production ():

24. Examples of radioactive decay
Gamma ray production ():
Positron production:
Electron capture: (inner-orbital electron is captured by the nucleus)

25. 18.2 The kinetics of radioactive decay
Nuclear decay is a first order reaction
Rate  amount of radioactive isotope present
For a radioactive nuclides, the rate of decay, that is the negative change in the number of nuclides per unit time
is directly proportional to the number of nuclides N
That is
This is a first order process
# of nuclides
remaining at
time t
Original # of nuclides

26. Half-Life
The time required for the number of nuclides to reach half the original value (N0/2). 5

27. Isotope Half life C-15 2.4 sec Ra-224 3.6 days Ra-223 12 days
Examples of Half-Life
Isotope Half life
C sec
Ra days
Ra days
I days
C years
U years

28. Examples the original amount would be left after 21.04 years?
1. The half-life of Cobalt-60 is 5.26 years how much of
the original amount would be left after 21.04
years?
2. Tritium decays by beta emission with a half-life of
12.3 years. How much of the original amount
would be left after 30 years?
3. If a 1.0 g sample of tritium is stored for 5.0 years,
what mass of that isotope remains?
k = 0.563/year.

29. 18.3 Nuclear Transformation
The change of one element into another
Bombard nuclei with nuclear particles to convert element to another one to become more stable through radioactivity is transmutation.
Rutherford
Irene Curie 6

30. Nuclear transformation can occur by alpha or beta radiation, or
some other nuclear reactions such as nuclear bombardment
Nuclear transformation is achieved mostly using particle accelerator
Accelerators are needed when positive ions are used as the
bombarding particles
The particle is accelerated to a very high velocity thus it can
overcome the repulsion and can penetrate the target nucleus
Neutrons are also used often as bombarding particles
Neutrons are uncharged, thus they are not repelled and readily
absorbed by many nuclides
Using neutron and positive ion bombardment made possible to
extend the periodic table
Since 1940, elements with atomic numbers 93 through
112 have been synthesized
These elements are called transuranium elements

31. Schematic diagram of a cyclotron
Positive ion
Nucleus

32. A Schematic Diagram of a Linear Accelerator

33. 4. Detection and uses of radiation
Geiger counters
detect charged particles produced from interaction of gas with particles emitted from radioactive material. The device detects the current flow
Scintillation counters
detect particles from radioactive material by measuring intensity of light when these particles hit substances such as ZnS.
Units: 1 curie (Ci) = 3.7x1010 disintigrations×s-1

34. A representation of a Geiger-Müller counter.
High energy particles produced from radioactive decay produce ions when they travel through matter
Ar(g) Ar+(g) + e-

35. Scintillation counters

36. Dating by radioactivity
Carbon-14 Dating
Carbon-14 is formed naturally at a fairly constant
rate by bombardment of atmospheric nitrogen by
cosmic rays (high energy neutrons).
147N + 10n  146C + 11 H
and then over time C-14 decays
146C  147N + 0-1e

37. Age of organic material
As long the plant or animal lives the
C-14/C-12 ratio in its molecules remains the same as in the atmosphere (1/1012) because of the continuous uptake of carbon.
When the plant/animal dies, C-14 decays and the ratio decreases
t1/2 for C-14 = 5730 yr
If C-14/C-12 found in the old wood is ½ of that in a currently living plant, then its age is 5730 yr.
This assumes that the current C-14/C-12 ratio is the same as that in the ancient plant

39. Age of rocks/Age of earth
U-238 present in certain rocks slowly decays to
Pb-206
Pb-206 was not present originally
As time progresses the amount of U-238 decreases and Pb-206 increases
By measuring the ratio of Pb-206 / U-238 scientists can determine the age of a rock
The oldest rocks can then be used to determine the minimum age of the earth
It is assumed that
All of the decay products are retained

42. Medical applications of radioactivity
Radioactive nuclides can be introduced into organisms in food or drugs where their paths can be traced by monitoring their radioactivity
Radioactive tracers provide sensitive methods for:
learning about biological systems,
detection of disease,
monitoring the action and effectiveness of drugs,
early detection of pregnancy,

