Nuclear Change

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Nuclear Change

• Radiation and Radioactivity

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Change

• We have learned about
• Physical change when atoms retain their
  identities and chemical compositions
• Chemical change when atoms are rearranged
  forming new substances
• We are now going to learn about
• Nuclear change when atoms form new atoms with
  different identities and properties. A change in
  the atoms nucleus occurs!

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Comparison of Chemical Nuclear
Chemical Reaction Nuclear Reaction
Occur when bonds are broken and formed Involve only valence electrons Small energy changes Atoms keep the same identity Occur when nuclei combine, split, and emit radiation Can involve protons, neutrons, and electrons Large energy changes Atoms are converted into different elements
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Check for understanding

• What type of change are the following?
• H2O (l) ? H2O (s)
• 2H2 (g) O2 ? 2H2O (l)
• 21 H (g) 21H (g) ? 42He (g)
• He (g) ? He (l)

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Discussion

• What do you think of when we talk about
  radioactivity?
• Can you give some examples of materials that are
  radioactive?
• What are some uses for radioactivity?

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Common radioactive elements

• Uranium
• Plutonium
• Radon
• Radium
• Bismuth
• Carbon

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Uses of Radioactivity

• Medicine
• X-rays, PET scans, MRI
• Sterilizing bandages
• Cancer treatment
• Power
• Nuclear power plants generate 1/5 of our
  electricity
• Weaponry
• Bombs
• Sun
• Industry
• Mass spectroscopy
• Carbon dating

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Essential Questions

• Who discovered radiation or radioactivity?
• Why do we have radiation?
• What are the different types of radiation?

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Who discovered of radioactivity?

• In 1895, Wilhelm Roentgen discovered the
  existence of X-rays, though the mechanism behind
  their production was not yet understood. He
  named the radioactive rays after the traditional
  unknown X.

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Radioactivity

• In 1896, Henri Becquerel discovered that uranium
  salts emitted rays that resembled X-rays in their
  penetrating power.

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Radioactivity

• Marie Curie was a French-Polish physicist and
  chemist, famous for her pioneering research on
  radioactivity.
• She coined the word, radioactivity.
• Marie continued work on uranium salts and in
  1898, determined that the radiation was dependent
  on the quantity of the substance not an
  interaction with other molecules.

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Radioactivity

• In 1899, Ernest Rutherford, a British scientist,
  began to classify radiation alpha (a), beta (b),
  and gamma (g).

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Why do we have radiation?

• For most stable atoms, the number of protons
  roughly equals the number of neutrons.
• But, if an isotope has too many more neutrons
  than protons, the nucleus becomes unstable.
• When this happens, the nucleus needs to shed some
  neutrons in the form of decay.

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Remember - The Atom
The atom consists of two parts
1. The nucleus which contains
protons
neutrons
2. Orbiting electrons.
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Because of isotopes, there are many types of
uranium
Mass Number
Number of protons
Number of neutrons
Mass Number
Number of protons
Number of neutrons
235
238
92
92
143
146
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Band of Stability

• Graph of the number of protons against the number
  of neutrons.
• Stable atoms have the ideal ratio of protons to
  neutrons
• Unstable will release radiation until they are
  stable

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Isotopes

• Unstable isotopes can become stable by releasing
  radiation in the form of energy and different
  types of particles.

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Types of Radioactive Decay
Radioactive decay results in the emission of
either

• Alpha decay When the nucleus spins off 2
  protons and 2 neutrons
• Beta decay When a neutron in the nucleus
  converts to a proton and electron and then keeps
  the additional proton while shooting off the
  electron.
• Gamma decay accompanies the other two types of
  decay but does not change the mass or charge.

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Types of Radioactive Decay
Radiation Type Symbol(s) Charge



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Radiation
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Types of Radioactive decay

• Rutherfords experiment
• http//www.youtube.com/watch?vvuGvQjCOdr0 (144)

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Alpha Decay

• When an alpha particle, containing two protons
  and two neutrons, is ejected from the nucleus.
• An alpha particle is identical to the nucleus of
  a helium atom
• The mass number decreases by 4 and the atomic
  number decreases by 2.

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Alpha Decay
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Beta Decay

• Beta particle is a fast moving electron which is
  emitted from the nucleus of an atom undergoing
  radioactive decay.
• Beta decay occurs when a neutron changes into a
  proton and an electron.

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Beta Decay

• As a result of beta decay, the nucleus has one
  less neutron, but one extra proton.
• The mass number stays the same and the atomic
  number increases by 1 and

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Beta Decay
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Gamma Decay
Gamma rays are not charged particles like a and b
particles.
Gamma rays are electromagnetic radiation with
high frequency.
When atoms decay by emitting a or b particles to
form a new atom, the nuclei of the new atom
formed may still have too much energy to be
completely stable.
Gamma decay does not result in the creation of a
new element!
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Other Types of Nuclear Reactions
positron
proton -
neutron -
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Nuclear Equations

• Because we know that mass is conserved in every
  chemical reaction, we know that the conservation
  of matter is maintained even in radioactive
  decay!
• So, can we use this information to write balanced
  nuclear equations?

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Alpha Decay
unstable atom
alpha particle
New more stable element
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Alpha Decay
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Alpha Decay
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Beta Decay
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Beta Decay
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Beta Decay
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Check for understanding

• Work on problems on worksheet Radioactive Decay
  Worksheet
• Then with a partner, use the cards to solve the
  decay series of uranium. Be sure to indicate the
  type of decay in each step.

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Radioactive Decay Rates

• Calculating half lives

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Radioactive decay

• Radioactive decay rates are measured in
  half-lives
• Half life the time required for one half of the
  nuclei to decay into its products
• The decay continues until there is a negligible
  amount of the radioactive isotope remaining.

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Calculating Halflives Example

• An Isotope of cesium-137 has a half-life of 30
  years. If 1.0 g of cesium-137 disintegrates over
  a period of 90 years, how many grams of
  cesium-137 would remain?

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The solution
Time (yrs) Mass (g)
0 1.0
30 0.5
60 0.25
90 0.125
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Essential Question

• So youve talked about carbon-14 dating, how does
  it actually work?

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Carbon Dating

• In the 1950s W.F. Libby and others (University of
  Chicago) devised a method of estimating the age
  of organic material based on the decay rate of
  carbon-14.
• Carbon-14 dating can be used on objects ranging
  from a few hundred years old to 50,000 years old.

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How does Carbon-14 dating work?

• http//videos.howstuffworks.com/discovery/29401-as
  signment-discovery-carbon-dating-artifacts-video.h
  tm

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Decay Rate

• Formula
• N N0(1/2)t/T
• Where
• N the remaining amount
• N0 the initial amount
• t elapsed time
• T duration of the half life

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Practice

• 1. The half life of cobalt-57 is 270 days. How
  much of a 5.000mg sample will remain after 810
  days?
• N ??
• N0 5.000 mg
• t 810 days
• T 270 days

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Practice

• 2. Krypton-85 is used in indicator lights of
  appliances. The half-life of kypton-85 is 11
  years. How much of a 2.000 mg sample remains
  after 33 years?
• N ??
• N0 2.000 mg
• t 33 years
• T 11 years