Transcript Nuclear Chemistry

Nuclear Chemistry
Unit 4
History
Wilhelm Conrad Roentgen (1845-1923)
 Awarded a Nobel Prize in Physics in 1901
 Discovered X-Rays - November 8, 1895
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Saw a glowing fluorescent screen on a nearby
table and determined that the fluorescence
was caused by invisible rays that were able
to go through opaque black paper
Died of unrelated causes at 77, one of
few scientists to always use a lead shield
History
Antoine Henri Becquerel (1852-1908)
 Nobel Prize in Physics, 1903
 Discovered radioactivity
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Proved that uranium emitted radiation without an
external source of energy such as the sun
Discovered that radiation emitted by uranium shared
certain characteristics with X rays but could be
deflected by a magnetic field and therefore must
consist of charged particles
History
Pierre Curie (1859-1906)
 Nobel Prize in Physics, 1903
 Worked with his wife Marie to investigate
the phenomenon of radioactivity in
uranium ore
 Died in an accident crossing the street in
a rainstorm
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History
Marie Curie (1867-1934)
 Nobel Prize in Physics, 1903
 Nobel Prize in Chemistry, 1911
 Discovered the elements Polonium (Po)
and Radium (Ra)
 First person to win two Nobel Prizes
 Died of overexposure to radiation
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Transmutation
Involves reactions where the nucleus of
the atom is changed
 When the atom’s nucleus is changed,
radiation is emitted
 Transmutation:
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The process when the nucleus changes
atomic number and a new element is formed
Radioactive or not?
Not all elements are radioactive.
Isotopes (also called nuclides) that are
not radioactive are called stable isotopes
 Unstable isotopes:
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Isotopes that undergo nuclear reactions and
emit radioactivity
 All elements above atomic number 83 are
radioactive!
 Also known as radioisotopes or radionuclides
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Unstable Nucleus
In the nucleus, there is a lot of + charge,
so there should be an electrostatic force
pushing all of the protons apart
 This doesn’t happen – why?
 There is a second force that acts on the
protons when they are very close
together. It is called the strong force
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Strong Force
There is a “magic number” of protons
and neutrons that keep the nuclei stable
 When there are an even number of
protons and an even number of neutrons,
the nucleus is very stable
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Strong Force
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When there are an even number of
protons and an odd number of neutrons,
the nucleus is less stable
Strong Force
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When there are an odd number of protons
and an odd number of neutrons, the
nucleus is very unstable
Stability Factors
The ratio of neutrons to protons also
contributes to stability
 Maximum stability:
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Smaller atoms = Ratio of 1 proton for every 1
neutron (A 1:1 ratio.)
 Larger atoms = More neutrons than protons
(the extra mass keeps the atom stable)
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Increasing Atomic Number
It makes sense that as one adds more
protons, it would take more neutrons to
help hold the nucleus together
 Remember - all elements above Bismuth
(83) are radioactive!
 If the atomic number is less than 83,
radioactivity will be determined by the
number of protons and neutrons
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Radioactive Decay
Process where an unstable nucleus emits
particles and/or electromagnetic
radiation
 We say that the nucleus has
spontaneously disintegrated to produce a
new element
 Transmutation occurs naturally
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Artificial Transmutation
Also known as induced radioactivity
 A nucleus alters its identity by reacting
with or capturing a neutron or another
nucleus
 We say that we have changed or
transmuted the nucleus by bombarding it
with other particles
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Alpha Decay
Radioactive decay where an alpha
particle is emitted
 Alpha is the weakest type of radiation,
with the least penetrating power
 A sheet of paper can block alpha particles
 Alpha particles are Helium particles
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Alpha Decay, Continued
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There is always a conservation of mass
and charge
Mass number  238 = 4 + 234
 Atomic number  92 = 2 + 90
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Beta Decay
Radioactive decay where a beta particle
is emitted
 Beta particles have more penetrating
power than alpha. It would take a thin
sheet of aluminum or your hand to block
beta particles
 Beta particles are high speed electrons
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Beta Decay, Continued
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There is always a conservation of mass
and charge
Mass number  14 = 14 + 0
 Atomic number  6 = 7 + (-1)
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Gamma Decay
Radioactive decay where gamma
radiation is emitted
 Gamma radiation has the greatest
penetrating power
 Gamma rays are high energy photons
 Mass = 0, Charge = 0
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Other Radiation
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Neutron
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Proton
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Mass of 1, Charge is neutral
Mass of 1, Charge is positive
Positron (Opposite of Beta)
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Mass of 0, Charge is positive
Nuclear Symbols
Relative Strength of Radiation
Radiation Review
Separation of Particles
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Use an electric field to separate a mixture of
alpha, beta, and gamma radiation
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Alpha is +, so they are attracted to the - plate
Beta is -, so they are attracted to the + plate
Gamma is neutral, so it passes straight through
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Half Life
The amount of time needed for one half of
the nuclei of a substance to decay
 Any substance that is radioactive will
disappear over time as it changes into
other substances
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Half Life
Example
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A radioactive substance has a half-life of
20 minutes. If we begin with a 500 g
sample, how much of the original sample
remains after two hours?
Solution
The easiest way to attack these questions
is to start with the original amount of the
sample, then draw arrows representing
each half-life
 Two hours is 120 minutes, so that’s six
half-lives:
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500  250  125  62.5  31.25  15.625  7.8125
At the end of the stated time period, 7.8 g
remains
Half Life
Nuclear Reactions
Cause transmutation of elements with
the release of a large amount of energy
 These reactions are the source of electric
energy at nuclear power plants as well as
the energy from the Sun and stars
 This immense amount of energy comes
from the conversion of matter to energy
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Nuclear Reactions
The mass of a nucleus is not exactly
equal to the sum of the masses of its
nucleons
 This difference in mass means that some
nuclear mass is converted to energy
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Nuclear Fission
Nuclear Fission
When a neutron strikes a Uranium-235
nucleus it can cause the nucleus to break
apart into smaller nuclei
 The fission reaction produces smaller
nuclei as well as loose neutrons
 The loose neutron can strike the smaller
nuclei, causing that nuclei to divide
 This is known as a nuclear chain reaction
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Nuclear Fission
Nuclear Fission
Splitting a nucleus into smaller pieces
increases overall stability
 Fission = division (of the nucleus)
 Nuclear fission is used as a source of
electricity in nuclear power plants
 The most common fission reaction is the
fission of Uranium-235
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Nuclear Power
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When the chain reaction is controlled,
the energy can be captured and
converted into electricity
Nuclear Power
Advantages:
 Not as much fuel
needed
 No pollutants or
greenhouse gases
released
Disadvantages:
 Waste material is
extremely
radioactive, and stays
that way for
thousands of years
 They need fuel to
operate
Nuclear Fusion
Nuclei of smaller atoms join together to
form a larger atom
 Converts matter into large amounts of
energy
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Nuclear Power
Fission Animation
 Animations
 Fusion Animation
 Vision Learning
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