Transcript Nuclear Chemistry - Mona Shores Blogs

Nuclear Chemistry
10.1 Radioactivity
• Radioactivity: process in which an
atomic nucleus emits charged
particles and energy
• Radioisotope: any atom containing an
unstable nucleus
During nuclear decay, atoms of one element
can change into atoms of a different element
altogether
Uranium – 238 decays into Thorium – 234
(also a radioisotope)
• Nuclear Radiation: charged particles
and energy that are emitted from the
nuclei of radioisotopes
• Common nuclear radiation types
– alpha particle
– beta particle
– gamma rays
Alpha Decay
• Alpha Particle (): a positively
charged particle made up of 2
protons and 2 neutrons (the SAME
as a HELIUM NUCLEUS)
Common symbol = 
Example of alpha decay of uranium –
238
Dangers of Nuclear Radiation
• Least penetrating type of nuclear
radiation, can be stopped by sheet of
paper or clothing
Beta Decay
• Beta Particle (): an electron
emitted by an unstable nucleus
– Written as: 
• Assigned atomic # of -1 mass of 0
(zero)
• How can a nucleus (which is positive),
emit a negatively charged particle?
During beta decay, a neutron
decomposes into a proton and an e-
• Proton stays trapped in the nucleus,
e- released
Example of beta decay of thorium –234
• Product isotope has 1 proton more
and 1 neutron fewer than the
reactant isotope
• Mass number of the isotopes are
equal because the emitted beta
particle has essentially NO MASS
• Beta particles pass through
paper, but stopped by thin sheet
of metal
Gamma Decay
• Gamma Ray (): a penetrating ray of
energy emitted by an unstable nucleus
Gamma Decay
• NO mass and NO charge
• During Gamma Decay:
• Atomic number and mass number of
the atom remains the same
• Energy of nucleus decreases
• Gamma decay often accompanied by
alpha or beta decay
• Example of thorium – 234 emitting
both beta particles and gamma rays
as it decays:
• Gamma rays much more
penetrating – takes several
centimeters of lead or several
meters of concrete to stop
gamma radiation
Effects of Nuclear Radiation
• Background Radiation: nuclear
radiation that occurs naturally in the
environment
• When nuclear radiation exceeds
background levels, it can damage the
cells and tissues of your body
Effects of Nuclear Radiation
• Nuclear radiation can ionize atoms
(when cells are exposed to nuclear
radiation, the bonds holding together
proteins and DNA molecules may
break cells may no longer function
properly)
Effects of Nuclear Radiation
• , , and  are all forms of ionizing
radiation
• The extent of the damage of
external nuclear radiation is
dependent on the penetrating power
of the radiation…
• Beta particles cause more damage
than alpha particles, but less than
gamma rays
• Gamma rays can penetrate deeply into
the human body, potentially exposing
all organs to ionization damage
Detecting Nuclear Radiation
• Although you can’t see, hear, or feel
the radioactivity
around you, scientific instruments can
measure nuclear radiation
• Geiger Counters
• Film Badges
10.2  Rates of Nuclear Decay
Nuclear Decay
• By studying the radioactive nuclei of an
object we can determine how old the object
is.
• Because most materials contain at least
trace amounts of radioisotopes, scientists
can estimate how old they are based on
rates of nuclear decay.
Half-Life
• Half-life: the time required for one half of a
sample of a radioisotope to decay
– After one half-life, half of the atoms in a
radioactive sample have decayed, while the
other half remain unchanged
– After two half-lives, half of the remaining have
decays, leaving one quarter of the original
sample unchanged
Half-Life Example
• Iodine Half-life= 8.07 days
– After one half-life (8.07 days) half of the
original sample remains
– After 2 half-lives (16.14 days) one quarter of
the original remains
– After 3 half-lives (24.21 days) one half of one
quarter remains, or 1/8 (one eighth)
– …and so on
Half-Lives Vary
• Half-lives can vary from fractions of a
second to billions of years
• Unlike chemical reaction rates, which vary
with the conditions of a reaction, nuclear
decay rates are constant!!!
Radioactive Dating
• Method used for determining the age of
objects using the half-lives of Carbon – 14
• Radiocarbon dating: determining the age
of an object by comparing its carbon-14
levels with carbon-14 levels in the
atmosphere.
Radioactive Dating
• Carbon-14 has a half-life of 5,730 years.
• Carbon-14 is formed in the upper
atmosphere when neutrons produced by
cosmic rays collide with nitrogen-14 atoms.
• The radioactive carbon-14 undergoes beta
decay to form nitrogen-14.
Using Carbon-14 to Date
• Living organisms absorb the carbon (CO2)
from the atmosphere, but when they die
they stop absorbing it and the levels do not
change.
• From this point levels start to decrease as
the radioactive carbon decays.
• The levels in the object are then compared
with levels in the atmosphere.
Example: if an object has half the amount of carbon-14 in it
as in the atmosphere, then we know the object is about 5,
730 years old (which is one half-life for carbon-14)
Carbon-14 Dating
• Carbon-14 or radiocarbon dating can be
used to date any carbon-containing object
less than 50,000 years old.
