Chapter 18 The Nucleolus: A Chemist’s View Topics Nuclear stability and radioactive decay The kinetics of radioactivity Nuclear transformations Detection and use of.
Download ReportTranscript Chapter 18 The Nucleolus: A Chemist’s View Topics Nuclear stability and radioactive decay The kinetics of radioactivity Nuclear transformations Detection and use of.
Chapter 18
The Nucleolus: A Chemist’s View
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
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.
Representation of atomic nuclei
Mass number- A Atomic number- Z
6 6 12
C
14
C C
12
Isotopes
C
14
Nucleus components
Nucleon : any nuclear particle, e.g. protons, p, and neutrons, n.
A Z X
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
Radioactivity
Radioactivity is a nuclear reaction in which an unstable nucleus decomposes spontaneously
Natural radioactivity Natural unstable nuclei decompose Decay Parent nuclei more stable nuclei Daughter nuclei
Artificial radioactivity Synthetic unstable nuclei decompose more stable nuclei
Radioactive Decay Series
Decay of P-32 to S-32
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
Nuclear Stability
•
The stability of a nucleus the mass number and depends mainly on A , 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 protons. As Z gets bigger, than 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.
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,
Empirical rules for predicting stability of nuclei
Neutron-to-proton ratio number varies with atomic Light isotopes (small atomic number) have a Neutron-to-proton ratio almost = 1(almost stable)
12 14 6
C
7 N 8 16 O
Nuclei are held together by strong attractive forces; but electrostatic repulsion causes large atoms (>83 protons) to be unstable.
206 82
Pb
; n p 1 .
51
Nuclides with
even
number of nucleons (
p +n) odd
are
more
number stable than those with
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)
Types of radiation emitted in natural radioactivity
Types of radioactive decay
radiation = attracted towards negatively charged plate
Positively charged
4 2
He
radiation = attracted towards positively charged plate
Negatively charged = 1e -
0 1
e
radiation = not attracted to either plate Neutral.
When emitted it does not change atomic or mass numbers Very high energy photons; very short wavelength . Positron is a positive electron
1
e
Positron emission is equivalent to a fall of e -1 in nucleus
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
: 1. Proton 2. Neutron 3. Electron 4. Positron 5. Gamma ray 1 1 H 1 0 n 0 1 e or 0 1 0 0 e or 0 1 0 1
Balancing nuclear equations Protactinium 1) Total Nucleon Number (TOP VALUES) =Total number of protons and neutrons 2) Total electric charge (BOTTOM VALUES) Are kept the same.
•
Nuclear reaction is written maintaining mass and charge balance.
E.g.
14 6 C 14 7 N
+
0 1 e
+
•
Examples of adioactive decay
Beta emission: Converts neutron into a proton by emission of energetic electron; atomic # increases:
1 0 n 1 1 p 0 1 e
E.g. Determine product for following reaction:
40 19 K 0 1 ?
Alpha emission
: emits He particle .
E.g. Determine product:
226 88 Ra ?
4 2 He
Positron emission : Converts proton to neutron:
1 1 p 1 0 n 0 1 e 94 53 Tc ?
0 1 e
Gamma emission : no change in mass or charge but usually part of some other decay process.
E.g.
14 6 C 14 7 N 0 1 e
Electron capture : electron from electron orbitals captured
1 1
to convert proton to neutron
.
p 0 1 e 1 0 n
E.g. Determine product
40 19 K 0 1 e ?
More examples of radioactive decay Alpha production (
): helium nucleus,
238 92 U 4 2 He 234 90 Th
Beta production (
):
234 90 Th 234 91 Pa 0 1 e 0 1 e
Examples of radioactive decay Gamma ray production
(
):
238 92 U 4 2 He 234 90 Th 2 0 0
Positron production
:
22 11 Na 0 1 e 0 1 10 e Ne
Electron capture: (inner-orbital electron is captured by the nucleus)
201 80 Hg 0 1 e 201 79 Au 0 0
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 unit time
(
N
t
)
in the number of nuclides per is directly proportional to the number of nuclides
Rate ΔN α N Δt
That is N
Rate ΔN Δt kN
Original # of nuclides This is a first order process
ln( N o ) N kt
# of nuclides remaining at time t
Half-Life
The time required for the number of nuclides to reach half the original value (N 0 /2).
t
k k
N N o e kt
N
N o e 0693t t 1 /2
Examples of Half-Life Isotope Half life
C-15 2.4 sec Ra-224 Ra-223 3.6 days 12 days I-125 C-14 U-235 60 days 5700 years 710 000 000 years
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
.
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.
14 7 N 4 2 He 17 8 O 1 1 H
Rutherford
27 13 Al 4 2 He 30 15 P 1 0 n
Irene Curie
249 98 Cf 18 8 O 263 106 X 4 0 1 n
•
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
238 92 U 1 0
n
238 U 92 238 N
p
93 0 1 p •
Since 1940, elements with atomic numbers 93 through 112 have been synthesized
•
These elements are called transuranium elements
Schematic diagram of a cyclotron Nucleus Positive ion
A Schematic Diagram of a Linear Accelerator
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) = 3.7x10
10 disintigrations ×s
1
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 -
Scintillation counters
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).
