Nuclear Chemistry

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Nuclear Chemistry

• DHS Chemistry
• Chapters 4 and 25

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Little House on the Prarie
3
Review Structure of an Atom
4
Structure of an Atom

• An atom consists of three types of subatomic
  particles, protons, neutrons, and electrons.

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Structure of an Atom

• Protons and neutrons are located in the nucleus
• Electrons are in a cloud surrounding the nucleus.
  
• The number of protons is equal to the atomic
  number.

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Structure of an Atom

• If the atom is neutral, the number of electrons
  is equal to the number of protons.
• The mass number is equal to number of protons
  number of neutrons.

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Structure of an Atom

• Elements with the same number of protons, but
  have different numbers of neutrons is called an
  isotope.

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Structure of an Atom
Protons Neutrons Electrons
Symbol P no e-
Charge 0 -
Location Nucleus Nucleus Electron cloud
Relative mass 1 amu 1 amu negligible
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Whats a chemical reaction?
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Chemical Reactions
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Why Call it Nuclear Chemistry?
See unitedstreaming.com
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Chemical VS. Nuclear

• Nuclear reactions are caused from unstable nuclei
  becoming stable through radioactive decay.
• Releasing particles and high-energy waves
• Alters the number of nuclear particles (neutrons
  and protons).
• Nuclear reactions are very different from
  chemical reactions.

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Chemical Changes

• 1. Occur when bonds are broken and formed.
• 2. Atoms remain unchanged, though they may be
  rearranged.
• 3. Involve only valence electrons.
• 4. Associated with small energy changes.
• 5. Reaction rate is influenced by temperature,
  pressure, concentration, and catalyst.

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Nuclear Changes

• 1. Occur when nuclei emit particles and/or rays.
• 2. Atoms are often converted into atoms of
  another element. Thus their identity changes.
• 3. May involve protons, neutrons, and electrons.
• 4. Associated with large energy changes.
• 5. Reaction rate is not normally affected by
  temperature, pressure, or catalysts.

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What is an isotope?
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Isotope Examples
Mass Numbers
Atomic protons
Lithium - 6
Lithium - 7
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Isotopes

• Isotopes are atoms of the same element that vary
  in their number of neutrons, thus they have
  different mass numbers.
• The convention for writing isotope names is first
  the element name dash and then the mass number.

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For example Carbon-14, carbon-12 carbon-13

• How many protons and neutrons does each of the
  isotopes of carbon have?
• Carbon 14 8 neutrons, 6 protons
• Carbon 12 6 neutrons, 6 protons
• Carbon 13 7 neutrons, 6 protons

Mass Number (protons neutrons)
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More on Isotopes

• When an isotope is unstable it is called a
  radioisotope.
• To gain a more stable configuration, the nuclei
  emit radiation.
• The resulting stable atom is called the daughter
  product.
• This is called radioactive decay.

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Strong Nuclear Force

• The dense nucleus has two different kinds of
  nuclear particles (protons neutrons) closed
  packed together.
• The protons are positively charged nucleons.
• The neutrons are neutral nucleons.
• All of the protons in the nucleus repel each
  other and cause an electrostatic force that
  pushes the nucleus apart.

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• However, there is a force holding the nucleus
  together.
• The strong nuclear force is a force that acts
  only on subatomic particles that are extremely
  close together.
• If the strong nuclear force overcomes the
  electrostatic force, the nucleus stays together.

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Neutron to Proton Ratio

• The strong nuclear force is not always strong
  enough to overcome the electrostatic force. When
  this happens the nucleus breaks apart. The
  stability of the nucleus can be determined by the
  ratio of neutrons to protons.(n/p)

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Neutron to Proton Ratio

• Elements with low atomic numbers are most stable
  when the neutron to proton ratio is 11.
• As the atomic number increases, more and more
  neutrons are needed to overcome the electrostatic
  force.
• Thus, the stable ratio of neutron to proton
  increases as the atomic number increases.
• 1.51 is the largest ratio for a stable nucleus.

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Example Determine the neutron to proton ratio
for Lead-206. Is it stable?

