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Module P7 L8

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Rutherfords Alpha Scattering Experiment

The is one of the most important experiments inthe history of science.

Professor Rutherford wanted to see whathappened to alpha particles* when they collidedwith atoms.

He asked two ofhis students (Hans Geiger and

Ernest Marsden) to carry out the experiment.

*alpha particles very small, heavy +vely charged particles emitted by someradioactive substances

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Shield

Source of

alpha particles Very thin gold foilVacuum

(air pumped out)

Moveable

Microscope

Zinc sulphide

screen(glows when hit by alpha particles)

Rutherford

Experiment:

the set up

Geiger and

Marsden

counted the

tiny green

flashes in the

microscopeproduced when

alpha particleshit the screen.

Both Geiger

and

Marsden

described

this as one

of the mostdifficult and

boring

experiments

theyd ever

had to do.

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Geiger and Marsden

placed the microscope

as shown.

As expected,most of the

alpha particles

went straight

through

A few alpha

particles were

scattered by

angles less than

90, also as

expected.

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Geiger and Marsden took their results to Professor Rutherford.

Rutherford: Hmmsome nice results.Geiger and Marsden: Thanks, Professor.Rutherford: But tell me, why didnt you put the

microscope behindthe gold foil?

Geiger and Marsden: Because theres no point! There is

absolutely nothing inside anatom that could reflect an alpha

particle! It would be a complete and

utter waste of time!

Rutherford: Remind me whos the professor

here? Go and do the experiment!

Geiger and Marsden: Awwwwdo we have to?

Rutherford.: Yes and stop pulling faces

behind my back![PS this conversation probably neverhappened]

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Under protest, Geiger

and Marsden placed the

microscope behind the

gold leaf.

Much to their

surprise, a very

small number of

alpha particles(about 1 in

8000) bounced

off the gold

atoms!

They handed

the results to

Professor

Rutherford who

now had to

explain whatwas going on.

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Rutherfords experiment proved that

most of the mass of an atom was

concentrated in an incredibly tiny central

nucleus it was the beginning of the

nuclear age.

Up until Rutherfords

experiment, it was thought that

atoms were like a plum

pudding the electrons were

negative plums embedded in

a ball of positive pudding

Plum Pudding Anyone?

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Nobel Prize? Aw, do I have to

The top scientific honour in the

world in the Nobel Prize.

Ernest Rutherford was awarded

the Nobel Prize in 1908 for

discovering the atomic nucleus.(He deserved it it was his

calculations based on the data that

measured the size of the nucleus.

Geiger and Marsden were given full

credit in the published scientificpaper.)

However, he was slightly

disappointed because he was

given the Nobel Prize for

Chemistry instead of Physics

All science is

either

Ph

ysics orstamp

collecting

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Rutherfords Model of the Atom

Please note that the electrons live in specific energy levels.

An electron will move up to a higher energy level if it is given energy.

An electron will move down to a lower energy level if it loses energy.

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Inside the Nucleus

The protons inside a

nucleus repel each other

because they eachhave a

positive charge.

It takes an incredibly strong

force to hold the nucleus

together.

This incredibly strong force

is called . . .

. . . the Strong force.

Unlike the electromagnetic

force, it has a very, very

short range.

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FusionThe Sun shines because it is

fusing atoms ofhydrogen intohelium.

This releases energy.

Originally the Sun was almost

100% hydrogen. Now it is 71%

hydrogen and 29% helium.

Fusion is the opposite of fission

(splitting the atom)

High temperatures are needed so

that the nucleii are moving fast

enough to overcome the

electrical repulsion of the protons.

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Spectra

Continuous Spectrum no

gaps

Absorption spectrum some wavelengths have

been absorbed

Emission spectrum

some wavelengths areemitted

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Rutherfords Model of the Atom - revisited

Please note that the electrons live in specific energy levels.

An electron will move up to a higher energy level if it is given energy.

An electron will move down to a lower energy level if it loses energy.

Nucleus

Electron

higher energy levels

Lowest Energy Level

(ground state)

The energy levels

can be shown on a

diagram like this

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Understanding Line Spectra 1

Electrons inside atoms

can only have fixed levels

of energy i.e. they arequantised.

The ladder of allowed

energy levels is unique to

each element.

Lowest energy

Highest energy

Electron starts here

Photon of just the

right energy emitted

This spectral line

produced

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Understanding Line Spectra 2

Electrons inside atoms

can only have fixed levels

of energy i.e. they arequantised.

The ladder of allowed

energy levels is unique to

each element.

The pattern of dark lines

is a fingerprint for each

element. Lowest energy

Highest energy

Electron starts here

Photon of just the

right energy arrives

Electron jumps to

higher energy level

Absorption creates

dark line here

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Solar Spectrum

The numbers s

how t

he wavelengt

hin nanometers. 1 nm = 10

-9

m.

Scientists were able to identify all the elements in the Sun (and other stars) by

looking at the dark absorption lines.

For example, the element helium was discovered in the Sun in the 1890s manyyears before it was discovered on Earth.

Scientists can also use the position of the dark absorption lines (Fraunhofer lines)

to calculate red and/or blue shift.