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The Thomson model

answer

positively charged globule negatively charged electrons sprinkled in it plum pudding model

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Rutherford's scattering experiment

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stream of alpha particles from a radioactive source fired at very thin gold foil Geiger and Marsden recorded the no of alpha parties scattered at different angles alpha particles hit a fluorescent screen producing a tiny visible flash of light

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outcomes of the Rutherford experiment if the Thomson model was correct

answer

all flashes should have been seen within a small anger of the beam little deflection

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actual outcomes and conclusions of the rutherford experiment

answer

most of the fast, charged alpha particles went straight through the gold foil- atom mostly empty space some particles deflected back through significant angles- centre of atom tiny but very massive alpha particles repelled- nucleus has a positive charge atom are neutral overall- electrons must be on the outside of the atom separating one atom from the next

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nuclear model of the atom

answer

nucleus containing protons and neutrons (nucleons) electrons orbiting the core nucleons 2000 times more massive than electrons nucleus 10000th size of the whole atom

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proton number

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no of protons and electrons results in chemical properties

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nucleon number

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mass number number of nucleons is about the same as the atom's mass as electrons weigh virtually nothing

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diameter of an atom

answer

0.1nm

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diameter of the smallest nucleus

answer

2fm

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isotopes

answer

atoms with the same no of protons but different numbers of neutrons

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effect of changing the number of neutrons

answer

doesn't affect chemical properties affects stability of the nucleus unstable nuclei may be radioactive

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nuclear density

answer

around 1.5x10^17kg/m3

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nuclear density > atomic density shows that

answer

most of an atoms mass is in its nucleus nucleus is small compared to the atom atom contains a lot of empty space

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electrostatic force in the nucleus

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protons have an equal positive charge repel each other FR = 2.3N

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gravitational force in the nucleus

answer

two massive objects will attract each other FA = -1.86x10-36N

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why must the strong nuclear force exist

answer

electrostatic force is far bigger than the gravitational force nucleons would fly apart must be another attractive force

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features of the strong nuclear force

answer

must be an attractive force larger than the electrostatic force short range- up to 10fm strength between nucleons quickly falls beyond this distance works equally between all nucleons repulsive at very small separations (less than 0.5fm)- otherwise nothing to stop it crushing the nucleus to a point

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radioactive decay

answer

unstable atoms break down to become more stable decay b releasing energy and/or particles until reaches a stable form

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instability caused by

answer

too many neutrons not enough neutrons too many nucleons too much energy in the nucleus

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alpha radiation

answer

helium nucleus +2 charge mass of 4u

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Beta-minus

answer

electron -1 charge negligible mass

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Beta-plus

answer

positron +1 charge negligible mass

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gamma radiation

answer

short-wavelength, high frequency electromagnetic wave 0 charge 0 mass

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ionising radiation

answer

radioactive particle hits an atom knocks of electrons, creating an ion

question

features of alpha

answer

strongly ionising slow speed absorbed by paper or a few cm of air affected by magnetic field

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features of Beta-minus

answer

weakly ionising fast speed absorbed by 3mm of aluminium affected by magnetic field

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features of Beta-plus

answer

annihilated by electron virtually 0 range

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features of gamma

answer

very weakly ionising speed of light absorbed by many cm of lead or several m of concrete not affected by magnetic field

question

uses of radioactive substances

answer

dating organic material diagnosing medical problems sterilising food smoke alarms

question

radiocarbon dating

answer

carbon-14 living plants take in carbon-14 during photosynthesis activity of carbon-14 starts to fall when they die half-life of 5730 years archeological finds can be tested to find the current amount of carbon-14 anthem and date them

question

smoke detectors

answer

weak source of alpha radiation close to two electrodes radiation ionises air and a current flows smoke absorbs the radiation current stops and alarm sounds alpha allows current to flow but doesn't travel very far

question

ionising property of alpha

answer

strongly positive easily pull electrons of atoms losing an atom transfers some of the energy from the alpha particle to the atom alpha particle quickly ionises many atoms and loses all its energy 10,000 ionisations per particle

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ionising power of Beta-minus

answer

lower mass and charge but higher speed 100 ionisations per particle cause less damage to the body tissue than alpha

