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The Scattering The Scattering experimentexperiment

L/O :- To know the structure of an atom and to understand Rutherford and Marsden’s scattering experiment

What does an atom look What does an atom look like? like?

Draw and label a diagram Draw and label a diagram of heliumof helium

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The structure of the atomThe structure of the atom

ELECTRON – negative,

mass nearly nothing

PROTON – positive,

same mass as neutron

(“1”)

NEUTRON – neutral,

same mass as proton

(“1”)

Is this what we

have always thought?

What is this atom mostly made of?

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The structure of the atomThe structure of the atom

Particle Relative Mass Relative Charge

Proton 1 +1

Neutron 1 0

Electron 0 -1

MASS NUMBER = number of protons + number of neutrons

SYMBOL

PROTON NUMBER = number of protons (obviously)

The Ancient GreeksThe Ancient Greeks21/04/23

The ancient Greeks realised that all matter must be made up of tiny ‘indivisible’ particles.

(they figured that you couldn’t just keep chopping something in half forever)

So they came up with the idea of the ‘atom’

They also correctly thought that there are lots of types of atom.

(periodic table of elements)

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Structure of the atomStructure of the atom

A hundred years ago people thought that the atom looked like a “plum pudding” – a sphere of positive charge with negatively charged electrons spread through it…

I did an experiment (with my colleagues Geiger and Marsden)

that proved this idea was wrong. I called it the “Scattering

Experiment”

Ernest Rutherford, British scientist:

Example QuestionExample Question21/04/23

How does Rutherford and Marsden’s gold leaf experiment confirm our model of the

structure of the atom? (4 marks)

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The Rutherford Scattering ExperimentThe Rutherford Scattering Experiment

Thin gold foil a few

atoms thick

Helium Nuclei were fired at a thin gold ‘leaf’. Gold can be rolled out

as flat as a few atoms thick.

A detector was placed in front and behind the leaf to see how

the helium atoms path was affected.

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The Rutherford Scattering ExperimentThe Rutherford Scattering Experiment

Alpha particles (positive

charge, part of helium

atom)

Thin gold foil

Most particles passed through, 1/8000 were

deflected by more than 900

Conclusion – atom is made up of a small, positively charged nucleus surrounded by

electrons orbiting in a “cloud”.

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If the nucleus was not positively charged, what would have happened?

Positively charged helium nuclei (alpha particles) are fired at a thin layer of gold

leaf.

Most of the alpha particles went through, and only a small number deflected back

towards the source.

This proves that the atom is mostly empty space and that the nucleus is small and

positively charged.

QuestionQuestion21/04/23

How does Rutherford and Marsden’s gold leaf experiment confirm the structure of the atom? (4

marks)

QuestionQuestion21/04/23

How does Rutherford and Marsden’s gold leaf experiment confirm the structure of the atom? (4

marks)

To answer this question we need to say ‘what the current model of the atom is’ and how the experiment

proves this.

Nuclear model is:Postively charged small nucleus.

Most of atom is empty space.

How can we write this in full?

QuestionQuestion21/04/23

How does Rutherford and Marsden’s gold leaf experiment confirm the structure of the atom? (4

marks)

It proves that most of the atom is empty space, (1 mark) because most of the helium nuclei went

through the leaf un-deflected (2nd mark)Only a small number of particles were reflected back

towards the source. (3rd mark)The gold nuclei must be positively charged because it

repelled the positively charged helium nuclei. (4 marks)

Exam Exam questioquestio

nn

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Rutherford's teamfired small particlesat gold leaf

nearly all theparticles wentstraight through

very few particlesbounced back

Scientists thought that all the parts in atoms were evenly spread.(a)What did scientists expect to happen in this experiment? (2 marks)

(b) Complete the sentence.The experiment showed that atoms must be mostly empty ....................... (1 mark)

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Explain how Rutherford’s and Marsden’s experiment changed our understanding

of atoms

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The structure of the atomThe structure of the atom

Particle Relative Mass Relative Charge

Proton 1 +1

Neutron 1 0

Electron 0 -1

MASS NUMBER = number of protons + number of neutrons

SYMBOL

PROTON NUMBER = number of protons (obviously)

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IsotopesIsotopesAn isotope is an atom with a different number of neutrons:

Each isotope has 8 protons – if it didn’t then it just wouldn’t be oxygen any more.

Notice that the mass number is different. How many neutrons does each isotope have?

A “radioisotope” is simply an isotope that is radioactive – e.g. carbon 14, which is used in carbon dating.