– After this point, there is too little carbon-14 left
to be measurable
• Objects older than this use radioisotopes
with longer half-lives
• The older the object the lower the levels of
radioisotopes present
10.3 Artificial Transmutation
Transmutation
• Transmutation: the conversion of atoms of
one element to atoms of another.
• It involves a nuclear change, not a chemical
change.
• Transmutations can either occur naturally
(nuclear decay) or artificially.
• Scientists can perform artificial
transmutations by bombarding atomic
nuclei with high-energy particles such as
protons, neutrons, or alpha particles.
Transuranium Elements
• Transuranium Elements: Elements with
atomic numbers greater than 92 (uranium)
– All transuranium elements are radioactive and
generally not found in nature
• Scientists can create a transuranium element by
the artificial transmutation of a lighter
element
• Useful transuranium elements
– Americium-241: used in smoke detectors
– Plutonium-238: energy source for space probes
Particle Accelerators
• Sometimes transmutations will not occur
unless the bombarding particles are moving
at extremely high speeds.
• To achieve these high speeds scientists use
particle accelerators.
Particle Accelerator
• These accelerators move charged particles
at speeds very close to the speed of light
• The particles are then guided toward a
target, where they collide with atomic
nuclei and transmutations are allowed to
occur
• These collisions have also lead to the
discovery of more subatomic particles
– Quarks: protons and neutrons are made up of
these even smaller particles
Large Hadron Collider (LHC)
10.4  Fission & Fusion
Question
• What holds the nucleus together?
• It’s full of positive particles, so why don’t
they push each other away?
• What keeps the protons and neutrons
together?
• Clearly, there must be an attractive force
that binds the particles
Answer
• Strong Nuclear Force: the attractive force
that binds protons and neutrons together in
the nucleus
– Over very short distances, the strong nuclear
force is much greater than the electric forces
among protons
Forces in the Atom
Electric Force
• Question: What determines the strength of
the electric force?
• Answer: The number of protons
Electric Force
• The greater the number of protons, the
greater is the electric force that repels the
protons
• Larger nuclei have a stronger repulsive
force than a smaller nuclei
• As a result, the nucleus will become
unstable (or radioactive) when the strong
nuclear forces can’t overcome the repulsive
electric forces among protons.
Nucleus Size & Radioactivity
• Because of the size issue, there is a point
beyond which all elements are radioactive.
• Once they become large enough, the
repulsive forces overcome. This occurs
with all nuclei with 83 or more protons.
• Therefore, all elements with an atomic
number greater than 83 are radioactive!
Fission
• FISSION: the splitting of an atomic
nucleus into two smaller parts
• In nuclear fission, tremendous amounts of
energy can be produced from very small
amounts of mass
Converting Mass into Energy
• During a fission reaction, some of the mass
of the reactants is lost!
• The Law of Conservation of Mass says this
is illegal, highly illegal!
• This “lost” mass is converted into energy!
Converting Mass into Energy
• Since we bent the law a little…we use a
revised version of the law: Law of
Conservation of Mass and Energy
– It basically says: The total amount of mass and
energy remains constant!!!
E=
2
mc
• 30 years before the discovery of fission,
Albert Einstein introduced the mass-energy
equation.
• E=mc2describes the relationship between
mass and energy:
– E = energy
– m = mass
– c = the speed of light (3.0 x 108m/s)
• It shows that the conversion of a small
amount of mass releases an ENORMOUS
amount of energy.
Lots of Energy!!!
• Example: the explosion of the first atomic
bomb contained 5kg of plutonium, but
created an explosion equivalent to18, 600
tons of TNT!!!
• So, since we bent the law a little, just a
little, we use a revised version of the law:
– This law is referred to as the Law of
Conservation of mass and energy.
• It basically says:
– THE TOTAL AMOUNT OF MASS AND
ENERGY REMAINS CONSTANT!
Chain Reaction
• Nuclear fission reactions act like rumors
being spread throughout school:
• One person tells a few friends, they tell a
few friends, and on and on…
• During a fission reaction each reactant
nucleus splits into 2 smaller nuclei and
releases 2-3 neutrons.
• If one of these neutrons is absorbed by
another nucleus, fission can result again,
releasing more neutrons.
Triggering a Chain Reaction
• CHAIN REACTION: neutrons released
during the splitting of an initial nucleus
trigger a series of nuclear fissions.
• Uncontrolled chain reactions occur when
each released neutron is free to cause other
fissions
Chain Reaction
Chain Reaction
• Nuclear weapons are designed to produce
uncontrolled chain reactions
• In order for a chain reaction to keep going,
the nucleus that splits needs to produce one
neutron that causes the fission of another
nucleus
– The material reacting uncontrolled needs to
have a critical mass.
– CRITICAL MASS: the smallest possible mass
of a fissionable material that can sustain a chain
reaction.
Fusion
• Another type of nuclear reaction can release
huge amounts of energy is fusion:
• FUSION: a process in which the nuclei of
two atoms combine to form a larger
nucleus.
• Just like fission, a small fraction of the mass
is converted into energy
Example of Fusion
• The sun and stars are powered by the fusion
of hydrogen into helium
– Fusion requires extremely high temperatures
where matter exists as plasma.
• This is a problem for scientists wanting to
use fusion for an energy source
– They cannot get high enough temperatures and
have trouble containing plasma here on Earth
Fusion in the Core