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ATOMIC STRUCTURE · CHAPTER 3 OF 12

How did scientists discover the nucleus?

You cannot see an atom’s nucleus through an ordinary microscope. Scientists worked out its structure by sending particles towards atoms and studying what happened next. A few unexpected results changed the model completely.

By the end, you’ll be able to…

  • compare the early solid-sphere, plum pudding and nuclear models
  • describe the alpha-particle scattering experiment
  • link the observations to conclusions about the nucleus
  • describe how Bohr, proton discoveries and Chadwick’s neutron evidence developed the model.

A model changes when the evidence changes

A scientific model is a representation used to explain observations and make predictions. It can be useful without being complete. When reliable new evidence disagrees with its predictions, scientists may need to change or replace it.

Early atomic models treated atoms as tiny, indivisible solid spheres. The discovery of the electron showed that atoms contained smaller particles, so that idea had to change.

INTERACTIVE MODEL COMPARISON

Before the nucleus was discovered

Compare the diagrams. Open each explanation to connect the model with the evidence.

Early solid sphere

Early indivisible solid sphere model
What did this model suggest?

An atom was a tiny sphere that could not be divided into smaller parts. Discovering electrons showed that atoms had internal structure.

Plum pudding model

Plum pudding model: positive charge spread throughout a sphere with negative electrons embedded+++++

Positive charge spread throughout; negative electrons embedded. Overall neutral.

Where was the positive charge?

Positive charge was spread throughout the atom, with negative electrons embedded in it. The total charges balanced, making the atom neutral. There was no tiny central nucleus in this model.

* Historical models, not photographs. Pink represents positive charge in the plum pudding model; blue − symbols represent electrons. White is used for the early sphere because that model did not specify internal charges. Sizes and positions are illustrative.

Testing the plum pudding model

In the gold-foil experiment, Hans Geiger and Ernest Marsden, working with Ernest Rutherford, directed alpha particles at a very thin sheet of gold. A detecting screen produced tiny flashes where particles arrived.

+ +

Alpha particle (α)
Relative charge: +2

Why use alpha particles?

An alpha particle is a helium nucleus: two protons and two neutrons.

Its positive charge makes it sensitive to repulsion from positive nuclear charge.

It has much more mass than an electron, so electrons have little effect on its direction. Large deflections therefore provide evidence about the nucleus.

If the positive charge were spread throughout each atom, most alpha particles should pass through with little deflection. Large changes of direction would be difficult for that model to explain.

How did Rutherford discover the nucleus?

Scientists used the gold-foil experiment to test the plum pudding model of the atom. First, explore the equipment. Then predict what the model would suggest before investigating the actual paths taken by alpha particles.

INTERACTIVE EXPERIMENT DIAGRAM

1. What did the apparatus do?

Click a numbered label. Its pink arrow points to the part being explained.

α Alpha-particle beam α = alpha particle

1 · Shielded source
A radioactive source emits alpha particles. Shielding with a small opening produces a narrow beam aimed at the foil. The opening in the screen allows the beam to enter.

2 · Thin gold foil
The beam encounters gold atoms. Very thin foil reduces repeated scattering through many layers, making the results easier to interpret.

3 · Detecting screen
The screen produces a tiny flash when an alpha particle arrives. Observations at different angles show which particles pass through, deflect or return towards the source.

* Simplified apparatus, not to scale. Pink arrows identify parts; black dashed lines summarise scattering directions. Orange dots represent detector flashes. The foil is enlarged. This is not a home experiment.

INTERACTIVE PREDICTION

2. Which result would be most surprising?

Before exploring the particle paths, think about what scientists expected from the plum pudding model.

Use the plum pudding model: positive charge spread throughout the atom, rather than concentrated at a tiny centre.

Which observation would be hardest for that model to explain?
Try again. Passing through with little deflection was broadly expected.
Try again. Small deflections were possible, but a reversal was much more surprising.
Correct. Turning a fast alpha particle back required a much more concentrated source of repulsion than the model predicted.
INTERACTIVE SCATTERING EXPLORER

3. Follow an alpha particle

Now investigate what actually happened when alpha particles passed through the gold foil. Compare the different paths with your prediction.

Choose a path. Follow the α marker, then reveal what it tells us. The fourth example shows a head-on approach to the nucleus.

Most particles: little or no deflection

+ α α Positive nucleus A greatly enlarged view inside the foil
What does this observation tell us?

