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)

Any Burning Questions?

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?