The development of the model of the atom - Teacher Slides.pptx

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The development of the model of the atom

Atomic structure

Lesson 3 of 10

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Warm-up

Atomic structure

Lesson 3 of 10

Answer each one, then check.

1

Who discovered the electron?

2

What is a scientific model?

3

What is an alpha particle?

4

What is evidence?

5

Who discovered the neutron?

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Warm-up - Answers

Atomic structure

Lesson 3 of 10

1

Who discovered the electron?

J. J. Thomson

2

What is a scientific model?

A simplified representation used to explain observations

3

What is an alpha particle?

Two protons and two neutrons (a helium nucleus)

4

What is evidence?

Results from experiments that support or reject an idea

5

Who discovered the neutron?

James Chadwick

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Learning Objectives

Atomic structure

Lesson 3 of 10

1

Describe how the atomic model has changed over time.

2

Describe the plum pudding model and the nuclear model.

3

Explain why the alpha particle scattering experiment led to the nuclear model.

4

Describe the contributions of Bohr, the discovery of the proton and Chadwick's neutron.

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Atomic structure

Lesson 3 of 10

CHANGING MODELS

New experimental evidence may lead to a scientific model being changed or replaced.

The atom has been thought of as a solid sphere, a plum pudding, a nuclear atom, then with electrons in specific orbits, and finally with protons and neutrons in the nucleus.

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How the Atom Model Changed

Atomic structure

Lesson 3 of 10

Each new model came from new evidence.

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The Alpha Scattering Experiment

Atomic structure

Lesson 3 of 10

Most alpha particles passed straight through.

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Evidence and Conclusions

Atomic structure

Lesson 3 of 10

Match each result to its explanation.

Observation

Conclusion

Most alpha particles went straight through

Most of the atom is empty space

Some were deflected through small angles

The nucleus is positive and repels the positive alpha particles

A very few bounced back

The mass is concentrated in a tiny, dense, charged nucleus

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Plum Pudding or Nuclear?

Atomic structure

Lesson 3 of 10

PLUM PUDDING MODEL

• A ball of positive charge.

• Negative electrons embedded in it.

• Charge and mass spread throughout the atom.

NUCLEAR MODEL

• A tiny, positively charged nucleus at the centre.

• Electrons orbit at a distance.

• Most of the atom is empty space.

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Why the Model Changed

Atomic structure

Lesson 3 of 10

Explain why the results of the alpha scattering experiment led to the plum pudding model being replaced.

1

Plum pudding prediction

Alpha particles should pass through with only small deflections

2

Result

Some were deflected by large angles and a few rebounded

3

Conclusion

A concentrated, positive nucleus must exist, so the nuclear model replaced the plum pudding model

ANSWER

The scattering results could not be explained by the plum pudding model, so the nuclear model replaced it.

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The Story

Atomic structure

Lesson 3 of 10

Learn the sequence.

Before the electron

Atoms were thought to be tiny spheres that could not be divided.

Discovery of the electron

Led to the plum pudding model.

Alpha scattering

Led to the nuclear model; the nucleus was charged.

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The Story (cont.)

Atomic structure

Lesson 3 of 10

Bohr

Adapted the nuclear model with electrons orbiting at specific distances; his calculations agreed with experiments.

Protons

The nucleus's charge was divided into whole-number units: protons.

Chadwick

Provided evidence for neutrons about 20 years after the nucleus was accepted.

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Key Terms

Atomic structure

Lesson 3 of 10

Model

A representation used to explain observations.

Plum pudding model

A ball of positive charge with electrons embedded.

Alpha scattering

An experiment in which alpha particles are fired at gold foil.

Nuclear model

An atom with a small, dense, positive nucleus.

Bohr model

Electrons orbit the nucleus at specific distances.

Evidence

Experimental results that support or contradict a model.

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Atomic structure

Lesson 3 of 10

YOUR TASK

Predict the Result

12 minutes

If the plum pudding model were correct, what would happen to alpha particles fired at gold foil? What actually happened, and what did it show?

1

Predict with plum pudding.

2

Compare with the result.

WHAT A GOOD ANSWER SHOWS

They would pass through with small deflections. In fact most passed straight through, some were deflected by large angles and a few bounced back, showing a small dense positive nucleus.

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Can I...?

