The Nucleolus: A Chemist

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Chapter 18

• The Nucleolus A Chemists View

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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

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IntroductionNuclear 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.

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Representation of atomic nuclei
Mass number- A
Atomic number- Z
Isotopes
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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
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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
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Radioactive Decay Series
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Decay of P-32 to S-32
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18.1 Nuclear stability and radioactive decay

• Nuclear stability
• 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

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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.

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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,

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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 (gt83 protons) to be unstable.

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• 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)

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Types of radiation emitted in natural
radioactivity
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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

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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

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Balancing nuclear equations
Protactinium

• Total Nucleon Number (TOP VALUES) Total number
  of protons and neutrons
• Total electric charge (BOTTOM VALUES)
• Are kept the same.

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• Nuclear reaction is written maintaining mass and
  charge balance.
• E.g.

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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

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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
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More examples of radioactive decay
Alpha production (?) helium nucleus, Beta
production (?)
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Examples of radioactive decay
Gamma ray production (?) Positron
production Electron capture (inner-orbital
electron is captured by the nucleus)
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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
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Half-Life

• The time required for the number of nuclides to
  reach half the original value (N0/2).

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Examples of Half-Life

• Isotope Half life
• C-15 2.4 sec
• Ra-224 3.6 days
• Ra-223 12 days
• I-125 60 days
• C-14 5700 years
• U-235 710 000 000 years

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Examples

• 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.

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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
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• 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

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Schematic diagram of a cyclotron
Positive ion
Nucleus
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A Schematic Diagram of a Linear Accelerator
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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 disintigrationss-1
  

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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-
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Scintillation counters
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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
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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

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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
• Pb-206 was not present originally
• All of the decay products are retained

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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,

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Medical applications of radioactivity
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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
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• 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

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• 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
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• 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

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Calculation of binding energy

• Calculate the binding energy per nucleon
• for nucleus.
• (Atomic masses 4.0026 amu,
  
• 1.0078 amu)
• We must calculate the mass defect for
• He-4

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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

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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.

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The Binding Energy Per Nucleon as a Function of
Mass Number
Fusion of light nuclei and fission of heavy
nuclei are exothermic processes
Highest stability
Nuclear fission
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• 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.

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Both Fission and Fusion Produce More Stable
Nuclides
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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
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Fission

• 23592U 10n ? 23692U
• and 10-14 seconds later...
• 23692U ? 9236Kr 14156Ba 3 10n 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)

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Fission Process
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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

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Fission Produces a Chain Reaction
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Nuclear Fission
A self-sustaining fission process is called a
chain reaction.
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Fission Produces Two Neutrons
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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

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• 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

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A Schematic Diagram of a Nuclear Power Plant
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A Schematic Diagram of a Reactor Core
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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

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Breeder Reactors

• Fissionable fuel is produced while the reactor
  runs ( is split, giving neutrons for the
  creation of )

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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

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Proposed mechanism for reactions on the sun
T ? 1X109 oC E ? 1X1019 kJ/day
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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

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Effects of RadiationFactors 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.

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• 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)

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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