46. Medical applications of radioactivity

47. 18-5 Thermodynamic Stability of the Nucleus
We can determine the thermodynamic stability of a
nucleus by calculating the change in potential
energy that would occur if that nucleus were formed
from its constituent protons and neutrons.
For example, the hypothetical process of forming
nucleus from eight neutrons and eight protons:

51. What is the change in energy that correspond to the formation of 1 mol of O-16 from its protons and neutrons?
Thus,
= (-1.366X10-4 kg/mol)(3.00X108 m/s2) = -1.23X1013J/mol
Consequently:
Nuclear processes are accompanied with extremely large energy compared to chemical and physical changes
Nuclear processes constitute a potentially valuable energy resource

52. Calculate the energy released per a nucleon of O-16
Thermodynamic stability of a particular nucleus is represented as energy released per nucleon
Calculate the energy released per a nucleon of O-16
Thus, 7.98 MeV of energy per nucleon would be released if O-16 were
from neutrons and protons

53. Thus, 7.98 MeV of energy per nucleon would be
released if O-16 were from neutrons and protons
The energy required to decompose the above nucleus into its components has the same quantity but with +ve sign : This is the binding energy per nucleon for O-16

54. Calculation of binding energy
Calculate the binding energy per nucleon
for nucleus.
(Atomic masses = amu,
amu)
We must calculate the mass defect for
He-4

58. Nuclear binding energy
It is the energy required to decompose nucleus into protons and neutrons or it is the energy released when protons and neutrons combine together to form nucleus
The NBE is a measure of the stability of the nucleus towards decomposition. Large NBE means more stability. Atoms of intermediate masses have larger NBE than either the very light atoms or the very heavy ones

59. 18.6 Nuclear fission and nuclear reaction
The graph above has very important implications for the use of nuclear processes as sources of energy.
Energy is released, that is, E is negative, when a process goes from a less stable to a more stable state nuclei
The higher a nuclide is on the curve, the more stable it is.
This means that two types of nuclear processes will be exothermic
1. Combining two light nuclei to form a heavier, more
stable nucleus. This process is called fusion.
2. Splitting a heavy nucleus into two nuclei with smaller
mass numbers. This process is called fission.
Because of the large binding energies involved in holding the nucleus together, both these processes involve energy changes more than a million times larger than those associated with chemical reactions.

60. The Binding Energy Per Nucleon as a Function of Mass Number
Fusion of light nuclei and fission of heavy nuclei are exothermic processes
Stability of nuclei increasing
56Fe has highest Eb and is most stable isotope.
Energy sources:
Fission for large radioactive elements, such as U-235
Fusion of very light nuclei such as deuterium producing He. Not yet accomplished.
Atoms of Z=50-80 (intermediate masses have the largest NBE.
Nuclear fusion
Highest stability
Nuclear fission

61. Nuclei of heavy atoms will gain more stability if they are fragmented (fission into intermediate ones). They will give off energy when the fission occurs
Nuclei of light atoms will gain more stability if they are fused together (fusion) to give atoms of intermediate NBE. Energy will be given off when fusion occurs.

62. Both Fission and Fusion Produce More Stable Nuclides

63. Nuclear Fission
Several isotopes of the heavy elements undergo fission if bombarded with neutrons of high enough energy
In practice attention was paid to
and
The discussion will focus on
That is only 0.7% of the naturally occurring U
is most abundant isotope and does
not go fission

64. Fission 23592U + 10n  23692U* and 10-14 seconds later...
23692U*  9236Kr Ba n + ENERGY
50 possible sets of fission products (sum of atomic numbers = 92)
3 neutrons released for ONE 23592U (too many for stability, thus fragmentation continues to reach stability)

65. Fission Process

66. Chain Fission Reactions
Produced neutrons will attack more and more forming chain reaction
This chain reaction occurs in the atomic bomb. Energy is evolved in successive fissions that will lead to tremendous explosion
For the chain reaction to occur must be large (critical mass), thus most neutrons are captured
Critical mass for is 1 to 10 Kg
If the sample is too small most neutrons escape braking the chain

