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Atomic Models and Atomic Models and RadioactivityRadioactivity
NCEA AS 2.5NCEA AS 2.5
Text Chapters:Text Chapters:
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HistoryHistory
Greeks:Greeks:
4 types of atoms, earth, air, fire, water4 types of atoms, earth, air, fire, water
Used these atoms to explain why things Used these atoms to explain why things happenedhappened
Eg stones fell to the earth because they Eg stones fell to the earth because they were made of earth atomswere made of earth atoms
Atomos = “indivisible”Atomos = “indivisible”
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HistoryHistory
Early 1800sEarly 1800s John DaltonJohn Dalton, an observer of weather and , an observer of weather and
discoverer of color blindness among other discoverer of color blindness among other things, came up with things, came up with atomic theoryatomic theory
All matter is made up of All matter is made up of small indivisble small indivisble particlesparticles known as known as “atoms”“atoms”
Atoms were solid spheresAtoms were solid spheres Drew the first molecular diagramsDrew the first molecular diagrams
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HistoryHistory
J.J. Thompson (1856 -1940) J.J. Thompson (1856 -1940) Studied the “mysterious cathode rays In 1903 he proposed the “Plum pudding
model” for the atom the atom is a sphere of positively charged
matter with electrons embedded like the currents in a “plum pudding”
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Thompson’s ModelThompson’s Model
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HistoryHistory
Ernest RutherfordErnest Rutherford Famous for his Famous for his gold foil experimentgold foil experiment Atom is mainly empty spaceAtom is mainly empty space Small dense positively charged nucleusSmall dense positively charged nucleus Electrons orbiting the nucleusElectrons orbiting the nucleus
(This is the model you have to be able to (This is the model you have to be able to explain for this achievement standard)explain for this achievement standard)
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Since Rutherford….Since Rutherford….
With the help of With the help of quantum theoryquantum theory that was being that was being developed by developed by PlanckPlanck, , EinsteinEinstein and others, the and others, the model continued to evolve…model continued to evolve…
Neils BohrNeils Bohr (1913): electrons occupy fixed (1913): electrons occupy fixed energy levels (not fixed positions)energy levels (not fixed positions)
Louis de BroglieLouis de Broglie (1924): electrons are waves (1924): electrons are waves Erwin SchrodingerErwin Schrodinger (1925): electrons are matter (1925): electrons are matter
waves whose position is based on a statistical waves whose position is based on a statistical probability (enter quantum mechanics)probability (enter quantum mechanics)
ChadwickChadwick (1935): Discovers the neutron. (1935): Discovers the neutron.
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Rutherford’s Gold Foil ExperimentRutherford’s Gold Foil Experiment
He fired alpha He fired alpha particles at a very thin particles at a very thin piece of gold foil and piece of gold foil and measured the angles measured the angles they were scattered they were scattered at.at.
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The ResultsThe Results
Observation 1Observation 1
Most passed right through the gold foilMost passed right through the gold foil
Explanation 1Explanation 1
Atoms are mostly empty spaceAtoms are mostly empty space
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The ResultsThe Results
Observation 2Observation 2
Some were deflected Some were deflected
Explanation 2Explanation 2
The atom contains a positive charge in The atom contains a positive charge in its centre or nucleus that deflects alpha its centre or nucleus that deflects alpha particles (which are positively charged)particles (which are positively charged)
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The ResultsThe Results
Observation 3Observation 3
A rare few bounced directly backwardsA rare few bounced directly backwards
Explanation 3Explanation 3
The positive charge must be small and The positive charge must be small and densely packed so only a few alpha densely packed so only a few alpha particles hit it directly head-on and particles hit it directly head-on and bounce backbounce back
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Rutherford’s ModelRutherford’s Model
Positive nucleus Positive nucleus surrounded by surrounded by orbiting negatively orbiting negatively charged electronscharged electrons
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Nuclear ReactionsNuclear Reactions
3 types:3 types:
Radioactive DecayRadioactive Decay – the spontaneous – the spontaneous emission of particles from the nucleus of emission of particles from the nucleus of an atoman atom
Nuclear FissionNuclear Fission – splitting one large – splitting one large nuclei into two smaller onesnuclei into two smaller ones
Nuclear FusionNuclear Fusion – combining two small – combining two small nuclei into one large one. nuclei into one large one.
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RadioactivityRadioactivity
3 types:3 types:Alpha Alpha Beta Beta GammaGamma
Named in order of their discovery.Named in order of their discovery.Alpha and beta decay don’t usually occur by Alpha and beta decay don’t usually occur by
themselves, there is usually some gamma themselves, there is usually some gamma that occurs with them.that occurs with them.
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The NucleusThe Nucleus
In small atoms, the number of protons and In small atoms, the number of protons and neutrons are usually the same (roughly)neutrons are usually the same (roughly)
In larger atoms, there are usually many In larger atoms, there are usually many more neutrons than protons, in order to more neutrons than protons, in order to keep the nucleus stable. keep the nucleus stable.
If a nucleus is unstable, it may If a nucleus is unstable, it may spontaneously decay to something more spontaneously decay to something more stable by emitting alpha, beta or gamma stable by emitting alpha, beta or gamma radiationradiation
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Alpha ParticlesAlpha Particles
Helium nucleusHelium nucleusCharge of +2Charge of +2Mass of 4 (a.m.u)Mass of 4 (a.m.u)Travel slowly ie. 10% of light speedTravel slowly ie. 10% of light speedDon’t travel very far ie. A few cms in airDon’t travel very far ie. A few cms in airLow penetration power – can be stopped Low penetration power – can be stopped
by a piece of paperby a piece of paperVery good ionising power – because Very good ionising power – because
they’re big and slow.they’re big and slow.
