I: The Nucleus
nucleons = protons + neutrons
nuclide = an atom
ways of representing a nuclide
22888Ra or radium - 228
A: Mass Defect and Nuclear Stability
mass defect - definition
1. Nuclear Binding Energy
loss in mass when nucleons combine - why
mass is converted to energy according to Einsteins theory E = mc2.
nuclear binding energy - definition
it can also be thought of as the energy required to break apart the nucleus and thus is a measure of the stability of the nucleus.
2. Binding Energy per Nucleon
this is used to compare the stability of different nuclides
figure 22-1 page 702
binding energy per nucleon can be thought of as the binding energy of the nucleus divided by the number of nucleons
the higher it is the more tightly the nucleons are held together
elements with intermediate atomic masses have the greatest binding energies per nucleon and thus are more stable.
B: Nucleons and Nuclear Stability
Stable nuclides have certain characteristics.
Figure 22-2 page 703 plots the number of protons against the number of neutrons. We get a belt-like graph.
Band of stability - definition
Atoms with low atomic number have a ratio of one to one; as atomic numbers increase, the ratio increases to about 1.5 to 1 as in lead-206 with 124 neutrons and 82 protons.
Due to relationship between the nuclear force and the electrostatic forces between protons. Nuclear force is a short range force.
Figure 22-3 page 702 Short range nuclear force v electrostatic repulsion
Protons in a nucleus repel all other protons through electrostatic repulsion, but the short range of the nuclear force allows them to attract only protons very close to them.
As the number of protons in a nucleus increases, the electrostatic force between protons increases faster than the nuclear force. More neutrons are required to increase the nuclear force and stabilize the nucleus.
Beyond the atomic number 83, bismuth, the repulsive fore of the protons is so great that no stable nuclides exist.
Stable nuclei tend to have even numbers of nucleons.
Out of 265 stable nuclides, 159 have even numbers of both protons and neutrons. Only four nuclides have odd numbers of both. Thus, the stability of a nucleus is greatest when the nucleons - like electrons- are paired.
The most stable nuclides are those having 2, 8, 20, 28, 50, 82, or 126 protons, neutrons, or total nucleons. this extra stability at certain numbers supports a theory that nucleons- like electrons - exist at certain energy levels.
According to the nuclear shell model, nucleons exist in different energy levels, or shells, in the nucleus. The numbers of nucleons that represent completed nuclear energy levels, 2, 8, 20, 28, 50, 82 and 126 are called magic numbers.
C: Nuclear Reactions
Unstable nuclei undergo spontaneous changes.
Their number of protons and/or neutrons changes.
This permits them to release energy and become more stable.
Nuclear Reaction - definition
In such a reaction, the total of the atomic numbers on each side must be equal and the total of the mass numbers on each side must be equal.
e.g. 94Be + 42He ---> 126C + 10n
Transmutation - definition
Sample problem 22-1 page 704
Identify the product that balances the following nuclear reaction:
21284Po ---> 42He + ??
Solution: 4 + x = 212; x = 208
2 + x = 84; x = 82
Look up the atomic number 82 on the periodic table to get the symbol and it is lead, Pb.
II: Radioactive Decay
1896 Henri Becquerel, uranium compound, photographic plate, nuclear radiation due to radioactive decay.
Radioactive decay - definition
Nuclear radiation - definition
Radioactive nuclide - definition
Marie and Pierre Curie in 1896 discovered polonium and radium, which brought to four the number of radioactive nuclides known at that time, the other two were uranium and thorium.
All nuclides beyond atomic number 83 are unstable and radioactive.
A: Types of Radioactive Decay
A nuclides type and rate of decay depend on the nucleon content and energy level of the nucleus.
Table 22-1 page 705 Radioactive Nuclide Emissions
alpha particle, 42He, 2+, 4.002 60
beta particle, 0-1B, 1-, 0.000 5486
positron, 0+1B, 1+, 0.000 5486
gamma ray, g, 0, 0
1. Alpha Emission
alpha particle - definition
they are helium nuclei and have a charge of 2+.
symbol is 42He
emitted by very heavy nuclei since these nuclei must decrease their mass to become stable
e.g. 21084Po ---> 20682Pb + 42He
2. Beta Emission
Elements above the band of stability have too many neutrons and are unstable. A neutron can be converted into a proton and an electron. The electron can be emitted and this is a beta particle.
Beta particle - definition
10n ---> 11p + 0-1B
3. Positron Emission
Elements below the band of stability have too many protons. A proton can be converted into a neutron by emitting a positron.
