Atomic Models and Atomic Models and RadioactivityRadioactivity

NCEA AS 2.5NCEA AS 2.5

Text Chapters:Text Chapters:

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”

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

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”

Thompson’s ModelThompson’s Model

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)

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.

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.

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

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)

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

Rutherford’s ModelRutherford’s Model

Positive nucleus Positive nucleus surrounded by surrounded by orbiting negatively orbiting negatively charged electronscharged electrons

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.

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.

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

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.

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

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

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:

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

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

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

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

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

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

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

Uses of RadioactivityUses of Radioactivity

Radiation therapy to treat cancerRadiation therapy to treat cancerSterilisationSterilisationCarbon datingCarbon datingNuclear medicine eg tracersNuclear medicine eg tracersSmoke detectorsSmoke detectors

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

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

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

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

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.