Chapter 2 of 8

All 8 chapters
  1. 1Inside the atom, readunderstood
  2. 2How the model of the atom changed, readunderstood
  3. 3Isotopes, readunderstood
  4. 4Electron shells, readunderstood
  5. 5The periodic table, readunderstood
  6. 6Ionic bonding, readunderstood
  7. 7Covalent bonding, readunderstood
  8. 8Metals and alloys, readunderstood

S4.6.2 Chapter 2

How the model of the atom changed

How can you find out what is inside something you cannot see?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

  • You picked A. Read on. At the end, the chapter comes back to your guess.

  • You picked B. Read on. At the end, the chapter comes back to your guess.

  • You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

  • Describe how the atom model changed from Dalton to Bohr
  • Explain how the gold foil experiment showed a tiny nucleus

In Chapter 1 you met the nucleus, the protons, the neutrons and the electrons. Nobody has seen an atom as a tiny ball with a nucleus in it. So how do we know?

A model is a picture or an idea that explains what we measure. A good model matches all the evidence, which means the results of careful experiments. When a new result does not fit, the model is changed or replaced. This is how the model of the atom grew.

Draw it first

Four models of the atom

  1. Four models of the atom, one at a time. First Dalton, a solid ball. Then Plum pudding, a hatched positive ball with dots for electrons, and a key that names the positive charge and the electron. Then Nuclear, a tiny nucleus with a few electrons far away, and a key that names the nucleus and the electron. Then Bohr, a nucleus with electrons on shells, and a key that names the nucleus, the electron and the shell.Dalton

    1 Dalton, about 1803: an atom is a tiny solid ball that cannot be split.

  2. Four models of the atom, one at a time. First Dalton, a solid ball. Then Plum pudding, a hatched positive ball with dots for electrons, and a key that names the positive charge and the electron. Then Nuclear, a tiny nucleus with a few electrons far away, and a key that names the nucleus and the electron. Then Bohr, a nucleus with electrons on shells, and a key that names the nucleus, the electron and the shell.Plum puddingpositive chargeelectron

    2 Thomson, 1897: he found electrons, so an atom is a positive ball with electrons in it.

  3. Four models of the atom, one at a time. First Dalton, a solid ball. Then Plum pudding, a hatched positive ball with dots for electrons, and a key that names the positive charge and the electron. Then Nuclear, a tiny nucleus with a few electrons far away, and a key that names the nucleus and the electron. Then Bohr, a nucleus with electrons on shells, and a key that names the nucleus, the electron and the shell.Nuclearnucleuselectron

    3 Rutherford, with Geiger and Marsden, 1911: a tiny positive nucleus, with the electrons far outside it.

  4. Four models of the atom, one at a time. First Dalton, a solid ball. Then Plum pudding, a hatched positive ball with dots for electrons, and a key that names the positive charge and the electron. Then Nuclear, a tiny nucleus with a few electrons far away, and a key that names the nucleus and the electron. Then Bohr, a nucleus with electrons on shells, and a key that names the nucleus, the electron and the shell.Bohrnucleuselectronshell

    4 Bohr, 1913: the electrons move on fixed shells round the nucleus.

John Dalton said that each element is made of its own kind of atom. He pictured atoms as tiny, solid spheres. This fitted what chemists knew about how elements join to make compounds.

In 1897 J. J. Thomson showed that atoms contain electrons. Electrons are far lighter than atoms and have a negative charge. So an atom could not be a solid ball that cannot be split.

Atoms are neutral, so Thomson pictured a ball of positive charge with the electrons spread through it. People called it the plum pudding model.

About 1909 Hans Geiger and Ernest Marsden, who worked with Ernest Rutherford, ran an experiment. They fired alpha particles at very thin gold foil. An alpha particle is a small, fast particle with a positive charge. It comes from some radioactive substances.

The gold foil experiment seen from above. An alpha source, labelled alpha source, sends a beam of alpha particles, labelled alpha particles, at a thin sheet labelled gold foil. A ring, labelled detector, surrounds the foil. Most paths go straight through, labelled most pass straight through. Two paths bend a little. One path bounces back, labelled a few bounce back.alphasourcealphaparticlesgold foildetectormost passstraightthrougha few bounce back
The gold foil experiment. The detector shows where each alpha particle goes.

If an atom were a plum pudding, the alpha particles should all pass through with only a tiny bend. The results were different.

