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Module P7 L8

May 30, 2018

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geth-jones

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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.

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