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Transcript
NUCLEAR CHEMISTRY
Radioactivity
•
Radioisotopes
– A isotope that has an unstable nucleus
and undergoes radioactive decay
• Always accompanied by the emission of
large amounts of energy
• Nuclear reactions NOT Affected by:
– Changes in temperature, pressure or the
presence of catalyst
– Unaffected by compounds in which the
unstable isotopes are composed
– Can not be slowed down, speed up or turned
off
Radioactivity
Antoine Henri Becquerel (1852-1908):
1896- while studying uranium salts, observed
that they fogged up photographic film plates
Marie Curie (1867-1934) & Pierre Curie (18591906):
Demonstrated that fogging was caused by
rays emitted by uranium atoms in the ore
Radioactivity-process by which materials
give off such rays
Radiation-penetrating rays and particles
emitted by a radioactive source
Radioactivity
•Radioisotopes have unstable nuclei
•Stability dependent on proportion of
neutrons to protons and overall size
•Radioactive Decay: The spontaneous
disintegration of a nucleus into a slightly lighter and
more stable nucleus, accompanied by emission of
particles, electromagnetic radiation, or both
Types of Radioactive Decay
Alpha Radiation
Helium nuclei that have been emitted from a
radioactive source
1. Alpha particle (α) is a helium nucleus,
contains 2 proton and 2 neutron so it has a
2+ charge.
2. Loss of an alpha particle reduces atomic #
by 2 and atomic mass by 4
Beta Radiation
fast moving electrons formed by the decomposition
of a neutron in an atom
1. Beta particle (β) is an electron emitted from
the nucleus during nuclear decay
2. Beta particles are emitted when a neutron is
converted into a proton and an electron,
atomic # increases by 1
Gamma Emission
1. Gamma rays () are high-energy electromagnetic
waves emitted from a nucleus as it changes from
an excited state to a ground energy state
2. Gamma emission usually follows other types of
decay that leave the nucleus in an excited state
Nuclear Radiation
A. Penetrating Ability
1. Alpha Particles
a. Least penetrating ability due to large mass and charge
b. Travel only a few centimeters through air
c. Cannot penetrate skin
d. Can cause harm through ingestion or inhalation
2. Beta Particles
a. Travel at speeds close to the speed of light
b. Penetrating ability about 100 times greater than that of
alpha particles.
c. They have a range of a few meters in air.
3. Gamma rays
a. Greatest penetrating ability
b. Protection requires shielding with thick layers of lead,
cement, or both
B. Penetrating ability of radiation
C. Radioactive Elements
1. All isotopes of all man-made elements are
radioactive
2. Some naturally isotopes are radioactive
a. All isotopes of all elements beyond bismuth (atomic
#83) are radioactive
Half-Life
Half-Life (t1/2)
The time required for half the atoms of a radioactive
nuclei to decay
a. More stable nuclei decay slowly
b. Less stable nuclei decay rapidly
Half-Life Equations
Total mass of decay =
number of half-lives x number of years
Half - life
Fraction of sample remaining = final mass of sample
Initial mass of sample
1. If 100.0g of carbon-14 decays until only 25.0 g of carbon is
left after 11460 yrs, what is the half-life of carbon-14?
2. Thallim-208 has a half-life of 3.053 min. How long will it
take for 120.0g to decay to 7.50 g?
3. Gold-198 has a half-life of 2.7 days. How much of a 96 g
sample of gold -198 will be left after 8.1 days?
Radioactive Decay Modeling
Model the hypothetical nuclei of Mysterium (My) and it’s decay
1. Using toothpicks connect 1 large marshmallow + 1 small marshmallow
(large = proton ; small = electron) (proton + electron = Neutron)
2. Form 6 Neutrons in total
3. Combine 6 Neutron + 4 protons in one pile: Illustrate on poster and
write initial state AZ My
4. Illustrate alpha emission with equation and marshmallows (reduce
by 2 protons + 2 neutrons) AZ My  AZ X + 4 He
5. Illustrate beta emission using new isotope in step 4 with equation and
marshmallows AZ Y + 0-1 β
6. Label the following decay sequence as α or β emission:
 238 U  234 Th  234 Pa  230 Th  226 Ra  222 Rn
92
90
91
92
88
86
Neutron-Proton Ratios
•Any element with more than
one proton (i.e., anything but
hydrogen) will have repulsions
between the protons in the
nucleus.