A table of what the alpha particles did, with the columns Result and Share. Straight through: most. Bent a little: some. Turned back: 1 in 8 000, a share that is about one alpha particle in 8 000.ResultSharestraight throughmostbent a littlesometurned back1 in 8 000
About 1 alpha particle in 8 000 was turned back by more than a right angle. Geiger and Marsden measured this with platinum foil; gold gave the same pattern.

Each result said something about the atom. Most alpha particles went straight through, so most of an atom must be empty space. Some bent, so there is a small place where positive charge is packed. Like charges repel, so a positive particle that passes close is pushed aside.

A very few bounced back. Only something heavy and positive, in a tiny space, could do that. Rutherford called this centre the nucleus. The model with a nucleus is the nuclear model.

A close view of the foil. A column of tiny nuclei. Alpha particles, labelled alpha particles, pass between the nuclei and go straight on, labelled most pass straight through. One path passes close to a nucleus, labelled nucleus, and is pushed aside, labelled bent by a nucleus.alphaparticlesnucleusmost passstraightthroughbent bya nucleus
Zoomed in. A path bends only when it passes close to a nucleus.

Worked example

Suppose about 1 alpha particle in 8 000 bounces back. In one run 16 000 alpha particles hit the foil. About how many bounce back?

  1. The share that bounces back is 1 in 8 000.
  2. Count how many groups of 8 000 are in 16 000.
  3. 16 000 ÷ 8 000 = 2.

Answer: About 2 bounce back

An atom is between 10 000 and 100 000 times wider than its nucleus. The next example uses the top of that range, which keeps the numbers round.

Worked example

Suppose an atom is 100 000 times wider than its nucleus. Draw the nucleus 2 mm wide. How wide, in metres, must the atom be on the same scale?

  1. The atom is 100 000 times wider, so multiply the width of the nucleus by 100 000.
  2. 2 mm × 100 000 = 200 000 mm.
  3. There are 1 000 mm in 1 m, so 200 000 mm ÷ 1 000 = 200 m.

Answer: 200 m

The nuclear model had a gap. It did not say why the electrons stay out in space. In 1913 Niels Bohr pictured the electrons on fixed shells, each at a set distance from the nucleus. His model fitted the light that hydrogen atoms give out, and it is the picture we use for chemistry.

One more step came in 1932. Scientists knew the charge of the nucleus but its mass was too big. A helium nucleus has twice the charge of hydrogen, but four times the mass. James Chadwick found the neutron, a particle with no charge and the same mass as a proton. It filled the gap.

A table of the changing model. The columns are Year, Scientist and Idea. In 1803 Dalton: solid ball. In 1897 Thomson: electrons. In 1911 Rutherford: nucleus. In 1913 Bohr: shells. In 1932 Chadwick: neutron.YearScientistIdea1803Daltonsolid ball1897Thomsonelectrons1911Rutherfordnucleus1913Bohrshells1932Chadwickneutron
Each step came from new evidence that the old model could not explain.

See it three ways

As a picture

The nuclear model of the atom. A tiny black nucleus, labelled nucleus, sits in the middle. A few electrons, labelled electron, are far from it, with empty space all round the nucleus.nucleuselectron

In words

The gold foil results fit one picture: a tiny, heavy, positive nucleus, and empty space all round it. Nothing else explains a few bounces among thousands of straight paths.

In symbols

Most go straight: empty space. Some bend: a small positive centre. A few bounce back: the centre is heavy.

Try it

Find a hidden cup by rolling marbles

You need: a long cardboard box with both short ends cut open, and its lid, a small cup or a bottle top, about 20 marbles or small round stones, a flat floor, paper and a pencil for a tally.

Be safe: An adult cuts the box. Roll the marbles on a floor away from stairs, and pick up every marble when you finish, so nobody slips.

  1. Stand the cup upside down in the middle of the box, and close the lid. Do not look at it again.
  2. Lay the box on the floor. Roll a marble through it from one end, aiming at a different place each time.
  3. Make a tally. Mark each marble as straight through, bent, or bounced back.
  4. After 20 marbles, look at your tally. Which kind was most common?
  5. Open the lid. Compare the cup with what your tally told you.

What you will see: Most marbles go straight through, because most of the box is empty. A few hit the cup and bounce away. The more marbles that hit, the wider the cup must be. Scientists found the nucleus in the same way, but with alpha particles.

