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History of the Atomic Model

Worksheet Overview

QUESTION 1 of 10

Even scientists change their minds! In this story, you will learn about how the scientists involved used evidence to change their ideas.

The idea of atoms is very old. Philosophers in Ancient Greece imagined what would happen if you tried to cut something into smaller and smaller pieces. Eventually (they thought) the pieces would be so small that they couldn't be cut any more. They called those tiny pieces atoms. In their model, every different thing had its own atom- atoms of wood, atoms of skin, atoms of rock, even atoms of colour and taste. Although atoms are tiny, most of the other parts of this model turned out to be wrong.

The next person to develop the idea of atoms was John Dalton. He looked at chemical reactions, and worked out that some chemicals were compounds; they were made by joining different types of element together. For example, there aren't atoms of water- instead, you make water by joining hydrogen and oxygen atoms together. Dalton thought that each element was a different sort of sphere.

Later on, a physicist called J.J. Thompson discovered a particle called the electron. When electric current moves through a wire, it's because electrons are moving. Thompson showed that these electrons came out of atoms. This showed that Dalton's model of atoms as solid spheres was wrong, because atoms could have electrons pulled out of them. Thompson imagined the electrons scattered around the atom like pieces of fruit in a cake. Because of this, Thompson's model is called the plum pudding model.

In 1907, Ernest Rutherford did an experiment which had very surprising results. They fired a type of particle called an alpha particle (you'll learn more about those in some of the physics activities) at a thin sheet of gold. They expected the alpha particles to go straight through the gold sheet- and most did. But a few bounced off the sheet, which seemed impossible. Rutherford said "it was almost as incredible as if you fired a 15-inch shell (a shell is a type of bullet) at a piece of tissue paper and it came back and hit you".

The only way to make sense of the results of the alpha scattering experiment was if the plum pudding model was wrong. In Rutherford's model of the atom, all of the positive charge and most of the mass of the atom had to be squeezed into a tiny sphere at the centre of the atom. In Rutherford's nuclear model of the atom, there were positive protons in the nucleus at the centre of the atom, surrounded by a cloud of negative electrons flying around the rest of the atom.

The nuclear model of the atom explained the results of the alpha scattering experiment. Most alpha particles passed through the foil easily, because they passed through the gaps between nuclei. However, when an alpha particle hit a nucleus head-on, it bounced straight off it. Since only a few alpha particles bounced off the nucleus, the nucleus must be very small. Unfortunately, there were problems with the nuclear model. It was hard to explain why the electrons weren't pulled in towards the nucleus. Later on, scientists discovered that atoms can give off (emit) and absorb light, but only of specific colours; that's why fluorescent lamp tubes are often a strange purply-white colour.

These problems were solved by Niels Bohr. Instead of electrons flying around freely, the Bohr atom trapped them in specific orbits, or "shells". Since the electrons were trapped in shells, they couldn't fall into the nucleus. The gaps between the shells explained the effects linked to light and colour. Better still, the idea of electrons in shells explained some patterns about chemical reactions which people knew about but hadn't been able to explain.

There was one final step in thinking about the structure of the atom. In 1932, James Chadwick found that not all the particles in the nucleus were protons. There was another type of particle, with similar size and mass, but no electric charge. These particles were called neutrons. So instead of just protons in the nucleus, there was a mixture of protons and neutrons.

Everyone you've learned about in this activity had to use their imagination to think of a model of the atom which explained what they knew about what atoms do. When they found new information, they had to change their model to include the new information. That's OK because that's what scientists do.

These people are all linked to the story of atomic structure. Match them with the order they came in history.

Column A

Column B

Early
John Dalton
Middle
J. J. Thompson
Later
Ernest Rutherford

What was the nickname of J.J. Thompson's model of the atom?

 

 

Thompson model

Plum pudding model

Electron model

Match up these half-sentences about some early models of atoms.

Column A

Column B

Ancient Greeks thought
each material was made of a different atom
John Dalton thought
atoms could combine to make different materials
J.J. Thompson discovered that
electrons could be taken out of atoms

What turned out to be wrong with the plum pudding model?

Atoms are smaller than puddings

Atoms aren't nice to eat

Atoms have a nucleus in the middle, and puddings don't

Match up these part-sentences about Rutherford's scattering experiment.

Column A

Column B

What Rutherford did was
fire alpha particles at a sheet of gold foil
He expected
to see the particles go through the foil
What actually happened was
some particles bounced back

What was the difference between Rutherford's model of the atom and Thompson's model of the atom?