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use in medicine

answer

Beta-minus used to target and damage cancerous cells passes through healthy tissue without causing too many problems gamma used for diagnostic techniques even more weakly ionising so does less damage

question

features of radioactive decay

answer

completely random- can't predict which atom will decay when overall behaviour of a very large no of atoms shows a pattern any sample of a particular isotope has the same rate of decay

question

activity of a sample

answer

no of atoms that decay each second proportional to the size of the sample

question

decay constant ⋋

answer

measures how quickly an isotope will decay s-1

question

activity =

answer

decay constant x number of atoms

question

half-life (T1/2)

answer

the average time it takes for the number of undecided atoms in an isotope to halve

question

how is half-life measured

answer

measuring the time it takes for the activity to halve ln2/⋋

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N =

answer

No e^-⋋t

question

A =

answer

Ao e^-⋋t

question

similarities between discharging capacitors and radioactive isotopes

answer

exponential decay quantity that decays is Q- amount of charge left/ N- number of unstable nuclei remaining initially charge/ number of nuclei is Qo/No amount of charge takes CR seconds to fall to 37% number of nuclei takes 1/⋋ to fall to 37% half-life for charge is ln2xCR half-life for number of nuclei is ln2/⋋

question

stability graph

answer

Z (atomic number) against N (number of neutrons) line of stability plotted using a stable nuclei beta-minus emitters above line beta-plus emitter below line alpha emitters Z>82

question

when does alpha emission happen

answer

in very heavy atoms more than 82 protons e.g. uranium, radium nuclei too massive to be stable

question

When does Beta-minus emission happen

answer

electron and an antineutrino in isotopes that have many more neutrons than protons one of the neutrons in the nucleus is changed into a proton

question

when does gamma emission happen

answer

nucleus has excess energy- excited energy lost by emitting game ray often happens after alpha or beta decay

question

what is conserved in nuclear reactions

answer

energy momentum proton number/charge nucleon number

question

why is mass not conserved

answer

E = mc2 mass and energy are equivalent energy released when the nucleons bonded together account for the missing mass energy released = mass defect x c2

question

mass defect

answer

the mass of a nucleus is less than the mass of its constituent parts

question

hadrons

answer

feel the strong interaction not fundamental particles mad cup of quarks baryons and mesons

question

baryons

answer

protons and neutrons sigmas all decay to protons protons are the only stable baryon

question

baryon number

answer

number of baryons total baryon number in any particle reaction never changes

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mesons

answer

unstable baryon no B = 0 pions Kaons mesons interact with baryons via the strong interaction

question

pions

answer

lightest mesons 3 versions with different electric charges- π+, π0, π- discovered in cosmic rays lots in high energy particle collisions

question

Kaons

answer

heavier and more unstable than pions K+, K-, K0

question

leptons

answer

fundamental particles don't feel the strong interaction weak interaction, gravity and electromagnetic force if charged electrons, muons and taus each have they own neutrino

question

muons and taus

answer

heavy electrons unstable decay into stable electrons

question

neutrinos

answer

0 or almost 0mass 0 charge only take part in weak interactions can pass right through the Earth without anything happening to it

question

lepton number

answer

each lepton has a lepton number of +1 electron, muon and tau have to be counted separately

question

neutron decay

answer

neutron --> proton + electron + antineutrino caused by the weak interaction

question

prediction of antiparticles

answer

Paul Dirac predicted the positron like the electron but with opposite electric charge discovered in a cosmic ray experiment

question

antiparticles

answer

have the same mass but with opposite charge baryon/ lepton numbers of -1

question

creating matter and antimatter from energy

answer

E = mc2 when energy is converted into mass you have to make equal amounts of matter and antimatter

question

particle-antiparticle production

answer

one gamma ray photon has enough energy to produce that much mass tends to happen near nucleus- helps conserve momentum usually electron-positron pairs because they have a relatively low mass

question

why are the particle tracks curved

answer

usually a magnetic field present in particle physics experiments curve i opposite directions because of the opposite charges

question

annihilation

answer

particle meets an antiparticle mass converted to energy (gamma ray photons) antiparticles can only exist for a fraction of a second before this happens

question

antiparticles of mesons

answer

itself π- antiparticle of π+ π0 antiparticle is itself

question

quarks

answer

building blocks for hadrons up quark (u) down quark (d) strange quark (s) top bottom charm

question

antiquarks

answer

make up antiparticles of hadrons opposite properties to quarks

question

up quark

answer

u +2/3 charge +1/3 baryon number 0 strangenesss

question

down quark

answer

d -1/3 charge +1/3 baryon number 0 strangeness

question

strange quark

answer

s -1/3 charge +1/3 baryon number -1 strangeness

question

evidence for quarks

answer

hitting protons with high energy electrons the way electrons scattered showed there were three concentration of charge inside the proton

question

quarks in a proton

answer

uud

question

quarks in a neutron

answer

udd

question

quarks in pions

answer

up and own quarks and their antiquarks two quarks per pion

question

quarks in kaons

answer

have strangeness s quark + up/down quark or antiquarks two quarks per kaon

question

quark confinement

answer

cannot operate quarks if you blast a proton with enough energy, the energy just gets change into more quarks and antiquarks makes mesons

question

weak interaction

answer

can change a d quark into a u quark neutron --> proton

question

properties conserved in particle reactions

answer

total charge baryon number strangeness three types of lepton number seperately

question

binding energy

answer

energy needed to separate all of the nucleons in a nucleus equivalent to the mass defect measure in MeV