•A strong nuclear force helps
keep the nucleus from flying
apart.
•Neutrons play a key role
stabilizing the nucleus.
•Therefore, the ratio of
neutrons to protons is an
important factor.
Stable Nuclei
The shaded region in
the figure shows
what nuclides would
be stable, the socalled band of
stability.
Stable Nuclei
Nuclei above this belt
have too many
neutrons.
They tend to decay by
emitting beta particles.
Stable Nuclei
Nuclei below the belt
have too many
protons. They tend to
become more stable
by positron (mass of an
electron but a positive
charge) emission or
electron capture.
Transmutation Reactions
•Transmutation: the conversion of an atom of one
element to an atom of another element
oHigh energy particles bombard the nucleus of an atom
•Naturally Occurring
oNitrogen-14  Carbon-14 (upper atmosphere)
oFlourine-18  Oxygen-17 and one proton
•Transuranium elements: elements in the periodic
table with an atomic # > 92, all undergo
transmutation
•None occure in nature
•All radioactive
Nuclear Fission
A very heavy nucleus splits into more stable nuclei
of intermediate mass
– Occurs by chain reaction
The mass of the products is less than the mass of the
reactants. Missing mass is converted to energy
Nuclear Chain Reaction
In a chain reaction, some of the neutrons
produced react with other fissionable atoms,
producing more neutrons, which react with
still more fissionable atoms…
Nuclear Reactors
Use controlled fission to produce useful energy
Energy genreated in form of heat
Coolant fluid, removes heat from the core
Heat generates steam which drives a turbine that generates
electricity
Two step process:
1. Neutron Moderation: reduces speed of neutrons to
continue chain reaction (water & carbon)
2. Neutron absorption: decreases the # of slow moving
neutrons, prevents chain reaction from going too fast
Nuclear Waste
300 fuel rods = assembly
100 assemblies = reactor core
Spent fuel rods are high-level
nuclear waste (highly radioactive)
All nuclear power plants have
holding tanks for spent rods
12 meter deep and filled with
water – cools rods and acts as a
radiation shield
Spend a decade or more in tank
Limited time for plant operation
due to contamination
Nuclear Fusion
Nuclear Fusion
Light-mass nuclei combine to form a heavier, more stable
nucleus
Fusion Reactions
1. More energetic than fission rxns
2. Occur only at high temperature – in excess of 40 000 000 °C
3. Source of energy of the hydrogen bomb
4. Potential energy source that is inexpensive and readily
available and not radioactive
Problems: achieving high temperatures, uncontrolled, plasma state of
products
Detecting Radiation
Ionizing Radiation:
radiation with enough energy to knock electrons off atoms of
bombarded substances to form ions
Monitoring Devices:
Geiger Counter:
a gas-filled metal tube used to detect radiation
ionizing radiation penetrates the thin window at one end ionizing
the gas which conducts electricity making audible clicks
Scintillation counter:
A device that uses a specially coated phosphor surface to detect
radiation; ionizing radiation striking the phosphor plate produces
bright flashes of light
Uses for Radiation
Nuclear Medicine
Thyroid imaging using Tc-99m
Food Irradiation
•Food can be irradiated with g rays from
60Co or 137Cs.
•Irradiated milk has a shelf life of 3 mo.
without refrigeration.
•USDA has approved irradiation of meats
and eggs.
Chernobyl Disaster
April 26, 1986
Reactor # 4 in a Nuclear Power
plant in Ukraine exploded.
Further explosions and resulting
fire sent a plume of highly
radioactive fallout into the
atmosphere and over an
extensive geographical location
http://www.youtube.com/watch?v=uoEgkGNO-sQ&feature=related
http://ngm.nationalgeographic.com/2006/04/inside-chernobyl/audio-interactive