Intro to Nuclear Chemistry

Chemistry

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How does a subatomic particle cause damage to human tissue?

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Isotopes of Carbon

• Isotopes of certain unstable elements that spontaneously emit particles and energy from the nucleus.

• Henri Beckerel 1896 accidentally observed radioactivity of uranium salts that were fogging photographic film.

• His associates were Marie and Pierre Curie.

Radioactive Isotopes

Marie Curie a Pioneer of Radioactivity

• Winner of 1903 Nobel Prize for Physics with Henri Becquerel and her husband, Pierre Curie.

• Winner of the 1911 Nobel Prize for Chemistry.

• 1898 discovered the elements polonium and radium.

•

3 Main Types of Radioactive Decay

• Alpha

• Beta

• Gamma

Emission of alpha particles :

• helium nuclei • two protons and two neutrons • charge +2e  • can travel a few inches through air• can be stopped by a sheet of

paper, clothing.

Alpha Decay

Alpha Decay

Uranium Thorium

Alpha Decay

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Beta Decay

• Beta particles : electrons ejected from the nucleus when neutrons decay

n -> p+ +-

• Beta particles have the same charge and

mass as "normal" electrons.

• Can be stopped by aluminum foil or a block of wood.

Beta Decay

Thorium Protactinium

• Gamma radiation electromagnetic energy that is released. 

• Gamma rays are electromagnetic waves.

• They have no mass.• Gamma radiation has no charge.

– Most Penetrating, can be stopped by 1m thick concrete or a several cm thick sheet of lead.

Gamma Decay

Examples of Radioactive DecayAlpha Decay

Po Pb + He

Beta Decay p n + e

n p + e

C N + e

Gamma Decay

Ni Ni + (excited nucleus)

• Nuclei with atomic number > 83 are radioactive

Radioactive Half-Life (t1/2 ):

• The time for half of the radioactive nuclei in a given sample to undergo decay.

Common Radioactive Isotopes

Isotope Half-Life Radiation Emitted

Carbon-14 5,730 years

Radon-222 3.8 days

Uranium-235 7.0 x 108 years

Uranium-238 4.46 x 109 years

Radioactive Half-Life

• After one half life there is 1/2 of original sample left.

• After two half-lives, there will be

1/2 of the 1/2 = 1/4 the original sample.

Graph of Amount of Remaining Nuclei vs Time

A=Aoe-t

A

Example

You have 100 g of radioactive C-14. The half-life of C-14 is 5730 years.

• How many grams are left after one half-life? Answer:50 g

• How many grams are left after two half-lives?

Problem

A sample of 3x107 Radon atoms are trapped

in a basement that is sealed. The half-life of

Radon is 3.83 days. How many radon atoms

are left after 31 days?

answer:1.2x105 atoms

Energy in Nuclear Reactions• There is a tremendous amount of

energy stored in nuclei.

• Einstein’s famous equation, E = mc2, relates directly to the calculation of this energy.

• In chemical reactions the amount of mass converted to energy is minimal.

• However, these energies are many thousands of times greater in nuclear reactions.

Examples of Nuclear Energy

Nuclear Fission• How does one tap all that energy?• Nuclear fission is the type of reaction carried

out in nuclear reactors.

Nuclear Fission

• Bombardment of the radioactive nuclide with a neutron starts the process.

• Neutrons released in the transmutation strike other nuclei, causing their decay and the production of more neutrons.

• This process continues in what we call a nuclear chain reaction.

Nuclear Fission

• If there are not enough radioactive nuclides in the path of the ejected neutrons, the chain reaction will die out.

• Therefore, there must be a certain minimum amount of fissionable material present for the chain reaction to be sustained: Critical Mass.

Nuclear ReactorsIn nuclear reactors the heat generated by the reaction is used to produce steam that turns a turbine connected to a generator.

Nuclear Reactors

• The reaction is kept in check by the use of control rods.

• These block the paths of some neutrons, keeping the system from reaching a dangerous supercritical mass.

Nuclear Fusion• Fusion would be a superior method of generating

power.• It occurs when small nuclei combine – releasing

much more energy than fission.– The products of the reaction are not radioactive.– The material must be in the plasma state at several

million kelvins.– Tokamak apparati, using magnetic fields to heat the

material, show promise for carrying out these reactions.