Most particles miss the tiny nucleus, so most of the atom’s volume is empty space. The nucleus occupies only a small fraction of the atom.

Some particles: direction changes

+ α α Positive nucleus A greatly enlarged view inside the foil
What does this observation tell us?

A positively charged alpha particle passing close to the positive nucleus is repelled. Its path bends away. The positive charge is concentrated at the centre.

Very few particles: large-angle scattering

+ α α Positive nucleus A greatly enlarged view inside the foil
What does this observation tell us?

A near head-on approach to the concentrated nucleus can turn an alpha particle back through electrostatic repulsion. The nucleus is small, dense and contains most of the atom’s mass.

A head-on approach: a 180° reversal

+ Approaches Returns along the same line α α Positive nucleus A greatly enlarged view inside the foil
Why does it come straight back?

The alpha particle travels directly towards the positive nucleus. Electrostatic repulsion slows it until it momentarily stops, then accelerates it back along the same line: a change in direction of 180°.

It turns before touching the nucleus. This is repulsion between positive charges, rather than a collision with a hard surface.

This is a special case of large-angle scattering, not a separate fourth category of experimental results. An almost head-on approach produces a deflection close to 180°.

* Each moving pink α marker represents one positive alpha particle, with relative charge +2. The dashed line shows its path. The pale outer circle indicates the approximate atom region, not a physical boundary. Gold’s electrons are omitted. Sizes and distances are not to scale, and the selectable examples do not represent equal frequencies. Motion is illustrative.

Three observations changed the picture

Most alpha particles passed straight through the foil. Some were deflected, and a very small proportion were scattered through large angles, including back towards the source.

The important evidence was the pattern of results, especially the rare large-angle scattering. It suggested a tiny, concentrated, positively charged centre containing most of the atom’s mass.

The alpha particles were repelled, rather than bouncing off a solid wall. Their positive charge interacts with the positive nucleus. A large change of direction does not require physical contact with it.
INTERACTIVE EVIDENCE PRACTICE

Match the observation to the conclusion

Choose an explanation for each observation. Feedback appears after selection.

Most alpha particles travel straight through. What does this suggest?
Correct. Most alpha particles do not pass close enough to the tiny nucleus for large deflections.
Try again. A nucleus filling most of the atom would make large deflections much more common.
Try again. The atom contains charges; its overall neutrality does not mean its parts are uncharged.
A very small number are scattered back. What is the best conclusion?
Try again. Spread-out positive charge could not account for these rare, very large deflections.
Correct. Rare large-angle scattering is evidence for a small, concentrated centre of charge and mass.
Try again. The results support an internal nucleus rather than a solid sphere filling the whole atom.

The nuclear model replaced plum pudding

In the nuclear model, positive charge and most of the mass are concentrated in a tiny central nucleus. Electrons occupy the region outside it. Most of the atom’s volume is empty space.

This model explained both the common straight-through paths and the rare large-angle deflections. It replaced plum pudding because it fitted the experimental evidence better.

INTERACTIVE COMPARE THE MODELS

What changed—and what stayed?

Compare the locations of positive charge and electrons. Both models describe an atom with no overall charge.

Plum pudding

Plum pudding model: positive charge spread throughout a sphere with negative electrons embedded+++++

Positive charge spread through the atom. Electrons embedded within it. No central nucleus.

Nuclear model

Nuclear model: tiny positive nucleus, with electrons outside+

Positive charge concentrated in a tiny nucleus. Electrons outside. Most of the volume is empty space.

Did the discovery of the nucleus remove the electrons?

No. Electrons remained part of the model. The major change was where the positive charge and mass were located.

* Schematic comparisons, not specific elements. Blue − symbols are electrons. Pink represents positive charge. Nuclei are enlarged; electron positions are illustrative, and the diagrams are not to scale.

Bohr added specific energy levels

Niels Bohr adapted the nuclear model by proposing that electrons occupy specific energy levels at set distances from the nucleus. His calculations agreed with experimental observations.

As you explored in Chapter 2, electrons can absorb energy to move higher and emit electromagnetic radiation when moving lower. You do not need details of the experiments supporting Bohr’s model here.

INTERACTIVE ATOM BUILDER

What did Bohr add?

The nuclear model placed a positive nucleus at the centre. Bohr proposed that electrons occupy specific energy levels, often called shells.

Reveal Bohr’s addition
Electrons can occupy only allowed energy levels. An electron absorbs energy to move to a higher level and emits electromagnetic radiation when moving to a lower level. Changing level is different from leaving the atom completely.

