Radio Activity and Carbon Dating

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Radio ActivityandCarbon Dating

• Mr. Bartelt

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Background on radioactivity

• Radioactivity was first discovered in 1896 by the
  French scientist Henri Becquerel while working on
  phosphorescent materials.
• Wrapped a photographic plate in black paper and
  placed phosphorescent minerals on them.
• All results were negative until he tried using
  uranium salts. The result with these compounds
  was a deep blackening of the plate.

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• The blackening of the plate had nothing to do
  with phosphorescence because the plate blackened
  when the mineral was kept in the dark.
• The key was uranium. Non-phosphorecent uranium
  turned the plate black. Even uranium metal
  worked.

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• At first it seemed that the new radiation was
  similar to the then recently discovered X-rays.
  However further research by Becquerel, Marie
  Curie, Pierre Curie, Ernest Rutherford and others
  discovered that radioactivity was significantly
  more complicated. Different types of decay can
  occur.

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Different types of decay

• By applying a magnet to radioactive material
  three distict types of radiation were observed.
  Alpha, Beta, and Gamma were the intial generic
  names. No one got around to changing the name,
  and so the names stuck.

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Whats going on here???

• Determine the charge of each of the three forms
  of decay
• Which one is the MOST massive?

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Alpha Decay

• The nucleus emits a helium nucleus (an alpha
  particle)
• Uranium alpha decays to Thorium
• Generic equation

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Alpha Decay

• Why does this happen?
• Alpha decay tends to take place when the nucleus
  grows too large.
• Uranium is the largest element found in nature.
• Lead-206 is the largest STABLE element

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Vocab

• The helium4 nucleus is referred to as an alpha
  particle.
• Alpha decay is caused when a nuclide is too large.

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Practice problem

• Write the equation for when Radium 226 undergoes
  alpha decay.
• Fill in the missing nuclide.

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Beta decay

• A neutron is converted into a proton and an
  electron. The electron is rocketed out of the
  nucleus with great speed.

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Warning!!!

• Do not think of a neutron as being made up of a
  proton and an electron! Also emitted in beta
  decay is a particle called an antineutrino.
  Wikipidia it if you want more information .
• Neutron ? Proton Electron

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Beta Decay

• Why does this happen?
• Beta decay tends to occur when there are too many
  neutrons.
• If the neutronproton ratio is too HIGH beta
  decay may occur.

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Vocab

• An electron is referred to as an beta particle.
  It is sometimes indicate as below

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Practice problem

• Write the equation for the beta decay of
  Cobalt60.
• Fill in the missing nuclide

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Positron emission

• The conversion of a proton into a neutron by the
  emission of a positron decreases the atomic
  number by 1
• Generic equation

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Electron capture

• The nucleus captures an orbiting electron
• A proton is converted into a neutron

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Positron emission and electron capture

• Look at the equations below
• Note Both types of decay result in the same
  product... They both do the same thing

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Positron emission and electron capture

• Why does this happen?
• Both occur when there are too many protons
• When the neutronproton ration is too LOW

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Nuclear stability

• This graph is called the band of stability
• You dont need to memorize this!!!

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But whats happening INSIDE?

• We havent talked about whats happening inside
  the nucleus yet.
• There are protons in the nucleus.
• Protons have positive charge
• Positive charges repel one another
• Why doesnt the nucleus explode?

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Strong nuclear force

• The strong nuclear force-
• A full discussion of this topic is not
  appropriate at this stage in your education.
• The SNF is the force that holds the nucleus
  together.
• This force acts at VERY short distances
• The presence of neutrons in the nucleus lends
  stability to the protons and helps hold the
  nucleus together.

If youre asked what holds the nucleus together
you should write, Strong nuclear force.
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Nuclear binding energy

• There is a repulsive force between the protons in
  the nucleus.
• The SNF attracts all particle in the nucleus to
  each other. This attractive forces is equal and
  opposite to the repulsive force between the
  protons.
• This attractive force requires energy, but where
  does this nuclear binding energy come from???

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The mass defect

• The mass defect is a little abstract.
• From experiments we have calculated the exact
  mass of a free proton and a free neutron.
• Strangely, the masses of a proton and a neutron
  are lower in the nucleus than when they are free.
• WHY???

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Emc2

• Energy and mass are interchangeable

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Mass defect explained

• Einstein's famous equation Emc2 states that
  energy is equal to mass times the speed of light
  squared.
• Thus, small amounts of mass can be converted into
  tremendous amounts of energy.
• The lost mass is converted into the energy
  required to hold the nucleus together (the
  nuclear binding energy)

Mass defect Nuclear binding energy
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Any Burning Questions?
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The decay series
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A potential test question
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No more lecture

• Now we need to balance nuclear equations

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A brief lesson in statistics

• AKA Avogadros number is huge

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Basic statistics

• Why is the world series of baseball a seven game
  series?
• Why not just have a one game play off?
• If you flip a coin once and it lands on heads,
  can you conclude that it will and on heads 100
  of the time?
• Why not?