Atomic structure

Lesson 3 of 10

Describe the plum pudding model.

Describe the nuclear model.

Describe the alpha scattering results.

Explain why the model changed.

Say what Bohr added.

Say what Chadwick showed.

Explain why models change.

Order the models.

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Summary & Exam Focus

Plum pudding: ball of positive charge with electrons in it.

Nuclear: small positive nucleus with electrons around it.

Alpha scattering evidence changed the model.

Bohr: specific orbits. Chadwick: neutrons.

EXAM FOCUS

Describe the difference between the plum pudding model and the nuclear model of the atom. (3 marks)

Say where the positive charge and mass are in each.

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Exam Practice: The development of the model of the atom

Atomic structure

Lesson 3 of 10

Answer all questions. Use the mark allocation as a guide to how much to write. · 18 minutes

Question 1 · 2 marks · Describe

Describe the plum pudding model of the atom.

Question 2 · 4 marks · Explain

The diagram shows the paths of alpha particles fired at thin gold foil. Explain how these results led to the nuclear model of the atom.

Question 3 · 3 marks · Describe

Describe how the nuclear model of the atom differs from the plum pudding model.

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Exam Practice: The development of the model of the atom (cont.)

Atomic structure

Lesson 3 of 10

Question 4 · 2 marks · State

State two ways in which Bohr changed the nuclear model, or what the later experiments showed.

Question 5 · 6 marks · Explain

Describe how the model of the atom has changed, from the idea of a solid sphere to the model with protons and neutrons in the nucleus…

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Question 1 · 2 marks · Describe

Atomic structure

Lesson 3 of 10

“

Describe the plum pudding model of the atom.

HOW TO ANSWER IT

Command word: Describe. Worth 2 marks, so plan before writing.

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Question 1 · mark scheme

Atomic structure

Lesson 3 of 10

2 marks available. Award a mark for each point made.

Ball of positive charge

1 mark

Electrons embedded in it

1 mark

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Question 2 · 4 marks · Explain

Atomic structure

Lesson 3 of 10

The diagram shows the paths of alpha particles fired at thin gold foil. Explain how these results led to the nuclear model of the atom. (4 marks)

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Question 2 · mark scheme

Atomic structure

Lesson 3 of 10

4 marks available. Award a mark for each point made.

Most went straight through so the atom is mostly empty space

1 mark

Some deflected: the nucleus is positive

1 mark

A few rebounded: mass concentrated in a small nucleus

1 mark

Plum pudding could not explain this

1 mark

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Question 3 · 3 marks · Describe

Atomic structure

Lesson 3 of 10

“

Describe how the nuclear model of the atom differs from the plum pudding model.

HOW TO ANSWER IT

Command word: Describe. Worth 3 marks, so plan before writing.

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Question 3 · mark scheme

Atomic structure

Lesson 3 of 10

3 marks available. Award a mark for each point made.

Nuclear: positive charge and mass in a small nucleus

1 mark

Electrons outside the nucleus

1 mark

Plum pudding: charge spread out

1 mark

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Question 4 · 2 marks · State

Atomic structure

Lesson 3 of 10

“

State two ways in which Bohr changed the nuclear model, or what the later experiments showed.

HOW TO ANSWER IT

Command word: State. Worth 2 marks, so plan before writing.

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Question 4 · mark scheme

Atomic structure

Lesson 3 of 10

2 marks available. Award a mark for each point made.

Electrons orbit at specific distances

1 mark

Protons and neutrons in the nucleus

1 mark

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Question 5 · 6 marks · Explain

Atomic structure

Lesson 3 of 10

“

Describe how the model of the atom has changed, from the idea of a solid sphere to the model with protons and neutrons in the nucleus. Explain why each change was made.

HOW TO ANSWER IT

Command word: Explain. Worth 6 marks, so plan before writing.

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Question 5 · mark scheme

Atomic structure

Lesson 3 of 10

6 marks available. Award a mark for each point made.

Level 3 (5 to 6 marks): a correct sequence (solid sphere, plum pudding, nuclear model, Bohr, protons and neutrons) with the evidence for at least three changes, including alpha scattering

5 to 6 marks

Level 2 (3 to 4 marks): at least three models named with some evidence

3 to 4 marks

Level 1 (1 to 2 marks): one or two models described

1 to 2 marks

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