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atomic structure (isotopes and nuclear radiation (isotopes are different…
atomic structure
isotopes and nuclear radiation
isotopes are different forms of the same element
all atoms of each element have a set number of protons (atomic number)
the mass number is the number of protons + neutrons
isotopes of an element are atoms with the same number of protons but different number of neutrons (different mass number)
all elements have isotopes but there are very few stable ones
the other unstable isotopes decay into other elements and give out radiation as they try to become stable- radioactive decay
radioactive substances release one or more types of ionizing radiation from their nucleus (gamma, alpha, beta)
they also release neutrons during decay as they rebalance their atomic and mass number
ionizing radiation gets rid of electrons creating positive charge
alpha particles
alpha radiation is when an alpha particle is emitted from nucleus. it contains 2 neutrons and 2 protons (like helium nucleus)
they only travel a few cm in air and are absorbed by paper- they don't penetrate far
they are strongly ionizing
beta particles
a beta particles is a high speed electron released by the nucleus. they have virtually no mass and a charge of -1
they are moderate ionizing. they penetrate moderately far into materials before colliding and have a range in air of a few meters. they are absorbed by a sheet of 5cm aluminum
for every beta particle emitted, a neutron in the nucleus has turned into a proton
gamma rays
gamma rays are waves of electromagnetic radiation released by nucleus
they penetrate far into materials without being stopped and travel a long distance through air
they are weakly ionizing because they tend to pass through rather than collide with atoms.
they can be absorbed by thick sheets of lead
nuclear equations
mass and atomic number have balance
nuclear equations are a way of showing radioactive decay by using element symbols
atom before decay---> atom after decay + radiation emitted
the total mass and the atomic numbers must be equal on both sides
alpha decay decreases the charge and mass of the nucleus
mass number reduces by 4 ad the atomic number reduces by 2
a proton is positively charged and a neutron is neutral so the charge of the nucleus will decrease
beta decay increases the charge of the nucleus
when beta decay occurs a neutron in the nucleus turns into a proton
the number of protons in the nucleus has increased by 1. this increases the positive charge of the nucleus (atomic number)
because the neutron has lost a neutron and gained a proton the mass of the nucleus does not change
gamma rays don't change the charge of the nucleus
gamma rays are a way of getting rid of excess energy from a nucleus
this means that there is no change to the atomic mass or number
background radiation and contamination
background radiation comes from many sources
background radiation is low level radiation that is around us all the time
sources:
-radioactivity of naturally occurring unstable isotopes
-radiation from space (cosmic rays)
-radiation from human activities e.g. nuclear fallout, nuclear waste
the radiation dosage tells you the risk of harm to body tissues. its measured in Sieverts. the dosage is very small so is often measured in millisieverts , radiation varies depending on job and location
exposure to radiation is called irradiation
objects near a radioactive source are irradiated by it (theyre exposed to it)
irradiating something does NOT make it radioactive
keeping sources in lead lined boxes, standing behind a barrier are ways of reducing the effects of irradation
contamination is radioactive particles getting into objects
if unwanted radioactive atoms get onto or into an object the object is contaminated
these contaminating atoms might then decay, releasing radiation which could cause harm
contamination is especially dangerous because radioactive particles could get inside the body
gloves and tongs could be used when handling sources or protective suits
the seriousness of contamination and irradiation depends on the source
contamination and irradiation can cause different levels of harm based on radiation type
outside the body beta and gamma sources are the most dangerous, they can get into the body and penetrate organs. alpha is less because it cant penetrate skin
inside the body alpha sources are the most dangerous because they do all their damage in a localized area. so contamination rather than irradiation is a concern when working with alpha sources. beta and gamma are less dangerous
uses and risk
there are risks to using radiation
radiation can enter living cells and ionize atoms and molecules within them. this can lead to tissue damage
lower doses tend to cause minor damage without killing cells. this can give rise to mutant cells which divide uncontrollably (cancer)
higher cells tend to kill cells completely causing radiation sickness (leading to vomiting, hairloss) if lots of cells get destroyed at once
gamma sources are usually used in medical tracers
certain radioactive isotopes can be injected into people and their progress around the body can be followed using an external detector. a computer shows where the strongest reading is from
isotopes which are taken into the body are usually gamma so that the radiation passes out the body without causing much ionization and a short half life so the radioactivity disappears fast
radiotherapy- treating cancer with radiation
since high doses of ionizing radiation will kill all living cells it can be used to treat cancers
gamma rays are directed carefully and at just the right dosage to kill the cancer cells without damaging too many normal cells
