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Radio Activityand
Carbon Dating
Mr. Bartelt
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.
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.
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.
Different types of decayDifferent types of decay By applying a magnet By applying a magnet
to radioactive to radioactive material three distict material three distict types of radiation types of radiation were observed. were observed. AlphaAlpha, , BetaBeta, and , and GammaGamma were the were the intial generic names. intial generic names. No one got around to No one got around to changing the name, changing the name, and so the names and so the names stuck. stuck.
What’s going on here???
Determine the charge of each of the three forms of decay
Which one is the MOST massive?
Alpha Decay
The nucleus emits a helium nucleus (an alpha particle)
Uranium alpha decays to Thorium:
Generic equation:
HeThU decayalpha 42
23490
_23892
HeYX AZ
decayalphaAZ
42
42
_
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
Vocab
The helium—4 nucleus is referred to as an “alpha particle”.
Alpha decay is caused when a nuclide is too large.
particle Alpha42 He
Practice problem
Write the equation for when Radium –226 undergoes alpha decay.
Fill in the missing nuclide.
HeRa decayalpha 42
????
_22688 __
A neutron is converted into a proton and an electron. The electron is rocketed out of the nucleus with great speed.
Beta decay
eTh decaybeta 01
23491
_23490 Pr
eYX AZ
decaybetaAZ
011
_
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
Beta Decay
Why does this happen?Beta decay tends to occur
when there are too many neutrons.
If the neutron:proton ratio is too HIGH beta decay may occur.
Vocab
An electron is referred to as an “beta particle”. It is sometimes indicate as below:
particle beta01
01 e
Write the equation for the beta decay of Cobalt—60.
Fill in the missing nuclide
Practice problem
eCo decaybeta 01
????
_6027 __
Positron emission
The conversion of a proton into a neutron by the emission of a positron decreases the atomic number by 1
Generic equation:
eBC emissionpositron 01
115
_116
eYX AZ
emissionpositronAZ
011
_
Electron capture
The nucleus captures an orbiting electron
A proton is converted into a neutron
MgeAl captureelectron 2612
_01
2613
YeX AZ
captureelectronAZ 1
_01
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
MgeAl captureelectron 2612
_01
2613
eMgAl emissionpositron 01
2612
_2613
Positron emission and electron capture
Why does this happen?Both occur when there are too
many protonsWhen the neutron:proton ration is
too LOW
Nuclear stability
This graph is called the band of stability
You don’t need to memorize this!!!
But what’s happening INSIDE?
We haven’t talked about what’s happening inside the nucleus yet.
There are protons in the nucleus.Protons have positive chargePositive charges repel one another
Why doesn’t the nucleus explode?
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 distancesThe presence of neutrons in the nucleus
lends stability to the protons and helps hold the nucleus together.
If you’re asked what holds the nucleus together you should write, “Strong nuclear force”.
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???
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???
E=mc2
Energy and mass are interchangeable
Mass defect explained
Einstein's famous equation E=mc2 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)
A potential test question
No more lecture
Now we need to balance nuclear equations
A brief lesson in statistics
AKA: Avogadro’s number is huge
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?
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?
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?
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?
Teacher statistics
For random events:As your basis set goes to infinity, your
margin of error goes to zeroAvogadro’s 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
Carbon Dating
The carbon-14:carbon-12 ratio
Atomic mass of carbon is 12.0107g/molThis means most carbon found in nature
has a mass number of 12A small percentage of carbon atoms
have a mass number of 14This is a good thing because…
Carbon-14
Carbon-14 undergoes beta decay at a predictable rate.
The half-life of this reaction is ≈ 5730 years
eNC decaybeta 147
_146
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.
This fits in perfectly with the
Once we die we cease to consume carbon.
Without the constant uptake of C-14 from the atmosphere any flesh or decaying plant material’s 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 math…We know more or less how old it is.
How does it work?
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…
Why is this important?
Archeologists can find the ages of bonesWhat is paper made out of?We can find the age of anything written
on paperAnything painted on canvas Many dyes come from fresh plantsThe uses go on and on
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)
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)
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?
Uses of Radioactive Uses of Radioactive MaterialMaterial
Where do I even begin?Where do I even begin?
Carbon Dating
Medicinal
Radioactive Isotopes can be used as medicinal tracer element to pinpoint tumors and other bodily abnormalities
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.
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?
Weapons
Atomic weapons have been in use since the mid 1940’s
Not all uses of radioactive isotopes are beneficial
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)
Vocabulary you need to know
Parent nuclide-The nuclide before it has undergone a form of radioactive decay.
HeThU decayalpha 42
23490
_23892
Daughter nuclide-The nuclide that results from after decay has taken place
eTh decaybeta 01
23491
_23490 Pr
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
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
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?