• Lead 206
• 82 protons
• 124 neutrons (206 82)
• 124 1.51 ? stable
• 82 1 ratio

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Nuclear StabilityNeutron Protons

• Stable if
• Smaller than Fe 1
• 1
• Between Fe Pb
• 1-1.5
• 1
• Bigger than Pb
• 1.5
• 1

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Extra Practice

• Calculate the neutron to proton ratio, and
  determine if the isotope is stable.
• 1.
• 2. U- 293

22 ? 11
stable
20192 ? 2.181
unstable
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The Band of Stability

• The graph on the right plots the neutrons versus
  protons. The band created is called the band of
  stability. Anything that falls outside of that
  band is radioactive.
• All elements with an atomic number greater than
  83 are radioactive.

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III. Radioactivity
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Radioactive Substances

• Bananas
• Atomic fire ball candies
• Colored gemstones (blue topaz)
• Fiesta Ware
• Table ware that contained unsafe amounts of
  radioactive Uranium
• Uranium for color glaze. Up to 14 can be Uranium

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Geiger Counter
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Types of Radiation
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The experiment
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Types of Decay

• Nuclear reactions change an atom of one element
  to an atom of another element. This process is
  called transmutation. In a nuclear reaction
  there are three common types of radiation that
  are emitted alpha, beta, and gamma. The first
  two are involved in transmutation, changing the
  identity of the atom.

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Properties of Alpha, Beta, and Gamma Radiation Properties of Alpha, Beta, and Gamma Radiation Properties of Alpha, Beta, and Gamma Radiation Properties of Alpha, Beta, and Gamma Radiation
Property Alpha(a) Beta (ß) Gamma (?)
Composition Alpha particles Beta particles High-energy electromagnetic radiation
Description of Radiation Helium Nuclei Electrons Photons
Charge 2 1- 0
Relative Mass heaviest lightest 0
Relative Penetrating Power Blocked by paper Blocked by metal foil Not completely blocked by lead or concrete
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• Emit means to give off or release

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Alpha Particles

• Alpha particles contain the same composition as a
  helium nucleus. Out of all of the radiation
  particles, alpha particles move the slowest and
  are the least penetrating. As a result of alpha
  decay, the mass number decreases by 4 and the
  atomic number decreases by 2.

or
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Alpha decay
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Beta Particles

• Beta particles are similar to an electron except
  they come from the unstable nucleus of the atom.
  Beta particles are formed and ejected when a
  neutron decays to a proton and an electron. The
  proton stays in the nucleus and the electron is
  the beta particle. Beta emission is a constant
  flow of quick moving electrons that can be
  stopped by a metal foil. As a result of beta
  decay the atomic number increases by one. The
  mass number does not change.

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Gamma Rays

• Gamma rays are short wavelengths (photons) that
  move the quickest of all the types of decay, and
  the most harmful! They are very high-energy
  electromagnetic radiation. These rays often are
  released at the same time as an alpha or beta
  particle. The ray is the energy lost in the
  reaction. Gamma emission does not affect the
  atomic number or the mass number of the isotope.
  

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Gamma Rays

• Are very dangerous!!
• (write that down)

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C. Gamma ( ) Emission

• usually occurs along with other forms of
  radiation.
• Gamma particle is emitted
• No change in mass number
• No change in atomic number

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Practice Problems
1. Why is radiation given off? 2. What is the
most penetrating particle? 3. What is a main
difference between a nuclear reaction and a
chemical reaction? 4. Is Carbon-14 radioactive?
Why or Why not?
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III. Balancing Nuclear Reactions
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Balancing Nuclear Reactions

• In nuclear reaction equations we account for all
  of the changes in the mass number and atomic mass
  that occur through the decay of the nucleus.
• To verify this, we include the mass number and
  atomic mass of every particle or atom involved in
  the reaction.

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Solving Problems

• When solving/balancing a nuclear reaction,
• Look to find the difference of the mass numbers
  and atomic numbers between the reactants and the
  products.
• This will indicate the particle that was released
  or the atom that was formed.
• Make sure you have the same total mass number and
  atomic number on both sides of the equation.