67. Fission Produces a Chain Reaction

75. Nuclear Fission
A self-sustaining fission process is called a chain reaction. 10

76. Fission Produces Two Neutrons

77. Nuclear reactors
Because of the tremendous energies involved, it is desirable to develop the fission process as an energy source to produce electricity.
To accomplish this, reactors were designed in which controlled fission can occur.
The resulting energy is used to heat water to produce steam to run turbine generators, in much the same way that a coal-burning power plant generates energy.
A schematic diagram of a nuclear power plant is shown

78. In the reactor core, uranium that has been enriched to approximately 3% U-235(natural uranium contains only 0.7% U-235) is housed in cylinders.
A moderator surrounds the cylinders to slow down the neutrons so that the uranium fuel can capture them more efficiently.
Control rods, composed of substances that absorb neutrons, are used to regulate the power level of the reactor. The reactor is designed so that should a malfunction occur, the control are automatically inserted into the core to stop the reaction
A liquid that is usually water is circulated through the core to extract the heat generated
The energy can then passed on via a heat exchanger to water in the turbine system

79. A Schematic Diagram of a Nuclear Power Plant

81. A Schematic Diagram of a Reactor Core

82. Breeder Reactors Fissionable fuel is produced while the reactor runs
is changed (split) to fissionable
This reaction involves absorption of neutrons
As the reactor runs and U-235 is split some of the excess
neutrons are absorbed by U-238 to produce Pu-239
Pu-239 is then separated and used to fuel another reactor
This reactor, thus breeds nuclear fuel as it operates 12

83. Breeder Reactors
Fissionable fuel is produced while the reactor runs ( is split, giving neutrons for the creation of ): 12

84. Fusion
Large quantities of energy are produced by the fusion of two light nuclei to give a heavier one
Stars and sun produce their energy through nuclear fusion.
Our sun, which presently consists of 73% hydrogen, 26% helium, and 1 % other elements, gives off vast quantities of energy from the fusion of protons to form helium:

85. Proposed mechanism for reactions on the sun
T  1X109 oC; E  1X1019 kJ/day

86. How does fusion take place?
The major stumbling block in having these fusion reactions feasible is that high energies are required to initiate fusion.
The forces that bind nucleons together to form a nucleus are effective only at very small distances (10-13 cm).
Thus, for two protons to bind together and thereby release energy, they must get very close together.
But protons, because they are identically charged, repel each other electrostatically.
This means that to get two protons (or two deuterons) close enough to bind together (the nuclear binding force is not electrostatic), they must be "shot" at each other at speeds high enough (106 m/s) to overcome the electrostatic repulsion.
High temperatures are expected from various sources that are under study

87. Effects of Radiation Factors that make the biological effects
The energy of the radiation.
The higher the energy the more damage it can cause. Radiation doses are measured in rads (radiation absorbed dose), where 1rad corresponds to 10-2 J of energy deposited per kilogram of tissue.
2. The penetrating ability of the radiation.
The particles and rays produced in radioactive processes vary in their abilities to penetrate human tissue:  rays are highly penetrating,  particles can penetrate approximately 1 cm, and  particles are stopped by the skin.

88. 3. The ionizing ability of the radiation
Extraction of electrons from biomolecules to form ions is particularly detrimental to their functions. The ionizing ability of radiation varies dramatically. For example,  rays penetrate very deeply but cause only occasional ionization. On the other hand,  particles, although not very penetrating, are very effective at causing ionization and produce a dense trail of damage.
Thus ingestion of an  particle producer, such as plutonium, is particularly damaging.
4. The chemical properties of the radiation source
When a radioactive nuclide is ingested into the body, its effectiveness in causing damage depends on its residence time. For example, Kr-85 and Sr-90 are both -particle producers.
However, since krypton is chemically inert, it passes through the body quickly and does not have much time to do damage.
Strontium, being chemically similar to calcium, can collect in bones, where it may cause leukemia and bone cancer.
The energy dose of the radiation and its effectiveness in causing biological damage form the source for the term rem (roentgen equivalent for man)
Number of rems = (number of rads X RBE (relative effectiveness of radiation in causing biological damage)

93. The two models for radiation damage
In the linear model, even a small dosage
causes a proportional risk.
In the threshold, risk begins only after a
certain dosage