Positron - definition
11p ---> 10n + 0+1B
e.g. 3819K ---> 3818Ar + 0+1B
4. Electron Capture
Occurs in electron decay for nuclides with too many protons.
electron capture - definition
The inner orbital electron combines with a proton to form a neutron.
0-1e + 11p ---> 10n
e.g. 10647Ag 0-1e ---> 10646Pd
5. Gamma Emission
Gamma rays - definition
Figure 22-6 page 707 - Position of gamma radiation in electromagnetic radiation spectrum.
This type of emission supports the nuclear shell model as gamma rays are produced when nuclear particles undergo transition in nuclear energy levels. Similar to emission of light when an electron goes from an excited state to its ground state.
Gamma emission usually occurs immediately following other types of decay, when other types of decay leave the nucleus in an excited state.
B: Half Life
Half Life - definition
Each radioactive nuclide has its own half life. More stable nuclides decay slowly and have longer half lives.
Table 22-2 page 708 - Representative Radioactive Nuclides and Their Half Lives
Sample Problem 22-2 page 709
Phosphorus-32 has a half life of 14.3 days. How many milligrams of phosphorus-32 remain after 57.2 days if you start with 4.0 mg of the isotope?
Given: original mass = 4.0 mg; half life = 14.3 days; time elapsed = 57.2 days; mass of phosphorus remaining = ?
How many half lives have passed:
C: Decay Series
Decay series - definition
Necessary since it is not always possible to become stable after a single nuclear reaction
Parent nuclide - definition
Daughter nuclide - definition
All naturally occurring nuclides with atomic numbers greater than 83 are radioactive and belong to one of three natural decay series.
The parent nuclides are uranium-238, uranium-235, and thorium-232.
Figure 22-8 page 710 Transmutation of uranium-238 to lead-206.
Notice the color of the arrows to indicate the particle emitted and the change in position of the daughter nuclide to correspond to the new atomic mass and atomic number. Also note the half lives are in each circle.
D: Artificial Transmutations
Artificial radioactive nuclides - definition
Artificial transmutations - definition
Neutrons are not repelled by anything in the nucleus since they have no charge and can, thus, penetrate the nucleus.
Charged particles can be used to penetrate the nucleus but they need a lot of energy to overcome repulsion and so particle accelerators are used.
Figure 22-9 page 711 Particle Accelerator at the Fermi Labs in Illinois.
1. Artificial Radioactive Nuclides
Radioactive isotopes of all the natural elements have been produced by artificial transmutation.
Figure 22-10 page 712 Artificial transmutation and the periodic table
Artificial transmutation has also been used to produce the transuranium elements - those elements with more than 92 protons in their nuclei. They are all radioactive.
Table 22-3 page 712 Reactions for the first preparation of several transuranium elements
III: Nuclear Radiation
Different types of nuclear radiation have different penetrating abilities.
Alpha: range = few centimeters in air; low penetrating ability due to large mass and charge. Cannot penetrate skin. Harmful if ingested or inhaled.
Beta particles travel at close to the speed of light and penetrate 100 times more than alpha particles. Range is a few meters in air
Gamma rays have greatest penetrating ability.
figure 22-11 Penetrating and shielding for radiation
A: Radiation Exposure
Radiation can pass its energy to atoms or ions and cause electrons to be ejected.
roentgen - definition
Radiation can damage living tissue. This damage is measured in rems (roentgen equivalent man).
Rem - definition
Cancer and genetic effects caused by DNA mutations are long term radiation damage to living tissue.
DNA can be mutated directly by interaction with radiation or indirectly by interaction with previously ionized molecules.
Radiation is all around us and we are exposed to about 0.1 rem per year in the US.
0.5 rem is the maximum permissible exposure
Those who live or work at high altitudes have greater exposure because of the increased cosmic radiation at those altitudes.
Radon-222 in homes. It is a gas and moves through soil and through cracks in foundations and and floors of basements.
B: Radiation Detection
Figure 22-12 page 714 Film badges and Geiger counter
Film badges use exposure of film to measure the approximate radiation exposure of people working with radiation.
Geiger counters are instruments that detect radiation by counting electric pulses carried by gas ionized by radiation. Used mainly for beta particle detection.
Radiation can also be detected when it transfers its energy to substances that scintillate, or absorb ionizing radiation and emit visible light.
Scintillation counters are instruments that convert scintillating light to an electric signal for detecting radiation.