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Beta ParticlesBeta Particles
An electron from the nucleusAn electron from the nucleusCharge of -1Charge of -1Same mass as an electron (effectively 0)Same mass as an electron (effectively 0)Travel relatively fast – up to 95% of light Travel relatively fast – up to 95% of light
speed speed Travel about 30 cms in airTravel about 30 cms in airAverage penetration power – can be Average penetration power – can be
stopped by a few mm of Aluminiumstopped by a few mm of AluminiumAverage ionising powerAverage ionising power
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Gamma RadiationGamma Radiation
A wave of electromagnetic radiation A wave of electromagnetic radiation (energy)(energy)
No chargeNo chargeNo massNo massTravels at light speed Travels at light speed Travels several metres in airTravels several metres in airHigh penetration power – Several cms of High penetration power – Several cms of
lead needed to stop itlead needed to stop itLow ionising power – because no massLow ionising power – because no mass
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RadiationRadiationOne way that the different types of One way that the different types of
radiation can be distinguished is by radiation can be distinguished is by observing their behaviour in a magnetic observing their behaviour in a magnetic field:field:
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The NucleusThe Nucleus
Writing nucleiWriting nuclei
X = element symbolX = element symbol
A = mass number or A = mass number or nucleonnucleon number (the number (the number of p+n)number of p+n)
Z = atomic number Z = atomic number (the number of (the number of protons)protons)
XAZ
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IsotopesIsotopes
Atoms with the same atomic number but Atoms with the same atomic number but different mass numbersdifferent mass numbers
Eg:Eg:
)(
)(31
21
11
tritiumH
deuteriumH
H
C
C
C
146
136
126
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Alpha DecayAlpha Decay
Example: Radium 226 decays to Radon 222 by Example: Radium 226 decays to Radon 222 by alpha decay:alpha decay:
Note: Both mass and charge must be conservedNote: Both mass and charge must be conserved
(ie 226=222+4, 88=86+2(ie 226=222+4, 88=86+2
HeRnRa 42
22286
22688
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Beta DecayBeta Decay
Cobalt 60 decays by beta decay to Nickel Cobalt 60 decays by beta decay to Nickel 6060
Again, mass and charge are conservedAgain, mass and charge are conservedNB. the NB. the or or symbols can be used symbols can be used
instead of He or einstead of He or e
eNiCo 01
6028
6027
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Half-lifeHalf-life
The time it take for the decay rate to have The time it take for the decay rate to have halved, or….halved, or….
The time taken for half of the original The time taken for half of the original atoms to have decayedatoms to have decayed
Usually shown on a graphUsually shown on a graph
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Half-lifeHalf-lifeHalf Life
0
500
1000
1500
2000
2500
3000
3500
4000
4500
0 1 2 3 4 5
Time in days
No
. o
f A
tom
s
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Detecting RadioactivityDetecting Radioactivity
Geiger Counter – detects electrical current Geiger Counter – detects electrical current caused by the ionisation of atoms in a gascaused by the ionisation of atoms in a gas
400V DC Supply
Counter or speaker- Anode: central wire
+Cathode: metal cylinderGeiger-Muller tube filled with low pressure Ar
End: thin mica window
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Uses of RadioactivityUses of Radioactivity
Radiation therapy to treat cancerRadiation therapy to treat cancerSterilisationSterilisationCarbon datingCarbon datingNuclear medicine eg tracersNuclear medicine eg tracersSmoke detectorsSmoke detectors
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Nuclear FissionNuclear Fission
Breaking large unstable nuclei into smaller ones.Breaking large unstable nuclei into smaller ones. Releases a Releases a lotlot of energy of energy Lots of possible combinations of fragments from Lots of possible combinations of fragments from
one initial nucleusone initial nucleus Eg:Eg:
nKrBaUn 10
9236
14156
23592
10 3
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Nuclear FissionNuclear Fission
Only one Only one neutron is neutron is needed to needed to start the start the reaction, but reaction, but several are several are producedproduced
This starts a This starts a “chain “chain reactionreaction””
n
UBa Kr
n n n
UBa Kr
n n n
UBa Kr
n n n
UBa Kr
n n n
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Nuclear FissionNuclear Fission
If the chain reaction is controlled it can be If the chain reaction is controlled it can be used in a nuclear reactorused in a nuclear reactor
If it is uncontrolled it explodes as a nuclear If it is uncontrolled it explodes as a nuclear or atomic bombor atomic bomb
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Nuclear FusionNuclear Fusion
The joining of two small nuclei to form one The joining of two small nuclei to form one larger onelarger one
Again, a lot of energy is producedAgain, a lot of energy is producedThis is the process that powers the sunThis is the process that powers the sunEg:Eg:
nHeHH 10
42
31
21
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Nuclear FusionNuclear Fusion
Fusion requires extreme temperature and Fusion requires extreme temperature and pressure to occur, and has not practically pressure to occur, and has not practically and economically been used in power and economically been used in power generation (yet….)generation (yet….)
Hydrogen bombs have been successfully Hydrogen bombs have been successfully made, but require a fission reaction to made, but require a fission reaction to provide the necessary temp and pressure.provide the necessary temp and pressure.