Transcript Powerpoint - mvhs

Ch. 22: Nuclear Chemistry
San Onofre Nuclear Power Plant
San Clemente, CA
(61 miles south of Los Angeles)
• Nuclear forces, and
radioactive decay
• Alpha, beta, and gamma
particles
• Transmutation,
disintegration series, &
carbon dating
• Fission vs. fusion
• Effects of radiation
The Nucleus
• Atomic nuclei are made of protons and
neutrons, which are collectively called
nucleons
• An atom is referred to as a nuclide and is
identified by the number of protons and
neutrons in its nucleus.
• Two types of notations: Radium-228 or 228Ra
88
Factors Affecting the Stability
of Nuclei
• Why are some atoms radioactive and others
are not?
• There are three major factors:
1. Nuclear Binding Energy (Mass Defect)
2. Band of Stability (n/p ratios)
3. Magic Numbers
Mass Defect and Nuclear Stability
• The experimental observations show that the
mass of a nucleus is always less than the sum
of masses of its constituent protons and
neutrons. This “missing mass” is called as Mass
Defect. This “missing mass” is converted to
energy according to Einstein’s E=mc2 and this
energy is called as “Nuclear Binding Energy”.
The greater the nuclear binding energy, the
more stable is the atom.
Sample Problem
•
Calculate the nuclear binding energy of a
sulfur-32 atom. The measured atomic mass
of a sulfur-32 is 31.972 070 amu.
1. Find mass defect:
2. Convert amu to kg; 1 amu=1.6605 *10-27 kg
ANSWER: 4.36*10-11 J.
Band of Stability
Stable, naturally occurring isotopes
For low atomic
numbers the stable
nuclei are those with a
ratio of protons to
neutrons approx. 1:1
As the atomic #increases,
The stable ratio increases
to about 1.5: 1.
The trend explained by
nuclear forces.
1
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 126are called magic numbers.
Nuclear Reaction
• a reaction that affects the nucleus of an
atom.
• Occurs to increase stability
• Give off large amounts of energy
• Total of atomic #s & mass #s is equal on
both sides of equation.
• Identity of atom does not change till atomic
# changes, When atomic #changes, the
identity of element
changes=transmutation
Natural Radioactivity
Discovered in 1896 by Antoine Henri Becquerel, who
saw a uranium salt produce an image on a
photographic film.
The term radioactivity was coined in 1898 by Marie
Curie, a Polish physicist, who was doing research with
her husband Pierre. (They did much of the initial
work on radioactivity, and eventually died of radiationrelated illnesses.)
Radioactive Decay: spontaneous disintegration of a
nucleus into a slightly lighter & more stable nucleus,
accompanied by emission of particles,
electromagnetic radiation or both.
Half Life:
The time required for ½ the amount of a
radioactive material to disintegrate.
• Phosphorus-32 radioactively decays to form
Sulfur-32
• Half life
32P
= 14 days
Types of Radioactive Decay
Type
Consists of
1. alpha He nucleus:
A
2p&2n
2. beta High energy e-
b
3.
Photon (particle of
gamma light)
g
* Has essentially no
mass (m < 5.81 x 10-72
g…its complicated!)
Stopped by Interesting Fact
Paper or
skin
If ingested, is
harmful to lungs
Clothing,
glasses, or
thin sheet
of Al
Causes damage
to sensitive
tissues like eyes
A few feet
of dirt or
concrete,
or 6” of Pb
Causes severe
damage to body
tissues
• Alpha particle emissions:
– Helium nucleus: 2 protons and
2 neutrons, +2 charge.
– For large, unstable nucleus
which needs to reduce both
the number of protons and the
number of neutrons.
– Example:
210
84
Pο 
206
82
Pb  He
4
2
• Alpha emission
• Beta particle emissions:
– Electron emission, -1 charge.
– For unstable nucleus which
needs to reduce the number of
neutrons.
– A neutron is converted into a
proton and an electron, the
electron is given off as a beta
particle.
– Example:
14
14
0
6
7
1
C N
β
• Beta emission
• Positron emissions:
– Positron emission, +1 charge.
– For unstable nucleus which needs
to reduce the number of protons.
– A proton is converted into a
neutron and a positron, the
positron is emitted.
– Example:
10
6
C B
10
5
0
1
β
• Positron emission
• Electron capture:
– An inner orbit electron combines with a proton and
forms a neutron.
– For unstable nucleus which needs to reduce the
number of protons.
– Example:
7
4
Be 
0
1
e  Li
7
3
• Electron capture
• Gamma emissions:
– High energy electromagnetic
waves (photons) like visible light,
except with a shorter wavelength.
– For high energy nucleus when it
jumps down from an excited state
to a ground state.
– Example:
3
2
He  He  γ
3
2
• Gamma emission
• Electromagnetic spectrum
Arificial Transmutation
• First accomplished by Rutherford in 1919, even though
alchemists tried for hundreds of years.
• Transmutation of lead into gold was achieved by Glenn
Seaborg, who succeeded in transmuting a small quantity of
lead in 1980. He also first isolated plutonium for the atomic
bomb and discovered/”created” many elements. (NY Times,
Feb 1999)
• There is an earlier report (1972) in which Soviet physicists at a
nuclear research facility in Siberia accidentally discovered a
reaction for turning lead into gold when they found the lead
shielding of an experimental reactor had changed to gold.