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Nuclear Equations
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Nuclear Reactions
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Ex 1 Write a balanced equation for the alpha
decay of polonium-210
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Ex 2 Write a balanced equation for the beta
decay of carbon-14
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Practice

• Fill in the blank with the proper radiation
  particle or isotope
• 1)
• 2)
• 3)
• 4)

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Examples

• Ex 1. Write an equation for the alpha decay of
  Protactinium-231
• Ex. 2

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III. Radioactive Decay Rates
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Radioactive Decay Rates

• A. Half-Lives
• We measure radioactive decay in terms of half-
  lives.
• A half life is the time it takes for half of a
  radioactive sample to decay.

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Candy Bar Bandits

• There is a candy bar left in the teachers
  lounge. Every 5 minutes a teacher walks in, looks
  at the candy bar, breaks the candy bar in half
  and eats it. If the candy bar originally had a
  mass of 20 grams, how much is left after 4
  teachers have a taste?

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Keeping Track
A T
0 1 2 3 4 5 6 7 8 20g 10g 5g 2.5g 1.25g 0 5 min 10 min 15 min 20 min
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Candy Bar
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Keeping Track
A T
0 1 2 3 4 5 6 7 8 20g 10g 5g 2.5g 1.25g 0 5 min 10 min 15 min 20 min
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Example

• The half life of Carbon-14 is approximately 5730
  years. If you had 12g of Carbon-14 today, in
  5730 years you would only have 6g. The missing
  6g decayed and turned into Nitrogen 14. And, in
  another 5730 years you will have 3g of Carbon-14
  left and then in 5730 more years you would only
  have 1.5g of Carbon-14 left. Every 5730 years
  your sample is cut in half.

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Practice

• What is the half-life of the sample in the graph?

11 billion years
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Half-Lives
of HLs Remaining mass
1 Original Mass x (1/2) Org. Mass x (.5)1 0.5
2 Original Mass x (1/2) x (1/2) Org. Mass x (.5)2 0.25
3 Original Mass x (1/2) x (1/2) x (1/2) Org. Mass x (.5)3 0.125
4 Original Mass x (1/2) x (1/2) x (1/2) x (1/2) Org. Mass x (.5)4 0.0625
5 Original Mass x (1/2) x (1/2) x (1/2) x (1/2) x (1/2) Org. Mass x (.5)5 0.03125
6 Original Mass x (1/2) x (1/2) x (1/2) (1/2) x (1/2) x (1/2) Org. Mass x (.5)6 0.015626
Ratio of remaining mass to original mass
Remaining mass
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In the box

• Amount Remaining
• initial amount (1/2)HL
• initial amount (0.5HL)
• HL of half lives

Total Time Passed of Half Lives Time of one
HL
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Ex. 1 If gallium-68 has a half-life of 68.3
minutes, how much of a 10.0 mg sample is left
after 342 minutes?
HL A T
0 1 2 3 4 5 6 10mg 5mg 2.5mg 1.25mg .625mg 0.3125 0 68.3 136.6 204.9 273.2 341.5
0.3125 mg
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Ex. 1 If gallium-68 has a half-life of 68.3
minutes, how much of a 10.0 mg sample is left
after 342 minutes?
Amount Remaining (Initial amount) (0.5) n n
of half lives that have passed 342 total
minutes / 68.3 minutes half lives
Ans 0.3125 mg
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More Examples

• Ex. 2 If the passing of 116 years leaves 25.00
  mg of an original 400 mg sample of Strontium-90,
  what is the half life of Strontium-90?

HL A T
0 1 2 3 4 5 6 7 8 400 200 100 50 25mg 0 116y
116/4 29 years
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Dont get it? Here it is in words.

• If you are trying to solve for the amount of
  element left, divide the time passed from the
  half life and that will give you the amount of
  half lives your sample has had. You can then
  divide your original mass of sample by 2 as many
  times as you have half lives.
• If you are trying to solve for the half life of
  your sample, take the remaining mass and count
  how many times you have to multiple it by 2 to
  get your original mass. That will tell you how
  many half lives have elapsed. Take that number
  and divide it by the total time that has elapsed
  to get the length of just one half life.

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More on Half-lives

• Because the half-life is constant, radioisotopes
  can be used to date objects.
• Radiochemical dating is the process of
  determining the age of an object based on the
  amount of a particular radioisotope is
  remaining in the object.