Thompson's atoms could be made into thin foil, and Rutherford's couldn't

Rutherford's model had a nucleus, and Thompson's model didn't

Rutherford's model had smaller atoms than Thompson's

What was wrong with Rutherford's model of the atom?

There should be electrons in the nucleus

The electrons should be in shells

Atoms are invisible

Who discovered the neutron?

J.J. Thompson

Ernest Rutherford

Owen Chadwick

Complete the gaps in this paragraph about our modern model of the atom. Use these words:

electrons

neutrons

nucleus

seen

shells

J.J. Thompson

Ernest Rutherford

Owen Chadwick

Why did Rutherford's experiment show that most of the mass of the atom must be in a tiny nucleus?

Most alpha particles went through gaps between nuclei

Most alpha particles hit nuclei on the way

A few alpha particles bounced back, because they hit nuclei

Only a few alpha particles passed through

  • Question 1

These people are all linked to the story of atomic structure. Match them with the order they came in history.

CORRECT ANSWER

Column A

Column B

Early
John Dalton
Middle
J. J. Thompson
Later
Ernest Rutherford
EDDIE SAYS
Although the names aren't the most important part of this story, they are useful, because they will help make "hooks" in your mind, to help you remember the different models. These three people and models are the most important ones to remember.
  • Question 2

What was the nickname of J.J. Thompson's model of the atom?

 

 

CORRECT ANSWER
Plum pudding model
EDDIE SAYS
Don't worry if you've never seen a plum pudding, you can think of a fruitcake or a Christmas pudding instead. It's a good nickname because it gives a picture of what atoms could be like; the plums are like the electrons (and could be pulled out), and the stodgy pudding holds the fruits together- like positive charge in an atom.
  • Question 3

Match up these half-sentences about some early models of atoms.

CORRECT ANSWER

Column A

Column B

Ancient Greeks thought
each material was made of a diffe...
John Dalton thought
atoms could combine to make diffe...
J.J. Thompson discovered that
electrons could be taken out of a...
EDDIE SAYS
Check the introduction again if you aren't sure about these. As time passed, the model of the atom gradually got more complicated, but it described real atoms better.
  • Question 4

What turned out to be wrong with the plum pudding model?

CORRECT ANSWER
Atoms have a nucleus in the middle, and puddings don't
EDDIE SAYS
Scientific models aren't exactly real- they help us to think about something real that is hard to imagine. So for a model, it doesn't matter that electrons aren't really plums, but it does matter that the positive charge in an atom is all in the nucleus.
  • Question 5

Match up these part-sentences about Rutherford's scattering experiment.

CORRECT ANSWER

Column A

Column B

What Rutherford did was
fire alpha particles at a sheet o...
He expected
to see the particles go through t...
What actually happened was
some particles bounced back
EDDIE SAYS
This was a really surprising result, which is why Rutherford spoke about bullets and tissue paper. Once you know about the nuclear model of the atom, it becomes much easier to explain what happened.
  • Question 6

What was the difference between Rutherford's model of the atom and Thompson's model of the atom?

CORRECT ANSWER
Rutherford's model had a nucleus, and Thompson's model didn't
EDDIE SAYS
Look at the diagrams of the atomic models if you are unsure about these.
  • Question 7

What was wrong with Rutherford's model of the atom?

CORRECT ANSWER
The electrons should be in shells
EDDIE SAYS
A lot of the chemistry you will learn depends on electrons being in shells, but for a long time this wasn't known. Even really good scientists don't get everything right first time.
  • Question 8

Who discovered the neutron?

CORRECT ANSWER
Owen Chadwick
EDDIE SAYS
Check the introduction again if you're not sure. Thompson discovered the electron, and Rutherford had the idea of an atomic nucleus.
  • Question 9

Complete the gaps in this paragraph about our modern model of the atom. Use these words:

electrons

neutrons

nucleus

seen

shells

CORRECT ANSWER
EDDIE SAYS
This mental picture is really important; look at the diagram again if you are unsure.
  • Question 10

Why did Rutherford's experiment show that most of the mass of the atom must be in a tiny nucleus?

CORRECT ANSWER
Most alpha particles went through gaps between nuclei
A few alpha particles bounced back, because they hit nuclei
EDDIE SAYS
Imagine throwing lots of balls at a very small target. The chances are that most wouldn't hit, but a few would bounce off the target. That's basically what happened in Rutherford's experiment, but on a much smaller scale.
---- OR ----

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