• Accomplished with particle accelerators like the Stanford Linear
Accelerator (SLAC)
Disintegration Series (Decay Series)
• “Heavy” atoms (greater than Bismuth, #83) naturally decay to
smaller atoms along a consistent path, or series, of decays.
Atomic Number & Symbol
Mass Number
Radioactive U-238 → Th-234 + a
Th-234 → Pa-234 + b
Pa-234 → U-234 + b
U-234 → Th-230 + a
Th-230 → Ra-226 + a
Ra-226 → Rn-222 + a
Rn-222 → Po-218 + a
Po-218 → Pb-214 + a
Pb-214 → Bi-214 + b
Bi-214 → Po-214 + b
Po-214 → Pb-210 + a
Pb-210 → Bi-210 + b
Bi-210 → Po-210 + b
Po-210 → Stable Pb-206 + a
Source: http://www.frontiernet.net/~jlkeefer/uranium.html
Nuclear Fission
• Fission: process in which the nucleus of a large, radioactive
atom splits into 2 or more smaller nuclei
– Caused by a collision with a energetic neutron.
139 Ba + 94 Kr + 3 1n + energy
1 n + 235 U
56
36
0
0
92
• A neutron is absorbed by a U-235 nucleus. The nucleus is
now less stable than before. It then splits into 2 parts and
energy is released. Several neutrons are also produced; they
which may go on to strike the nuclei of other atoms causing
further fissions.
Fission animation: http://www.howstuffworks.com/nuclear-bomb3.htm
• *In a fission reaction that is working properly, more than one
neutron ejected from the fission reaction causes another
fission to occur. This condition is known as supercriticality.
• *The process of neutron capture and nucleus splitting happens
very quickly (takes about 1 x 10-12 seconds).
• An incredible amount of energy is released:
– As heat and gamma radiation
– Because the product atoms and neutrons weigh less than
the original U-235 atom; the “missing mass“ has been
converted to energy by E=mc2
*A Fission Chain Reaction
Harnessing Fission: A nuclear power plant
• Nuclear power plants utilize the energy released in a controlled
fission reaction in the core to heat water in one pipe, which is
used to vaporize water into steam in another pipe, which
drives a turbine and generates electricity.
•
The vaporized H2O is in a closed circuit, and is never exposed
to the radiation itself.
•
The speed of the fission chain reaction is regulated using
carbon control rods which can absorb extra neutrons.
The Atomic Bomb
• Uses an unregulated fission reaction in a very
fast chain reaction that releases a tremendous
amount of energy.
*Critical mass: the minimum amount of
radioactive, fissionable material needed to
create a sustainable fission chain reaction
* Site of fission reaches temperatures believed to
be about 10,000,000°C.
• Produces shock waves and a, b, g, x-rays, and
UV radiation.
*The classic “mushroom cloud” is a result of dust and
debris lifted into the air as a result of the detonation.
US Army aerial photograph from 80 km away, taken about 1
hour after detonation over Nagasaki, Japan, August 9, 1945.
Nuclear Fusion
Fusion: process in which 2 nuclei of small elements are united to
form one heavier nucleus
•
Requires temperatures on the order of tens of millions of
degrees for initiation.
•
The mass difference between the small atoms and the
heavier product atom is liberated in the form of energy.
•
Responsible for the tremendous energy output of stars (like
our sun) and the devastating power of the hydrogen bomb.
3H
1
+ 2H
1
4 He + 1 n + energy
2
0
*Stars & the Hydrogen Bomb
• The first thermonuclear bomb was exploded in 1952 in the
Marshall islands by the United States, the second was exploded
by the Russia (then the USSR) in 1953.
• “H bombs” utilize a fission bomb to ignite a fusion reaction.
Mass-Energy Relationship
•
*The energy that can be released from 2 kg
of highly enriched U-235 (as used in a
nuclear bomb) is roughly equal to the
combustion of a million gallons of gasoline. 2
kg of U-235 is smaller than a baseball; a
million gallons of gasoline would fill a
rectangular tank that is 50 feet per side.
Measurement of Radioactivity
ionizing radiation•
radiation from radioactive sources
•
When it strikes an atom or a molecule, one or more
electrons are knocked off and an ion is created
•
Measured with a Geiger counter, film badge or a
Scintillation counter
•
Curie-measures radioactivity emitted by a
radionuclide.
•
Roentgen & Rad - measures exposure to gamma
rays or X-rays
•
Rem- takes into account the degree of biological
effect caused by the type of radiation exposure.
*Biological Effects of Radiation:
Acute
• High level radiation (gamma ray & x–ray) causes death
• Damage centered in the nuclei of the cells; cells that are
undergoing rapid cell division most susceptible
• Gamma rays from a Co-60 source are often used to treat
cancer (cells that multiply rapidly)
Ionizing radiation: energy emitted from
radioactive matter; it can directly affect
(ionize) the structure of materials which it
passes through, including human tissue.
*Effects of Radiation: Long Term
• Long term exposure can weaken an an
organism and lead to onset of malignant
tumors, even after fairly long time delays.
• Largest source: X–rays
• Sr-90 isotopes are present in fallout from
atmospheric testing of nuclear weapons.
• Contaminated foods can increase incidence of
leukemia and bone cancers.