If you cannot try it, this is what happens.
A box seen from above with the lid off. A cup, labelled cup, stands in the middle. A marble path goes straight along the top, labelled passes straight through. Another path hits the cup and comes back, labelled bounces back.cuppasses straight throughbounces back

A wrong idea many people have

The Bohr model shows what an atom really looks like.

Why it feels right: Almost every book draws an atom with a nucleus and neat rings of electrons, so the picture seems to be a photograph.

The test that breaks it: Ask what each picture can explain. The shell picture explains the periodic table and bonding well. It cannot tell you where one electron is, and the rings are not real rings.

What is true: The Bohr model is a model. It is useful because it explains a lot, but electrons are not little balls on rings. Models are kept while they work and replaced when new evidence needs a better one.

Key words

model
a picture or idea that explains what we measure.
evidence
the results of careful experiments.
alpha particle
a small, fast, positive particle given out by some radioactive substances.
plum pudding model
the atom as a positive ball with electrons spread through it.
nuclear model
the atom as a tiny positive nucleus with electrons far outside it.

Back to your guess

The answer is A: Fire small particles at it and watch where they go.

Scientists fired fast particles at a thin sheet of gold. Where the particles went showed what is inside the atoms. A microscope cannot show an atom, and cutting never reaches inside it.

Check yourself

Try each one before you look. A wrong answer tells you where to go next.

  1. Which model of the atom is this?

    A model of an atom. A tiny black nucleus, labelled nucleus, sits in the middle. A few electrons, labelled electron, are far from it. A thin circle marks the edge of the atom. The electrons are scattered, not on shells.nucleuselectron
    Tap your answer.

    1. Not this one. The plum pudding model has electrons spread through a big positive ball. Here the positive part is a tiny nucleus. Try another.

    2. Not this one. Dalton drew a plain ball with nothing inside. This picture has a nucleus and separate electrons. Try another.

    3. Yes, that is right. It has a tiny nucleus and electrons far away in empty space, with no shells. That is the nuclear model.

    4. Not this one. Bohr put the electrons on shells. Here they are scattered, and the circle is only the edge of the atom. Try another.

    See the answer

    The nuclear model of Rutherford

    It has a tiny nucleus and electrons far away in empty space, with no shells. That is the nuclear model.

    • If you chose A: The plum pudding model has electrons spread through a big positive ball. Here the positive part is a tiny nucleus.
    • If you chose B: Dalton drew a plain ball with nothing inside. This picture has a nucleus and separate electrons.
    • If you chose D: Bohr put the electrons on shells. Here they are scattered, and the circle is only the edge of the atom.

  2. Which model did the gold foil results replace?

    Tap your answer.

    1. Not this one. Thomson had already replaced Dalton's ball when he found electrons, years before the gold foil experiment. Try another.

    2. Yes, that is right. A plum pudding atom has no tiny dense centre, so it could not make alpha particles bounce back.

    3. Not this one. Bohr came after the gold foil results. He built on the nucleus that they showed. Try another.

    See the answer

    The plum pudding model

    A plum pudding atom has no tiny dense centre, so it could not make alpha particles bounce back.

    • If you chose A: Thomson had already replaced Dalton's ball when he found electrons, years before the gold foil experiment.
    • If you chose C: Bohr came after the gold foil results. He built on the nucleus that they showed.

  3. Most alpha particles went straight through the gold foil. What does this show?

    Tap your answer.

    1. Yes, that is right. Particles pass freely only through empty space. The paths went straight, so there was nothing in the way.

    2. Not this one. If atoms were solid balls, alpha particles would be stopped or turned. Most went on as if nothing was there. Go to S4.6.1. Try another.

    3. Not this one. Atoms are neutral overall, but they hold charges. A few alpha particles were turned back, so something was repelling them. Go to S4.6.1. Try another.

    See the answer

    Most of an atom is empty space.

    Particles pass freely only through empty space. The paths went straight, so there was nothing in the way.

    • If you chose B: If atoms were solid balls, alpha particles would be stopped or turned. Most went on as if nothing was there. Go to S4.6.1.
    • If you chose C: Atoms are neutral overall, but they hold charges. A few alpha particles were turned back, so something was repelling them. Go to S4.6.1.

  4. A few alpha particles were turned back. What does this show?

    Tap your answer.