14 7 N + 1 0 n
14 6 C + 1 1 H
and then over time C-14 decays
14 6 C
14 7 N + 0 -1 e
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 (1/10 12 ) because of the continuous uptake of carbon.
When the plant/animal dies, C-14 decays and the ratio decreases t 1/2 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
Age of rocks/Age of earth
U-238 Pb-206 present in certain rocks slowly decays to Pb-206 was not present originally As time progresses the amount of and Pb-206 increases U-238 decreases By measuring the ratio of Pb-206 / U-238 can determine the age of a rock scientists 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
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,
Medical applications of radioactivity
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 from its constituent protons and neutrons.
formed
For example, the hypothetical process of forming
6 18 O
nucleus from eight neutrons and eight protons:
What is the change in energy that correspond to the formation of 1 mol of O-16 from its protons and neutrons?
E
mc
2
c
3 .
00 10 8
m
/
s
;
m
0 .
1366
g
/
mol
m
1 .
366 10 4
Kg
/
mol
Thus, = (-1.366X10
-4 kg/mol)(3.00X10
8 m/s 2 ) = -1.23X10
13 J/mol Consequently:
Nuclear processes are accompanied with extremely large energy compared to chemical and physical changes
Nuclear processes constitute a potentially valuable energy resource
Thermodynamic stability of a particular nucleus is represented as energy released per nucleon
Calculate the energy released per a nucleon of O-16
ΔE per 16 O nucleus 8 1 .
23
X
10 13
J
/
mol
6 .
022
X
10 23
nuclei
/
mol
2 .
04
X
10 11
J
/
nucleus
ΔE per nucleon for 16 8 O 1 .
X
2 .
04 / 16 1 .
6
X
10 13 10
J
11 1
J nucleons
/ /
nucleus
7 .
98
MeV nucleus
/ 1 .
28
X
10 12
J
/
nucleon
1 .
28
X
10 12
J
/
nucleonX
1 .
6
X
10 13
J
1
MeV
7 .
98
MeV
/
nucleon
Thus, 7.98 MeV of energy per nucleon would be released if O-16 were from neutrons and protons
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
Calculation of binding energy
Calculate the binding energy per nucleon
4
for nucleus.
2
He
(Atomic masses =4.0026 amu,
2 1 1
H
1.0078 amu)
We must calculate the mass defect for
He-4
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
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.
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
• 56 Fe has highest E b isotope.
and is most stable •Energy sources:
Nuclear fusion
–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.
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.
Both Fission and Fusion Produce More Stable Nuclides
Nuclear Fission
Several isotopes of the heavy elements undergo fission if bombarded with neutrons of high enough energy In practice attention was paid to
235 U 92
and
239 P
u
94
The discussion will focus on
235 U 92
That is only 0.7% of the naturally occurring U
238 U 92
is most abundant isotope and does not go fission
235 U 92
Fission
235 92 U + 1 0 n
236 92 U * and 10 -14 236 92 U *
seconds later...
92 36 Kr + 141 56 Ba + 3 1 0 n + ENERGY
50 possible sets of fission products (sum of atomic numbers = 92)
3 neutrons released for ONE 235 92 U (too many for stability, thus fragmentation continues to reach stability)
Fission Process
Chain Fission Reactions
Produced neutrons will attack more and
U 92
This chain reaction occurs in the atomic bomb. Energy is evolved in successive fissions that will lead to tremendous explosion large (critical mass),
235 92 U
thus most neutrons are captured
92 U •
If the sample is too small most neutrons escape braking the chain
Fission Produces a Chain Reaction
Nuclear Fission
A self-sustaining fission process is called a chain reaction .
Event
subcritical critical supercritical
Neutrons Causing Fission
< 1 = 1 > 1
Result
reaction stops sustained reaction violent explosion
Fission Produces Two Neutrons
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
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
A Schematic Diagram of a Nuclear Power Plant
A Schematic Diagram of a Reactor Core
Breeder Reactors
Fissionable fuel is produced while the reactor runs
238 92
U
is changed (split) to fissionable
239 94 Pu 1 0 92 n U 239 94 Pu U 239 92 U 239 92 U 239 93 Np 0 1 e 239 93 Np 239 94 Pu 0 1 e • • •
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
Breeder Reactors Fissionable fuel is produced while the reactor runs ( is split, giving neutrons for the 239 94 Pu 238 92
U
creation of ): 1 0 n 238 92 U 239 92 U 239 92 U 239 93 Np 0 1 e 239 93 Np 239 94 Pu 0 1 e
Fusion
Large quantities of energy are produced by the fusion of two light nuclei to give a heavier one
1 3 4 1 1
H
1
H
2
He
0
n
Energy
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:
Proposed mechanism for reactions on the sun T
1X10 9 o C; E
1X10 19 kJ/day
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 (10 6 m/s) to overcome the electrostatic repulsion.
High temperatures are expected from various sources that are under study
Effects of Radiation Factors that make the biological effects
1.
The energy of the radiation.
The higher the energy the more damage it can cause. Radiation doses are measured in rads (radiation absorbed dose), to 10 -2 where 1rad corresponds 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,
can penetrate approximately 1 cm, and particles
particles are stopped by the skin.
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, of damage.
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 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)
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