C: Applications of Nuclar Radiation
Non radioactive and radioactive forms of an element generally have the same physical and chemical properties. This is the basis for what follows.
1. Radioactive Dating
Radioactive Dating is the process by which the approximate age of an object is determined based on the amount of certain radioactive nuclides present. If we know the half life of the nuclide we can estimate the age of the object. Common using carbon-14 with a half life of 5715 years. Can estimate age of organic substances up to 50 000 years old.
2. Radioactive Nuclides in Medicine
e.g. cobalt-60 used to treat cancer.
radioactive tracers - radioactive atoms that are incorporated into susbtances so that movement of the substances can be followed by radiation detectors.
Figure 22-13 page 715 use of tracers
3. Radioactive Nuclides in Agriculture
tracers used in fertilizer to judge effectiveness of the fertilizer
also used to prolong shelf life of food; kills bacteria and insects that spoil or infest food.
D: Nuclear Waste
1. Nuclear Fission and Nuclear Fusion
Fission breaks apart particles, such as the nucleus of uranium being split into two or more nuclei. Used in nuclear reactors, nuclear ships, and nuclear missiles.
Fusion is the opposite - particles are put together. Need very high temperatures - rivaling those on the sun. Using light atoms combined to form heavier atoms e.g. using hydrogen to form helium. Happens on sun and stars.
Both processes release energy which can be used to produce electricity and heat and both produce nuclear waste.
Fission produces more wast than fusion.
2. Containment of Nuclear Waste
All radioactive substances have a half life. e.g. radioactive waste from medical research has a half life of a few months while waste that is produced in nuclear reactors will take hundreds of thousands of years to decay and so it has to be stored in containers that will shield living organisms from its radiation.
Two types of containment: on site storageand off site disposal.
3. Storage of Nuclear Waste
Most common form of nuclear waste is spent fuel rods from nuclear power plants.
Contained above ground in water pools or dry casks made of steel and concrete. Usually first stored in water pools and then moved to dry casks and then moved to permanent underground storage facilities.
4. Disposal of Nuclear Waste
The disposal process is done with the intention of never retrieving the materials.
77 disposal sites around the US.
IV: Nuclear Fission and Nuclear Fusion
A: Nuclear Fission
Nuclear fission involves a very heavy nucleus splitting into more stablenuclei of intermediate mass.
Lots of energy is released.
The process can be spontaneous or it can occur when nuclei are bonbarded by particles.
Slow neutron captured by Uranium-235, which splits into two intermediate mass particles whose mass is less than the uranium -- the missing mass is converted into energy -- and the medium mass particles emit more neutrons which are captured by other uranium-235 atoms, etc.
1. Nuclear Chain Reaction
The production of more neutrons in our example above allows for the possibility of a chain reaction.
A chain reaction is a reaciton in which the material that starts the reaction is also one of the products and can start another reaction.
Figure 22-14 page 717 Fission of uranium-235 and chain reaction
A chain reaction such as this continues until there are no more uranium-235 nuclei or until the neutrons produced do not strike any uranium nuclei.
Critical mass is the minumum amount of nuclide that provides the number of neutons needed to sustain a chain reaction.
Uncontrolled chain reactions occur in the explosion of atomic bonbs.
Nuclear reactors use controlled fission chain reactions to produce energy or radioactive nuclides.
2. Nuclear Power Plants
Nuclear power plants use heat from nuclear reactors to produce electrical energy.
Components include a) shielding - radiation absorbing material that is used to decrease exposure to radiation, expecially gamma rays, from nuclear reactors; b) fuel - uranium-235 is usually used as the fissionable fuel to produce heat; c) coolant which absorbs the heat from the fission reaction; d) control rods - neutron absorbing rods that help control the reaction by limiting the number of free neutrons; e) moderator - used to slow down the fast neutrons produced by fission.
Things to consider: a) environmental requirements; b) safety of operation; c) plant construction costs; d) storage and disposal of spent fuel and radioactive wastes.
Figure 22-15 page 718 Model of Nuclear Power Plant
B: Nuclear Fusion
Stability of nuclei with intermediate masses can also be used to explain nuclear fusion.
Figure 22-16 page 719 Fusion reaction
Hydrogen bomb is the uncontrolled fusion reaction involving hydrogen. A fission reaction is used to provide the heat and pressure necessary to trigger the fusion of nuclei.
Problems with controlled fusion: no known material can withstand the inital temperatures, about 108 K.
Areas of investigation include containing the fusion reaction within a magnetic field or inducing fusion at lower temperatures.