Physics 001.002.001 The Nuclear Model of the Atom - Model 1

Physics 001.002.001 The Nuclear Model of the Atom

Alignment

Learning Intentions

By the end of the lesson, students will be able to:

• Describe the nuclear model of the atom as a small, dense, positively charged nucleus surrounded by electrons.
• Identify the relative location, charge and approximate mass contribution of protons, neutrons and electrons.
• Use evidence from alpha-particle scattering to justify why the atom is mostly empty space.
• Distinguish the nuclear model from earlier simple atomic models, such as the plum pudding model.

Success Criteria

By the end of the lesson, students have successfully:

• Labelled a diagram of the nuclear model of the atom.
• Described the atom using the terms nucleus, proton, neutron, electron and empty space.
• Explained why most alpha particles passed through gold foil while a small number were deflected.
• Written a clear description of the nuclear model using correct physics language.

Syllabus Reference

• Unit 1: Thermal, Nuclear and Electrical Physics
• Topic 2: Ionising Radiation and Nuclear Reactions
• Nuclear Model and Stability: Describe the nuclear model of the atom characterised by a small nucleus surrounded by electrons.

Phenomenon

When alpha particles are fired at a very thin sheet of gold foil, most pass straight through, some are slightly deflected, and a very small number bounce back at large angles.

This was surprising because if positive charge and mass were spread evenly through the atom, as in the plum pudding model, the alpha particles should only have passed through with very small changes in direction.

The observation suggests that:

• most of the atom is empty space
• nearly all the mass is concentrated in a tiny nucleus
• the nucleus is positively charged
• electrons occupy the space around the nucleus

Key Idea

The nuclear model describes the atom as having a tiny, dense, positively charged nucleus made of protons and neutrons, surrounded by negatively charged electrons.

Concept

The concept and thought that best describes the cause of the phenomenon is below.

Atoms are not solid spheres. In the nuclear model, the atom contains a central nucleus that holds almost all the atom’s mass. The nucleus is extremely small compared with the total size of the atom. Electrons are found in the region surrounding the nucleus.

The nucleus contains:

• protons, which have positive charge
• neutrons, which have no charge

The surrounding region contains:

• electrons, which have negative charge and much smaller mass than protons or neutrons

A neutral atom has equal numbers of protons and electrons, so the total positive charge balances the total negative charge from a sufficient distance away.

Convention

The key conventions associated with the concept and in the branch of established knowledge is below.

• Proton charge is written as .
• Electron charge is written as .
• Neutron charge is written as .
• The nucleus is shown at the centre of an atom diagram.
• Electrons are shown outside the nucleus.
• Atomic diagrams are not drawn to scale because the nucleus is far too small compared with the whole atom.
• The number of protons determines the element.
• The atom is mostly empty space.

Misconceptions

Common misconceptions students have regarding the concept when applying to various situations and solving problems. It could be a conceptual, mathematical or logical misconception.

• The nucleus is large compared with the atom.
• Electrons are inside the nucleus.
• The atom is a solid ball of matter.
• Most of the atom’s mass is found in the electrons.
• A neutral atom has no charged particles.

Further Reading

• Rutherford gold foil experiment
• Development of atomic models
• Nuclear model of the atom
• Isotopes and nuclides
• Radioactive decay and nuclear stability

Explicit Instruction

1. Introduce the plum pudding model.

   • The atom was once modelled as a spread-out sphere of positive charge with electrons embedded throughout it.
   • This model suggested that mass and charge were distributed throughout the atom.

2. Describe Rutherford’s alpha-particle scattering experiment.

   • Alpha particles are positively charged.
   • They were fired at thin gold foil.
   • Most alpha particles passed straight through.
   • Some alpha particles were deflected.
   • A very small number bounced back.

3. Interpret the observations.

   • Most alpha particles passing through means the atom is mostly empty space.
   • A few alpha particles being deflected means there is a concentrated positive charge inside the atom.
   • A very small number bouncing back means this positive charge is very dense and contains most of the atom’s mass.

4. State the nuclear model.

   • The atom has a small, dense, positively charged nucleus.
   • The nucleus contains protons and neutrons.
   • Electrons surround the nucleus.
   • Most of the atom is empty space.

5. Connect the model to later nuclear physics.

   • The nuclear model is needed to understand nuclides, nuclear stability, radioactive decay and nuclear reactions.

Worked Examples

Worked Example 1

Question: Describe the nuclear model of the atom.

Answer:

The nuclear model describes the atom as having a very small, dense nucleus at the centre. The nucleus contains protons and neutrons and has a positive charge because of the protons. Electrons are negatively charged and are found in the region surrounding the nucleus. Most of the atom is empty space.

