atomic models and radioactivity ncea as 2.5 text chapter: 14

Atomic Models and Atomic Models and RadioactivityRadioactivity

NCEA AS 2.5NCEA AS 2.5

Text Chapter: 14Text Chapter: 14

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 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



Some were deflected Some were deflected


The atom contains a positive charge in itsThe atom contains a positive charge in its

centre or nucleus that deflects centre or nucleus that deflects

alpha particles (which are positively charged)alpha particles (which are positively charged)



A rare few bounced directly backwardsA rare few bounced directly backwards


The positive charge must be small and denselyThe positive charge must be small and densely

packed so only a few alpha particles hit it packed so only a few alpha particles hit it

directly head-on and bounce backdirectly head-on and bounce back

Rutherford’s ModelRutherford’s Model

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

The NucleusThe Nucleus

Writing nucleiWriting nuclei

X = element symbolX = element symbol

A = mass number or A = mass number or nucleonnucleon number (the number of p+n)number (the number of p+n)

Z = atomic number (the Z = atomic number (the number of protons)number of protons)

How many of each sub-atomic particle does potassium have?

XAZ39 K19

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

Just InterestingJust Interesting

In 1896 Henri Becquerel discovered by In 1896 Henri Becquerel discovered by chance that a rock containing uranium chance that a rock containing uranium when left on wrapped photographic paperwhen left on wrapped photographic paperwas able to darken it.was able to darken it.

Pierre and Marie Curie, French chemists Pierre and Marie Curie, French chemists did most of the early work on radio active did most of the early work on radio active substances. They discovered radium substances. They discovered radium which is far more active than uraniumwhich is far more active than uranium

RadioactivityRadioactivity Radioactivity is a natural decay processRadioactivity is a natural decay process Radioactivity is unaffected by external Radioactivity is unaffected by external

conditions like temperatureconditions like temperature Radioactivity involves the nucleus of Radioactivity involves the nucleus of

the atomthe atom Some elements spontaneously emit Some elements spontaneously emit

particles or energy from their nucleusparticles or energy from their nucleus In a nuclear reaction atoms can change In a nuclear reaction atoms can change

from one element to another.from one element to another.

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:

Balancing Nuclear Equations Balancing Nuclear Equations The atomic numbers are conserved. The atomic numbers are conserved.

That means they have to add up to the same That means they have to add up to the same number on both sides of the equation.number on both sides of the equation.

The same applies to the atomic masses.The same applies to the atomic masses.

XeI 13154

13153 01?

Note: Both mass and charge must be conservedNote: Both mass and charge must be conserved

Try thisTry this

(ie 226=222+4, 88=86+2)(ie 226=222+4, 88=86+2)

HeRn 42

22286Ra226

88

Another OneAnother OneCobalt 60 decays to Nickel 60 emitting aCobalt 60 decays to Nickel 60 emitting a

a gamma ray and ?a gamma ray and ?

eNiCo 01

6028

6027Again, mass and charge are conservedAgain, mass and charge are conservedNB. the NB. the or or symbols can be used instead symbols can be used instead of He or eof He or e

Why have 2 different atoms got the same atomic mass? (type 1 question)

Last one for nowLast one for now

That’s how the nucleus is able to emit That’s how the nucleus is able to emit electronselectrons

ep 01

11 n10

What is bigger a proton or neutron?(Type 1 question again)Think about it and don’t yell out

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

What ½ is being shown here?1 day

Answer These

The half-life of iodine-128 is 25 minutes. How long would it take the radioactivity of a sample to drop from…………….

3000 decays per second to 750 decays per second

2 half lives 50minutes

3000 to 375 decays per second 3 half lives 75 minutes

The half life of thorium-234 is 24 days. Initially a sample contains 6.4 x 1020 atoms.How many thorium atoms will remain after 72 days? 8 x 1019 atoms

How long will it take for 6.3 x 1020 thorium atoms to decay. 144 days

Answer These

Are studying for 2.7 yet?????

The background count in S2 averages out at 6 min-1 .

If a radio active source has a count of 246 min-1 and a week later 21 min-1 what is the ½ life?

240154 x ½ lives42 hours

A sample of atoms 16.5x109

has a 168 hour ½ life.How many decayed atomsare there 3 weeks later?

14.4 x 109

16.5x109 – (16.5x109 x ½ x ½ x ½) =

A radioactive sample has a ½ life of 3.6 days and 1.5 g of radioactive sample remain.

How much was there 14.4 days ago?

14.4/3.6=4So 4 half lives ago

1.5x2x2x2x2= ? 24g

Detecting RadioactivityDetecting RadioactivityGeiger 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 cancer - Radiation therapy to treat cancer - Power stations - Power stations - 22

44-1-100

Sterilization - Sterilization - Carbon datingCarbon dating - - -1-1

00 Nuclear medicine eg tracers - Nuclear medicine eg tracers - -1-1

00 Smoke detectors - Smoke detectors - 22

44 Detecting leaks in metal pipes - Detecting leaks in metal pipes - Blowing up your neighborsBlowing up your neighbors

(not)

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:

KrBaUn 9236

14156

23592

10 ?n103


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


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:

nHH 10

31

21 He4

2?

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.

atomic models and radioactivity ncea as 2.5 text chapter: 14

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