question

binding energy per unit of mass defect

answer

1u = 931MeV

question

binding energy per nucleon

answer

binding energy B/ nucleon number A

question

high binding energy per nucleon means

answer

more energy is needed to remove nucleons from the nucleus more stable

question

maximum point of a binding energy per nucleon against nucleon number graph

answer

nucleon number 56 iron

question

change in binding energy =

answer

energy released

question

fission

answer

large nuclei that are unstable (at least 83 protons) randomly split into two smaller nuclei

question

spontaneous fission

answer

happens by itself the larger the nucleus the more unstable so the more likely limits the no of nucleons a nucleus can contain limits the no of possible elements

question

induced fission

answer

we encourage it to happen make a neutron enter a uranium-235 nucleus causes it to become very unstable

question

thermal neutron

answer

a low energy neutron

question

why is energy released during fission

answer

new, smaller nuclei have a higher binding energy per nucleon increase in the binding energy per nucleon

question

nuclear reactors

answer

road son uranium that are rich in uranium-235- fuel fission reactions produce more neutrons which induce other nuclei to fission- chain reaction neutrons slowed down so they can be captured by the uranium nuclei- thermal neutrons fuel rods placed in a moderator to slow down and absorb neutrons critical mass control rods coolant- removes heat and used to make steam

question

choice of moderator

answer

slow down some of the neutrons enough to cause further fission reaction continues at a steady rate a moderator that absorbs more neutrons the higher the temperature will decrease the chance of meltdown if the reactor overheats

question

critical mass

answer

the amount of fuel needed for one fission to follow another at a steady rate any less is sub-critical mass

question

supercritical mass

answer

several new fissions normally follow each fission

question

control rods

answer

limit the no of neutrons in the reactor absorb neutrons so rate of fission is controlled made of boron can be inserted by varying amounts to control reaction rate released fully during an emergency to stop reaction as quickly as possible

question

atomic bombs

answer

runaway reaction large amounts of energy released in a very short time many new fission follow each fission

question

fission waste products

answer

larger proportion of neutrons than stable nuclei of a similar atomic number unstable and radioactive can be used for practical applications e.g. medical tracers

question

disposing of fission waste

answer

initially very hot placed in cooling ponds until the temperature falls to a safe level stored underground in sealed containers until the activity has fallen sufficiently

question

fusion

answer

two light nuclei combine to create a larger nucleus

question

why does fusion release energy

answer

increases binding energy per nucleon dramatically lot of energy released

question

requirement for fusion

answer

nuclei must have enough energy to overcome the electrostatic repulsion between them and get close enough for the strong interaction to bind them 1MeV of energy

question

fusion in stars

answer

temperature in the core of stars is so high atoms dont exist- electrons stripped away leaving a plasma energy released helps to maintain the temperature for further fusion reactions

question

plasma

answer

mixture of positively charged nuclei and free electrons

question

problem with experimental fusion reactors

answer

electricity generated is less than the amount needed ti get the reactor up to temperature

question

suggest why a alpha source is the most suitable for smoke detectors

answer

most strongly ionising more likely to be absorbed by smoke range is small so unlikely to constitute a hazard to the user

question

spontaneous nuclear decay

answer

decay of a particular nucleus is not affected by the presence of other nuclei decay of nuclei cannot be affected by chemical reactions or external factors such as temperature and pressure

question

random nuclear decay

answer

impossible to predict when a particular nucleus in a sample is going to decay each nucleus in a sample has the same chance of decaying per unit time

question

decay constant

answer

probability that an individual nucleus will decay per unit time interval

question

activity

answer

rate at which nuclei in a radioactive sample decay or disintegrate

question

reasons why count rate is lower than activity of a sample

answer

gamma-rays are not always detected as weakly ionising counter is inefficient some radiation absorbed within the sample before reaching the detector detector directional- some radiation will move away from detector rather than towards it

question

carbon dating

answer

constant fraction of carbon-14 in living tissue once organism decays, radioactive isotope decays gradually measure the fraction of carbon-14 nuclei determine time since death

question

problems with carbon dating

answer

can't be used on material that is less than a century old- material that died recently will show little change in carbon-14 modern living tissue may contain a reduced fraction of carbon-14 because we have burned so much fossil fuel- contains little carbon-14 fraction of carbon-14 is very small- activities are small small uncertainty in the half-life of carbon-14

question

similarities and differences between the decay of radioactive nuclei and decay of charge on a capacitor

answer

both decay exponentially equations same charge remaining on a capacitor after a certain time can be predicted exactly number of undecided nuclei left after a certain time shows statistical variation graph of N against t has random fluctuations because decay is random and spontaneous

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