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Intermediate statistics

• If you roll a six-sided die 36 times and it lands
  on the number six seven times can you conclude
  that the probability of rolling a six is 7/36?
• Why not?

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Advanced statistics

• If you flip a coin 1,000,000,000 times and it
  lands on heads 500,161,803 times, can you
  conclude that the probability of rolling heads is
  about 50?
• Why?

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Graduate statistics

• If half of your sample of a whopping
  602,214,150,000,000,000,000,000 iodine-131 atoms
  decay in 8 days, can you conclude that the half
  life of iodine 131 is about 8 days?
• Why?

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Teacher statistics

• For random events
• As your basis set goes to infinity, your margin
  of error goes to zero
• Avogadros number is so big, it give most (if not
  all) nuclear and chemical reactions a basis set
  that can be thought of as being infinite.
• Therein lies the beauty of chemistry

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Carbon Dating
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The carbon-14carbon-12 ratio

• Atomic mass of carbon is 12.0107g/mol
• This means most carbon found in nature has a mass
  number of 12
• A small percentage of carbon atoms have a mass
  number of 14
• This is a good thing because

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Carbon-14

• Carbon-14 undergoes beta decay at a predictable
  rate.
• The half-life of this reaction is 5730 years

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How does it work?

• All living creatures consume carbon.
• Plants take in CO2 from the atmosphere and then
  we eat the plants.
• Or we eat what ate the plants.
• Or we consume the predator that devoured the
  herbivore that ate the plant.

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This fits in perfectly with the
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How does it work?

• Once we die we cease to consume carbon.
• Without the constant uptake of C-14 from the
  atmosphere any flesh or decaying plant materials
  C-14 level will steadily drop from the moment it
  stops consuming.
• We then find it, measure the C-14 to C-12 ratio,
  and with a little mathWe know more or less how
  old it is.

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Important!

• We are constantly eating carbon that not too long
  ago came from the atmosphere.
• Atmospheric carbon has a fixed C-14 to C-12
  ratio.
• There is an important question that I hope one of
  you is thinking of right now

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Why is this important?

• Archeologists can find the ages of bones
• What is paper made out of?
• We can find the age of anything written on paper
• Anything painted on canvas
• Many dyes come from fresh plants
• The uses go on and on

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Sample problems

• The half life of carbon-14 is 5730 years. A
  sample of bone that would contain 800 g of
  carbon-14 were it recently dead is found to
  contain only 50 g of carbon-14?
• Use the equation below
• time (half-life)( of half-lives)

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Sample problems

• The half life of thorium-234 is 24.5 days. How
  long will it take for an 800 g sample to decay
  down to only 50 g?
• Use the equation below
• time (half-life)( of half-lives)

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Sample problems

• The half-life of protactinium-234 is 1.14
  minutes. How many milligrams of protactinium-234
  remain after 5.70 minutes if you start with 2.0
  mg of the isotope?

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Uses of Radioactive Material

• Where do I even begin?

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Carbon Dating
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Medicinal

• Radioactive Isotopes can be used as medicinal
  tracer element to pinpoint tumors and other
  bodily abnormalities

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Agriculture

• Radioactive Isotopes can be used as agricultural
  tracer elements as well. Scientist can use
  radioactively tagged elements to follow the
  flow of pesticides and fertilizers.

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Nuclear Power

• Pro Nuclear power has low green house gas
  emissions
• Con Radioactive waste
• Any one heard of a place called Chernobyl or
  Three Mile Island?

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Weapons

• Atomic weapons have been in use since the mid
  1940s
• Not all uses of radioactive isotopes are
  beneficial

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Vocabulary you need to know

• Mass defect- difference between the sum of all
  protons and neutrons and the actual mass of the
  nucleus. Mass is converted into the binding
  energy of the nucleus.
• Gamma rays-High energy electromagnetic radiation
  (photons)

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Vocabulary you need to know

• Parent nuclide-The nuclide before it has
  undergone a form of radioactive decay.

• Daughter nuclide-The nuclide that results from
  after decay has taken place

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Shielding

• Shielding- What is required to absorb nearly all
  radiations from a specific type of radioactive
  source.
• Alpha particles- Skin or a sheet of paper
• Beta particles- Lead plate, glass
• Gamma rays- Thick lead or cement, passes through
  nearly all material unless that material is quite
  thick

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Nuclear Fission vs. Fusion

• Fission- A large radioactive nuclide breaks down
  into a smaller daughter nuclide. Usually alpha
  decay.
• Fusion- Small elements are forced to create
  larger nuclides. Can only take place in a
  particle accelerator, a star, or when bombarded
  by high velocity particles

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AP Chemistry Problem

• The half-life of cromium-55 is about 2.0 hours.
  The delivery of a sample of this isotope from the
  reactor to a certain laboratory requires 12
  hours. About what mass of such material should
  be shipped in order that 1.0 mg of cromium-55 is
  delivered to the laboratory?