Teacher notes:

• A good description includes the nucleus, electrons, charge, mass and empty space.
• Students should avoid saying that electrons are “in” the nucleus.

Worked Example 2

Question: In the gold foil experiment, most alpha particles passed straight through the foil. What does this suggest about the structure of the atom?

Answer:

Most alpha particles passing straight through suggests that most of the atom is empty space. If the atom were solid or if its positive charge were spread throughout the whole atom, more alpha particles would have been deflected.

Teacher notes:

• Observation: most alpha particles passed straight through.
• Inference: atoms are mostly empty space.
• This supports the nuclear model.

Worked Example 3

Question: A small number of alpha particles were deflected through large angles or bounced back. What does this suggest about the atom?

Answer:

Alpha particles are positively charged, so they are repelled by positive charge. Large deflections suggest that the atom contains a very small, concentrated, positively charged nucleus. The fact that only a small number were deflected shows that the nucleus is tiny compared with the whole atom.

Teacher notes:

• Observation: a small number of alpha particles were strongly deflected.
• Inference: the nucleus is small, dense and positively charged.
• Link to electrostatic repulsion: positive alpha particles are repelled by the positive nucleus.

Check for Understanding

Check 1

Prompt:

Complete the sentence:

The nuclear model describes the atom as a small, dense, positively charged __________ surrounded by negatively charged __________.

Expected answer:

The nuclear model describes the atom as a small, dense, positively charged nucleus surrounded by negatively charged electrons.

Check 2

Prompt:

True or false: Most of the mass of an atom is found in the electrons.

Expected answer:

False. Most of the mass of an atom is found in the nucleus, which contains protons and neutrons.

Check 3

Prompt:

Why did most alpha particles pass straight through the gold foil?

Expected answer:

Most alpha particles passed straight through because atoms are mostly empty space. The nucleus is very small, so most alpha particles did not come close enough to the nucleus to be deflected.

Investigation (Alternative to Explicit)

Hypothesis

If the atom contains a small, dense, positively charged nucleus, then most alpha particles fired at thin gold foil will pass straight through, while a small number will be deflected by the nucleus.

Data Collection

Use a simulation, teacher demonstration or physical analogy.

Suggested model:

• Use a large open area to represent the atom.
• Place a small heavy object in the centre to represent the nucleus.
• Roll marbles or small balls through the area to represent alpha particles.
• Record whether each particle passes straight through, is slightly deflected, or is strongly deflected.

Data table:

Trial	Observation	Atomic interpretation
1	Passed straight through	Mostly empty space
2	Slightly deflected	Passed near the nucleus
3	Strongly deflected	Passed very close to the nucleus

Analysis

Students answer:

1. Which result occurred most often?
2. What does this suggest about the amount of empty space in the atom?
3. Why did only a small number of particles deflect strongly?
4. How does the model support the idea of a small nucleus?

Expected analysis:

Most particles pass through because the atom is mostly empty space. Only particles that pass close to the small central nucleus are deflected. This supports the nuclear model of the atom.

Evaluation

Students consider:

• The physical model is not to scale.
• The model shows deflection but does not fully represent electrostatic repulsion.
• Real alpha particles are affected by electric forces, not by direct contact.
• The model is useful for visualising why most particles pass through and only a few are deflected.

Problems

The following problems are designed to practise describing and interpreting the nuclear model of the atom.

1. Describe the nuclear model of the atom in two sentences.

2. Copy and complete the table.

Particle	Relative charge	Location in atom
Proton
Neutron
Electron

3. Explain why a neutral atom has no overall charge even though it contains charged particles.

4. Explain why the gold foil experiment showed that the atom is mostly empty space.

5. Explain why the gold foil experiment showed that the nucleus is positively charged.

6. A student says, “The nucleus takes up most of the atom’s volume.” Explain why this is incorrect.

7. A student says, “Electrons are found inside the nucleus.” Rewrite this statement correctly.

8. Compare the plum pudding model and the nuclear model of the atom.

9. Draw and label a simple diagram of the nuclear model of the atom. Include:

   • nucleus
   • protons
   • neutrons
   • electrons
   • mostly empty space

10. Write a paragraph that uses the following words correctly:

• nucleus
• proton
• neutron
• electron
• positive
• negative
• empty space

Followup

Self-check

Students rate their confidence:

• I can describe the nuclear model of the atom.
• I can identify protons, neutrons and electrons.
• I can state where most of the atom’s mass is located.
• I can explain why the atom is mostly empty space.
• I can connect Rutherford’s gold foil evidence to the nuclear model.

Exit ticket:

In one paragraph, describe the nuclear model of the atom and explain one piece of evidence that supports it.

Next Topic

Describe nuclides using nomenclature.