intro to nuclear chemistry december 20
TRANSCRIPT
How does a small mass contained in this bomb cause……
• Nuclear Bomb of 1945 known as “fat man”
http://www.travisairmuseum.org/assets/images/fatman.jpg
…this huge nuclear explosion?
http://library.thinkquest.org/06aug/01200/Graphics/705px-Nuclear_fireball.jpg
• 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: born 1867, in Poland as Maria Sklodowska
• Lived in France
• 1898 discovered the elements polonium and radium.
http://www.radiochemistry.org/nuclearmedicine/pioneers/images/mariecurie.jpg
Marie Curie a Pioneer of Radioactivity
• Winner of 1903 Nobel Prize for Physics with Henri Becquerel and her husband, Pierre Curie.
• Winner of the sole 1911 Nobel Prize for Chemistry.
Transmutation
• When the nucleus of one element is changed into the nucleus of another element. IT CAN ONLY HAPPEN IN A NUCLEAR REACTION!!!
Nuclear Reactions• The chemical properties of the nucleus are
independent of the state of chemical combination of the atom.
• In writing nuclear equations we are not concerned with the chemical form of the atom in which the nucleus resides.
• It makes no difference if the atom is as an element or a compound.
• Mass and charges MUST BE BALANCED!!!
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
• Mass changes by 4
• The remaining fragment has 2 less protons
• Alpha radiation is the less penetrating of all the nuclear radiation (it is the most massive one!)
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.
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
• Involves the conversion of a neutron in the nucleus into a proton and an electron.
• Beta radiation has high energies, can travel up to 300 cm in air.
• Can penetrate the skin
Gamma Emission:
Loss of a -ray (high-energy radiation that almost always accompanies the loss of a nuclear particle)
00
• 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)
What happens to an unstable nucleus?
• They will undergo decay
• The type of decay depends on the reason for the instability
Radioactive Half-Life (t1/2 ):
The time required for one half of the nuclei in a given sample to decay.
• After each half life the mass of sample remaining is half.
• Different Isotopes have different half lives. Use table N
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.
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?
• What is the total mass of Rn-222 remaining in an original mass 160 mg sample of Rn-222 after 19.1 days?
Measuring Radioactivity
• One can use a device like this Geiger counter to measure the amount of activity present in a radioactive sample.
• The ionizing radiation creates ions, which conduct a current that is detected by the instrument.
Transmutations
• To change one element into another.
• Only possible in nuclear reactions never in a chemical reaction.
• In order to modify the nucleus huge amount of energy are involved.
• These reactions are carried in particle accelerators or in nuclear reactors
Nuclear transmutations
• Alpha particles have to move very fast to overcame electrostatic repulsions between them and the nucleus.
• Particle accelerators or smashers are used. They use magnetic fields to accelerate the particles.
Particle Accelerators(only for charged particles!)
These particle accelerators are enormous, having circular tracks with radii that are miles long.
Cyclotron
Nuclear transformations can be induced by accelerating a particle and colliding it with the nuclide.
Neutrons
• Can not be accelerated. They do not need it either (no charge!).
• Neutrons are products of natural decay, natural radioactive materials or are expelled of an artificial transmutation.
• Some neutron capture reactions are carried out in nuclear reactors where nuclei can be bombarded with neutrons.
Mass defect• The mass of the nucleus is always
smaller than the masses of the individual particles added up.
• The difference is the mass defect.
• That small amount translate to huge amounts of energy E = (m) c2
• That energy is the Binding energy of the nucleus, and is the energy needed to separate the nucleus.
Energy in Nuclear Reactions
For example, the mass change for the decay of 1 mol of uranium-238 is −0.0046 g.
The change in energy, E, is then
E = (m) c2
E = (−4.6 10−6 kg)(3.00 108 m/s)2
E = −4.1 1011 J This amount is 50,000 times
greater than the combustion of 1 mol of CH4
Types of nuclear reactionsfission and fusion
• The larger the binding energies, the more stable the nucleus is toward decomposition.
• Heavy nuclei gain stability (and give off energy) if they are fragmented into smaller nuclei. (FISSION)
• Even greater amounts of energy are released if very light nuclei are combined or fused together. (FUSION)
Nuclear fission:
A large nucleus splits into several small nuclei when impacted by a neutron, and energy is released in this process
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.
Controlled vs Uncontrolled nuclear reaction
• Controlled reactions: inside a nuclear power plant
• Uncontrolled reaction: nuclear bomb
Nuclear Reactors
In 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.
Fusion
• 1H + 1H 2H + 1e + energy
• 1H + 2H 3He + energy
• 3He + 3He 4He + 21H + energy
• Reaction that occurs in the sun
• Temperature 107 K
• Heavier elements are synthesized in hotter stars 108 K using Carbon as fuel
Nuclear Fusion
• Fusion would be a superior method of generating power.– The good news is that the
products of the reaction are not radioactive.
– The bad news is that in order to achieve fusion, the material must be in the plasma state at several million kelvins.
Nuclear Fusion(thermonuclear reactions)
• Tokamak apparati like the one shown at the right show promise for carrying out these reactions.
• They use magnetic fields to heat the material.
• 3 million K degrees were reached inside but is not enough to begin fusion which requires 40 million K
Fission is the release of energy by splitting heavy nuclei such as Uranium-235 and Plutonium-239
Fusion is the release of energy by combining two light nuclei such as deuterium and tritium
How does a nuclear plant work?• Each fission releases 2 or 3
neutrons• These neutrons are slowed down
with a moderator to initiate more fission events
• Control rods absorb neutrons to keep the chain reaction in check
• The goal of fusion research is to confine fusion ions at high enough temperatures and pressures, and for a long enough time to fuse
• This graph shows the exponential rate of progress over the decadesControlled Fission Chain Reaction
Confinement Progress
• Magnetic Confinement uses strong magnetic fields to confine the plasma
• This is a cross-section of the proposed International Thermo-nuclear Experimental Reactor (ITER)
• Inertial Confinement uses powerful lasers or ion beams to compress a pellet of fusion fuel to the right temperatures and pressures
• This is a schematic of the National Ignition Facility (NIF) being built at Lawrence Livermore National Lab
Nuclear Power Plant
There are two main confinement approaches:The energy from the reaction drives a steam cycle to produce electricity
Nuclear Power produces no greenhouse gas emissions; each year U.S. nuclear plants prevent atmospheric emissions totaling:•5.1 million tons of sulfur dioxide•2.4 million tons of nitrogen oxide•164 million tons of carbon
Nuclear power in 1999 was the cheapest source of electricity costing 1.83 c/kWh compared to 2.04 c/kWh from coal
D
T
D-T Fusion4He3.52 MeV
Neutron14.1 MeV
Uses of radioisotopes
• Medicine• Medical imaging –
trace amounts of short half life isotopes can be ingested and the path of the isotope traced by the radiation given off
• cancer treatment – radiation kills cancerous cells more easily than healthy cells
• Sterilisation – γ – rays can be used to kill germs and hence sterilise food and plastic equipment
• Industry – used to trace blockages in pipes, or to test the thickness of materials (by putting a source on one side of the material and detector on the other)
• Carbon dating• Once a living organism dies, it is no
longer taking in any Carbon.• C14 is radioactive, and decays over time.• By measuring the activity of C14 in an
object and comparing it with the amount of C14 which was present initially you can estimate when the organism died