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Ch. 22: Nuclear Chemistry

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Ch. 22: Nuclear Chemistry Nuclear forces, and radioactive decay Alpha, beta, and gamma particles Transmutation, disintegration series, & carbon dating – PowerPoint PPT presentation

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Title: Ch. 22: Nuclear Chemistry


1
Ch. 22 Nuclear Chemistry
  • Nuclear forces, and radioactive decay
  • Alpha, beta, and gamma particles
  • Transmutation, disintegration series, carbon
    dating
  • Fission vs. fusion
  • Effects of radiation

San Onofre Nuclear Power Plant San Clemente,
CA (61 miles south of Los Angeles)
2
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

3
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

4
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 Einsteins Emc2 and this energy is
    called as Nuclear Binding Energy. The greater
    the nuclear binding energy, the more stable is
    the atom.

5
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.
  • Find mass defect
  • Convert amu to kg 1 amu1.6605 10-27 kg
  • ANSWER 4.3610-11 J.

6
Band of Stability
Stable, naturally occurring isotopes
  • For low atomic numbers the stable nuclei are
    those with a ratio of protons to neutrons approx.
    11
  • As the atomic increases,
  • The stable ratio increases to about 1.5 1.
  • The trend explained by nuclear forces.

1
7
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.

8
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 changestransmutation

9
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 radiation-related illnesses.)
  • Radioactive Decay spontaneous disintegration of
    a nucleus into a slightly lighter more stable
    nucleus, accompanied by emission of particles,
  • electromagnetic radiation or both.

10
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

11
Types of Radioactive Decay
Type Consists of Stopped by Interesting Fact



1. alpha A He nucleus 2 p 2 n Paper or skin If ingested, is harmful to lungs
2. beta b High energy e- Clothing, glasses, or thin sheet of Al Causes damage to sensitive tissues like eyes
3. gamma g Photon (particle of light) Has essentially no mass (m lt 5.81 x 10-72 gits complicated!) A few feet of dirt or concrete, or 6 of Pb Causes severe damage to body tissues
12
  • 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

13
  • Alpha emission

14
  • 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

15
  • Beta emission

16
  • 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

17
  • Positron emission

18
  • 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

19
  • Electron capture

20
  • 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

21
  • Gamma emission

22
  • Electromagnetic spectrum

23
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)

24
Disintegration Series (Decay Series)
  • Heavy atoms (greater than Bismuth, 83)
    naturally decay to smaller atoms along a
    consistent path, or series, of decays.

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/urani
um.html
25
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.
  • 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/n
uclear-bomb3.htm
26
  • 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 Emc2

27
A Fission Chain Reaction
28
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.

29
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 10,000,000C.
  • Produces shock waves and a, b, g, x-rays, and UV
    radiation.

30
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.

31
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.

32
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.

33
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.

34
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.

35
Biological Effects of Radiation Acute
  • High level radiation (gamma ray xray) 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.
36
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 Xrays
  • Sr-90 isotopes are present in fallout from
    atmospheric testing of nuclear weapons.
  • Contaminated foods can increase incidence of
    leukemia and bone cancers.
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