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what happens in chemical reactions
only valence electrons involved, atoms do not change their identity
atoms with different numbers of neutrons
isotopes
what do neutrons do to a nucleus
provides a strong nuclear force which binds the nucleus together despite the repulsion between protons
what happens in light nuclei
strong nuclear force counteracts the electric force (neutrons and protons are equal)
what happens in heavy nuclei
need more neutrons than protons to combat the repulsion between protons
what determines the stability of a nucleus
balance between strong nuclear force from neutrons and electrical force between protons
alpha particles
2 protons and 2 neutrons
blocked by paper
beta particles
electrons
blocked by aluminum
gamma rays
electromagnetic radiation given out due to nucleons reorganizing themselves
blocked by lead
what is nuclear binding energy
the energy required to break the nucleus into its constituent nucleons
what measures the stability of the nucleus
average binding energy per nucleon
relationship between binding energy per nucleon and stability of nucleus
greater, more stable
most stable
iron and nickel
what do fission and fusion do to binding energy per nucleon
increases
two deuterium nuclei
form an He nucleus and a neutron
forms tritium and hydrogen
what does the sun emit
a continuous spectrum in the visible region with some frequencies missing and analyzed with a spectroscope
what does the atmosphere of the sun do
lower temperature than the sun itself
gases can absorb some wavelengths of the sun’s radiation, producing dark absorption lines
what can dark bands do
identify elements present in gases around the sun
absorption lines vs emission lines
dark lines in an absorption spectrum have the same position as the lines in an emission spectrum of the same atom
how do lines in the visible spectrum form
caused from electrons moving from higher to lower energy levels
how do lines in the absorption spectrum form
electrons go from a lower to a higher energy level
how do lines in the emission spectrum form
electrons move from a higher to a lower energy level
why are the atomic energy levels of helium less clear than those of hydrogen
two electrons instead of 1, forming multiple energy states
energy levels of he vs h
helium have a more negative value due to increased nuclear charge pulling electrons closer to the nucleus
benefits of fusion
no massive transport or waste storage
no greenhouse gases are produced
why is a large amount of energy formed from the fission of 235u
mass of products is slightly less than mass of reactants
mass difference converts to energy
how are nuclear reactors designed
only one neutron from each reaction initiates a further reaction to prevent the chain reaction from running out of control
what does the chance that a neutron goes on to cause fission depend on
the number of potential nuclei in the neutron’s path
speed of the neutrons
what do moderators do
slow down neutrons produce by fission that are too quick to produce more reactions
what do control rods to
absorb neutrons to control the chain reaction
made of ca or cd
what is required for a fissile material to sustain a chain reaction
enough fissionable material for the neutron in one fission process to find another fissile nuclei
what is critical mass
the minimum mass of a fissionable material needed to sustain a chain reaction
what does the critical mass depend on
the exact nature of the fuel being used and the shape of the assembly
what is used to sustain the fission of uranium
enriched uranium fuel
what is enrichment
the process by which the percentage composition of uranium-235 is increased to make nuclear fission more likely
what was developed to increase the supply of fissionable 235u
breeder reactors
what do breeder reactors do
convert 238u to plutonium-239
how does 238u convert into 239pu
238u captures a neutron to form 239u
239u decays to 239np with the emission of a beta particle
239np decays into 239pu with the emission of another beta particle
why aren’t moderators needed in a plutonium reactor
plutonium requires faster neutrons than 235u
concerns for breeder reactors
plutonium is toxic and can be made into weapons
breeder reactors are less efficient and require liquid metal coolants
susceptible to accidents
radioactive by-products
what is half life
the time it takes for half of the atoms to decay
concentration of an aqueous isotope to fall to half its initial value
number of atoms of the isotope to fall to half their original value
activity of an isotope to fall to half its initial value
order of radioactive decay
first
rate of decay
rate=decay constant*[A]
does temperature affect the decay constant
no
what is the decay constant dependent on
starting concentrations
how is radioactive waste produced
nuclear power stations
research labs
military establishments
hospitals
problems with energy from nuclear fission
health issues
nuclear meltdown
nuclear weapons
two types of radioactive waste
high and low level
what is low level waste
low levels of activity and shorter half lives
fuel containers, hospital waste
what is high level waste
high levels of activity and longer half lives
spent fuel rods
military
low level waste water storage
stored in cooling ponds of water until the activity has fallen to safe levels
it is then passed through an ion exchange where isotopes are removes
water is diluted and released back into the sea
low level waste concrete storage
stored in steel containers inside concrete lined vaults
high level waste storage
problematic, may undergo further reactions
fuel rods are encased in glass or ceramic before being buried deep in the earth