however some damage is caused to normal cells which makes the patient feel ill
risks and benefits
for every situation its worth considering the pros and cons
prolonged exposure to radiation poses future risks and causes many side effects
half life
radioactivity is a random process
radioactive substances give out radiation from the nuclei of their atoms
this radiation can be measure by a Geiger- muller tube and counter, which records the count rate (the number of radiation counts reaching it per second)
radioactive decay is random. so you cant predict exactly which nucleus in a sample will decay next or when
half life= the time it takes for the amount of radiation emitted by the nucleus to halve
half life can be used to find the rate at which a source decays- its activity. measured in becquerels Bq
the radioactivity of a source decreases over time
each time a radioactive nucleus decays to become a stable nucleus, the activity as a whole will decrease (older sources emit less radiation)
for some isotopes it takes a few hours before nearly all the unstable nuclei decay, for others its millions of years
the problem with trying to measure this is that the activity never reaches 0
a short half life means that the activity falls quickly because the nuclei are very unstable and rapidly decay- are often dangerous because of high levels of radiation produced at start but quickly becomes safe
a long half life means the activity falls more slowly because most of the nuclei don't decay for a long time- this can be dangerous as nearby areas are exposed to radiation for a long time
can be measured using a graph
plot graph of activity against time
the half life is found by finding the time interval on the bottom of the axis corresponding to the halving of activity on vertical axis
developing the model of the atom
Rutherford replaced the plum pudding model with the nuclear model
1804- john Dalton- agreed with Democritus that matter was made up of tiny spheres that couldn't be broken up. he believed that each element was made up of different types of atom
100 years later- JJ Thompson- discovered particles called electrons that could be removed from atoms. Thompson suggested that atoms were positive spheres of charge with negative electrons embedded in them- plum pudding
1909- Ernest Rutherford- fired alpha particles as sheet of gold foil (alpha particle scattering experiment). he expected particles to go straight through however some deflected and some deflected backwards
because a few alpha particles were deflected back the scientists realized that most of the atoms mass was concentrated in the tiny nucleus which as a positive charge as it repelled alpha.
but seen as most of the alpha particles passed straight through they believed that most of the atom was made up of empty space
developments to modern nuclear model
also the idea that the nucleus was subdivided into positive particles called protons
Niels Bohr said that electrons orbited at set distances called energy levels/ shells
James Chadwick proved the existence of the neutron which explained the imbalance between the atomic and mass number
the current model
the nucleus is tiny but makes up most of the mass of the atom, it contains protons and neutrons- has an overall positive charge, its radius is 10 000 times smaller than the radius of the atom
the rest of the atom is mostly empty space. electrons orbit the nucleus in shells and give the atom its overall size of 1 x 10-10
the number of protons= the number of electrons which give the atom no overall charge
electrons in shells can move in the atom when they gain energy through absorbing EM radiation they move to a higher energy level, further from nucleus. if they release EM radiation they move to a lower energy level, closer to nucleus
fission and fusion
nuclear fission- splitting a large unstable nucleus
nuclear fission is a type of nuclear reaction that is used to release energy from large unstable atoms (uranium or plutonium) by splitting them into smaller atoms
spontaneous fission rarely happens. usually the nucleus has to absorb a neutron to spliy
when the atom splits it forms two new lighter elements that are roughly the same size and have energy in kinetic energy stores
2 or 3 neutrons are also released when an atom splits. if any of these neutrons are moving slow enough to be absorbed by another nucleus they cause more nuclear fission to occur
this is a chain reaction
the energy not transferred to the kinetic energy stores of the products is carried away by gamma rays
the energy carried away by gamma rays and in the kinetic energy stores of the neutrons can be used to heat water making steam to turn generators
the amount of energy produced by fission in a nuclear reactor is controlled by changing how quickly the chain reaction can occur. this done by using controlling rods which absorb neutrons and control the amount of energy released
uncontrolled chain reactions quickly lead to lots of energy being released as an explosion- how nuclear weapons work
nuclear fusion- joining small nuclei
fusion is the opposite of fission
in nuclear fusion two light nuclei collide at high speed and join to create a larger, heavier nucleus
the heavier nucleus produced by fusion does not have as much mass as the two separate light nuclei had. some of the mass is converted to energy which is released as radiation
fusion releases a lot of energy (more than fission)
scientists have not yet found a way of using fusion to generate energy for us to use. the temps and pressures needed for fusion are so high that the fusion reactors are really hard and expensive to build