    1. Not this one. An electron is far too light to turn back a fast alpha particle, as a marble cannot turn back a lorry. The cause must be heavy. Go to S4.6.1. Try another.

    2. Not this one. A thick foil would stop particles, not send them back. The few that bounced came from a repelling positive centre. Try another.

    3. Yes, that is right. A positive alpha particle is repelled by a positive nucleus. Only a heavy, tiny centre can turn it right round.

    See the answer

    An atom has a tiny, heavy, positive nucleus.

    A positive alpha particle is repelled by a positive nucleus. Only a heavy, tiny centre can turn it right round.

    • If you chose A: An electron is far too light to turn back a fast alpha particle, as a marble cannot turn back a lorry. The cause must be heavy. Go to S4.6.1.
    • If you chose B: A thick foil would stop particles, not send them back. The few that bounced came from a repelling positive centre.

  5. What was new in the model of Niels Bohr?

    Tap your answer.

    1. Not this one. That is the plum pudding model of Thomson. Bohr kept the nucleus and put the electrons on shells. Try another.

    2. Yes, that is right. Bohr put each electron at a set distance from the nucleus, on a shell. This fitted the light that hydrogen gives out.

    3. Not this one. Neutrons were found in 1932, after Bohr's model. The new idea of 1913 was the shells. Try another.

    See the answer

    The electrons move on fixed shells.

    Bohr put each electron at a set distance from the nucleus, on a shell. This fitted the light that hydrogen gives out.

    • If you chose A: That is the plum pudding model of Thomson. Bohr kept the nucleus and put the electrons on shells.
    • If you chose C: Neutrons were found in 1932, after Bohr's model. The new idea of 1913 was the shells.

  6. Why did scientists need a new particle, the neutron?

    Tap your answer.

    1. Not this one. Electrons are held by the pull of the positive nucleus. The neutron has no charge and does not pull them. Go to S4.6.1. Try another.

    2. Not this one. A neutron has no charge at all. The extra particles added mass, not charge. Go to S4.6.1. Try another.

    3. Yes, that is right. Helium has two protons but four times the mass of hydrogen. Neutral particles of the same mass as a proton fill the gap.

    See the answer

    The nucleus had more mass than its protons could explain.

    Helium has two protons but four times the mass of hydrogen. Neutral particles of the same mass as a proton fill the gap.

    • If you chose A: Electrons are held by the pull of the positive nucleus. The neutron has no charge and does not pull them. Go to S4.6.1.
    • If you chose B: A neutron has no charge at all. The extra particles added mass, not charge. Go to S4.6.1.

  7. From an earlier chapter

    Which particles are in the nucleus of an atom?

    Tap your answer.

    1. Yes, that is right. The nucleus holds protons (+1) and neutrons (0). The electrons (−1) are outside it.

    2. Not this one. Electrons are outside the nucleus. They move on the shells. Go to S4.6.1. Try another.

    3. Not this one. Most nuclei also hold neutrons. They add mass but no charge. Go to S4.6.1. Try another.

    See the answer

    Protons and neutrons

    The nucleus holds protons (+1) and neutrons (0). The electrons (−1) are outside it.

    • If you chose B: Electrons are outside the nucleus. They move on the shells. Go to S4.6.1.
    • If you chose C: Most nuclei also hold neutrons. They add mass but no charge. Go to S4.6.1.

  8. Suppose 1 alpha particle in 8 000 bounces back. A run sends 24 000 alpha particles at the foil. About how many bounce back?

    See the answer

    About 3 bounce back

    24 000 ÷ 8 000 = 3.

  9. A nucleus is drawn 3 mm wide. Suppose the atom is 100 000 times wider than its nucleus. How wide, in metres, is the atom on the same scale?

    See the answer

    300 m

    3 mm × 100 000 = 300 000 mm. Divide by 1 000 to get metres: 300 m.

Keep it

Copy this into your exercise book.

A model explains the evidence and changes when new evidence does not fit. Gold foil: most alpha particles pass through, a few bounce back. So atoms are mostly empty space with a tiny, dense, positive nucleus.

Draw: The gold foil experiment: a beam, the foil, and three paths: straight, bent and back.

Reading with a child? A short page for the adult: what this chapter teaches and three questions to ask.

No internet tomorrow? Save the whole book as one file (826 KB). It opens on any phone.