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Practice

1. Iron-59 is used in medicine to diagnose blood
   circulation disorders. The half-life of iron-59
   is 44.5 days. How much of a 2000 mg sample will
   remain after 133.5 days?

HL A T
0 1 2 3 4 5 6 7 8 2000 1000 500 250 0 44.5 89 133.5
250 mg of the sample
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Practice

• After 2 years, 1.99 g of a radioisotope remains
  from the sample that had an original mass of 2.00
  g. What is the half-life of this isotope?

200 years
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More Practice

• Solving Half-Life Problems
• Strontiums half life of the radioisotope
  strontium-90 is 29 years. If you had 10.0g of
  strontium-90 today, how much would you have left
  after 87 years? 1.25g
• If you start off with 64 grams of some substance,
  how much will you have after 3 half lives? 8g
• Iodine-131 has a half life of 8 days. What
  fraction of the original sample would remain at
  the end of 32 days? 1/16

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1) Strontiums half life of the radioisotope
strontium-90 is 29 years. If you had 10.0g of
strontium-90 today, how much would you have left
after 87 years?

• Sr-90 half-life 29 years
• How many half-lives is 87 years?
• 87years 29 years 3 half-lives
• 1
  half-life
• How much is left after 3 half-lives?
• 10.0g ? 5.0g ? 2.5g ? 1.25g
• start after 1 half-life
  after 2 half-lives after 3 half-lives

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1) Strontiums half life of the radioisotope
strontium-90 is 29 years. If you had 10.0g of
strontium-90 today, how much would you have left
after 87 years?
HL A T
0 1 2 3 4 5 6 0 29 58 87
same problem, different approach
10g 5g 2.5g 1.25g
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2) If you start off with 64 grams of some
substance, how much will you have after 3 half
lives?
HL A T
0 1 2 3 4 5 6 64g 32g 16g 8g
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3) Iodine-131 has a half life of 8 days. What
fraction of the original sample would remain at
the end of 32 days?
HL A T
0 1 2 3 4 5 6 1/1 1/2 1/4 1/8 1/16 0 8 16 24 32
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B. Carbon Dating
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Carbon Dating

• Carbon dating is a specific type of radiochemical
  dating
• All living organisms have the same ratio of
  carbon-14, carbon-13, and carbon-12 as the in
  atmosphere.
• However, once an organism dies and there is no
  new carbon intake, the unstable carbon-14 starts
  to break down.

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More on Carbon dating

• Scientists calculate the ratio of carbon-12 and
  carbon-13 to carbon-14 in dead organisms.
• Then they compare the ratio to the atmospheres
  ratio and determine how many half-lives have
  passed.
• With that information they can calculate how long
  the organism has been dead.

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IV. Nuclear Energy
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A. Fission
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Fission

• Fission means to break apart. Nuclear fission
  occurs when a nucleus splits apart into different
  fragments.
• This generally occurs with atoms that have a mass
  number heavier than 60.
• The nuclei do not always split the same way.
  Scientists have found 200 different products from
  the fission reaction of Uranium-235.

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More on Fission

• Another important factor of fission reactions is
  that they cause a chain of reactions.
• The products of the initial reaction can collide
  with other molecules and cause a new fission
  reaction to occur.
• This domino affect could go on for a long time.
  This is how an atomic bomb works.

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Fission Reaction
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Nuclear Power

• Nuclear power plants harness the energy released
  in fission reactions and turn it into
  electricity.
• One of the main issues the power plant has to
  deal with is keeping the chain reactions going,
  but not letting them speed out of control. To
  this date there have been two large nuclear
  accidents.

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Nuclear Power

• Nuclear power plants have to be very precise in
  their regulations of the reactions. Some of the
  products of the fission reaction are extremely
  radioactive.
• To ensure safety of all living things, the waste
  must be properly stored.
• It can take up to twenty half-lives for such
  radioactivity to reach levels safe enough for
  exposure.
• For some waste products this can be thousands of
  years.

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Fusion
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Fusion

• Fusion means to come together.
• Nuclear fusion is the combining of atomic nuclei.
  
• In fusion reactions, scientists bring together
  nuclei of atoms that have mass numbers less than
  60.

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Fusion

• This kind of a reaction also releases large
  amounts of energy.
• It useful to know, that the sun is powered by
  fusion reactions.