Build a neutral atom

Select “Next element” to add one proton and one electron. Watch the electrons fill the shells. The dashed pink ring highlights the last electron added.

Hydrogen · H

1 proton · 1 electron · neutral

Electron arrangement: 1

First element

Helium · He

2 protons · 2 electrons · neutral

Electron arrangement: 2

Lithium · Li

3 protons · 3 electrons · neutral

Electron arrangement: 2,1

Beryllium · Be

4 protons · 4 electrons · neutral

Electron arrangement: 2,2

Boron · B

5 protons · 5 electrons · neutral

Electron arrangement: 2,3

Carbon · C

6 protons · 6 electrons · neutral

Electron arrangement: 2,4

Nitrogen · N

7 protons · 7 electrons · neutral

Electron arrangement: 2,5

Oxygen · O

8 protons · 8 electrons · neutral

Electron arrangement: 2,6

Fluorine · F

9 protons · 9 electrons · neutral

Electron arrangement: 2,7

Neon · Ne

10 protons · 10 electrons · neutral

Electron arrangement: 2,8

Sodium · Na

11 protons · 11 electrons · neutral

Electron arrangement: 2,8,1

Magnesium · Mg

12 protons · 12 electrons · neutral

Electron arrangement: 2,8,2

Aluminium · Al

13 protons · 13 electrons · neutral

Electron arrangement: 2,8,3

Silicon · Si

14 protons · 14 electrons · neutral

Electron arrangement: 2,8,4

Phosphorus · P

15 protons · 15 electrons · neutral

Electron arrangement: 2,8,5

Sulfur · S

16 protons · 16 electrons · neutral

Electron arrangement: 2,8,6

Chlorine · Cl

17 protons · 17 electrons · neutral

Electron arrangement: 2,8,7

Argon · Ar

18 protons · 18 electrons · neutral

Electron arrangement: 2,8,8

Electron shells in a neutral atom The selected element and particle counts are given above. Pale blue electrons occupy shells around a positive pink nucleus. The first shell holds two electrons; the next two are filled to eight in this hydrogen-to-argon sequence. +
+ Positive nucleus − Electron Last electron added
Hydrogen has one electron in its first shell. Its nucleus contains one proton, so the charges balance.
The first shell now contains two electrons. It is full.

Predict: where will the next electron go? Select “Next element” to find out.

The third electron occupies the second shell because the first is full. This energy level already exists; it is now occupied.
Two electrons occupy the first shell and two occupy the second: 2,2.
The first shell stays at two electrons. The added electron goes into the second shell: 2,3.
Carbon has six electrons: two in the first shell and four in the second. Its six protons balance its six electrons.
Nitrogen has seven electrons arranged 2,5. Five occupy its outer shell.
Oxygen has eight electrons in total, but only six in its outer shell: 2,6.
Fluorine has seven electrons in its second shell. There is room for one more there.
The second shell now contains eight electrons. It is full.

Predict: where will electron number 11 go? Select “Next element” to find out.

Electron number 11 occupies the third shell. Sodium’s arrangement is 2,8,1. The third shell is now occupied.
Magnesium has two electrons in its third shell: 2,8,2. The inner shells keep their electrons.
Aluminium has 13 electrons arranged 2,8,3. Three occupy its outer shell.
Silicon has 14 electrons arranged 2,8,4. Four occupy its outer shell.
Phosphorus has 15 electrons arranged 2,8,5. Five occupy its outer shell.
Sulfur has 16 electrons arranged 2,8,6. Six occupy its outer shell.
Chlorine has seven electrons in its third shell: 2,8,7.

Predict: will the next electron need a fourth shell? Select “Next element” to check.

No fourth occupied shell is needed. Argon’s 18 electrons are arranged 2,8,8, giving it a complete outer shell in this model.
Why does the proton count increase too?
Each step shows a different neutral element. One extra proton changes the element; one extra electron keeps its overall charge at zero. Adding only an electron to the same atom would produce a negative ion.

* This sequence compares neutral atoms; it is not a method for turning one element into another. The pink centre represents the whole positive nucleus, not one proton. Neutrons are omitted. Circles represent electron energy levels, not physical tracks. Electron positions are schematic, and sizes and distances are not to scale. The 2,8,8 filling pattern shown here applies to this hydrogen-to-argon sequence; shell filling becomes more complex for heavier atoms.

Protons and neutrons completed the picture

Later work showed that nuclear positive charge comes in whole-number multiples of the same small charge. The positive particles were named protons. Their number identifies the element.

In 1932, James Chadwick provided experimental evidence for neutrons: particles in the nucleus with no electrical charge and approximately the same mass as protons. This came about twenty years after the nuclear model was accepted.

Neutrons help explain why isotopes can have the same proton number but different masses. You need to know Chadwick’s contribution, rather than the details of his experiment.

INTERACTIVE REVEAL THE EVIDENCE

Why did scientists need neutrons?

Compare carbon-12 and carbon-14. Each nucleus has six protons and the same positive charge, but their masses differ.

Predict what accounts for the extra mass before opening the explanation.

Carbon-12 and carbon-14 have the same six protons; carbon-14 has two extra neutrons++++++++++++Carbon-12Carbon-14
Reveal the explanation

Carbon-12 has six neutrons; carbon-14 has eight. The extra neutrons add mass without adding electrical charge. Both remain carbon because each has six protons.

* Enlarged nucleus diagrams: pink + circles are protons and white circles are neutrons. Faint circles mark approximate extent, not physical walls. Not to scale.

INTERACTIVE TIMELINE

How the atomic model developed

Follow the timeline from left to right. Compare the diagrams, then click “Explore” to open each explanation underneath. On a phone, swipe sideways.

Early solid
sphere

Explore
Atoms were treated as tiny, indivisible spheres. Discovering electrons showed they contained smaller particles.

Plum pudding
model

+ + + + +
Explore
Negative electrons were embedded in spread-out positive charge. The atom was neutral overall. Rare large-angle alpha scattering challenged this model.

Nuclear
model

+
Explore
A tiny positive nucleus contained most of the mass. Electrons were outside it; most of the atom’s volume was empty space. The dashed outer circle only indicates the atom’s approximate extent.

Bohr’s energy
levels

+
Explore
Bohr proposed specific allowed electron energy levels at set distances from the nucleus. His calculations agreed with observations. The rings represent energy levels, not physical tracks.

Protons and
neutrons

+ + + + Nucleus enlarged
Explore
Protons explained nuclear positive charge. Chadwick’s evidence established neutrons in the nucleus, helping explain masses and isotopes. This diagram zooms in on the nucleus; electrons are outside it.
+ Positive charge − Electron Neutron

* This is a sequence of developments, not a time scale. The gaps do not represent equal numbers of years. Diagrams are simplified and not to scale. Particle numbers are illustrative; the final diagram shows an enlarged nucleus rather than a whole atom.

INTERACTIVE QUICK CHECK

Choose one answer for each question. You will see feedback immediately.

1. Which observation was most difficult for the plum pudding model to explain?
Try again. Most particles passing through was broadly consistent with the model.
Correct. Rare reversals required a concentrated nucleus rather than spread-out positive charge.
Try again. Plum pudding already included electrons.
2. What did the scattering results suggest about most of an atom’s mass?
Correct. The large-angle deflections supported a small, dense central nucleus containing most of the mass.
Try again. The evidence challenged that spread-out picture.
Try again. Electrons have very little mass compared with protons or neutrons.
3. Which pairing is correct?
Try again. These contributions are reversed.
Try again. Neither pairing identifies their contributions correctly.
Correct. Bohr developed the electron-energy-level model; Chadwick provided evidence for neutrons.
INTERACTIVE CHALLENGE

A student says, “Most alpha particles passed straight through, so the plum pudding model was correct. We can ignore the few that came back.” Explain why this conclusion is wrong.

Use both the common and rare observations, and explain why a scientific model may need to change.

Click here to reveal the answer

A model must explain all reliable observations, including rare ones. Most alpha particles passing through suggested that most of the atom was empty space.

The very small number scattered back showed that positive charge and most of the mass were concentrated in a tiny nucleus. Spread-out positive charge could not account for those large deflections.

The nuclear model explained both results better, so the evidence justified replacing plum pudding.

CHAPTER 3

Chapter summary

  • Discovering electrons showed that atoms were divisible and led to the plum pudding model.
  • Gold-foil scattering tested how positive charge and mass were distributed.
  • Most alpha particles passed through: most of the atom’s volume is empty space.
  • Some deflected and very few returned: the nucleus is tiny, positive and contains most of the mass.
  • The nuclear model replaced plum pudding because it explained the evidence better.
  • Bohr proposed specific electron energy levels. Later work identified protons; Chadwick provided evidence for neutrons.
  • Scientific models develop when new evidence requires a better explanation.