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Understand the History of the Atomic Model

Worksheet Overview

QUESTION 1 of 10

Even scientists change their minds! When you read this story, think 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. Those tiny pieces were called atoms. In this 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 was a chemist in England in the early 1800s. By looking at chemical reactions, he 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 knew that atoms were too small to be seen, so he imagined them as tiny solid spheres. Each element was a different sort of sphere.

 

 

Dalton's model was accepted for most of the 1800s. Then in 1897, a physicist called J.J. Thompson (his actual name was Joseph John, but everyone calls him J. J.) 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. In other words, Dalton's model of atoms as solid spheres was wrong, because atoms could have electrons pulled out of them. Nobody had seen an atom yet, so 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.

 

 

The plum pudding model only lasted about ten years. In 1907, Ernest Rutherford (along with his students, Hans Geiger and Ernest Marsden) 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 indeed. 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 in 1913. 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.

 

Nobody has managed to make a direct picture of the structure of an atom- though some scientists think they they are very close to doing that. 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

First
Niels Bohr
Second
J.J. Thompson
Third
John Dalton
Fourth
Ernest Rutherford
Fifth
Ancient Greek Philosophers

Which part of the Ancient Greek model of the atom was right?

Atoms are very tiny

Atoms can't be cut into smaller pieces

Different things are made of different types of atom

Atoms have many different shapes

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

 

 

Electron model

Plum pudding model

Plum stone model

Joseph John model

Underline the sentence which contains a piece of evidence for the Plum Pudding model of the atom.

J. J. Thompson was doing experiments with cathode rays; nowadays we know these are beams of electrons. He saw that certain gases could be made to give off electrons. This showed that atoms were not indivisible, and were made of smaller parts.

What was the main flaw with the plum pudding model?

Atoms are smaller than puddings

Electons are not purple

Electrons are not tasty

The positive charge in the atom is not spread out

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

Column A

Column B

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

Match up these part sentences to explain why the nuclear model of the atom explained Rutherford's experiment. 

Column A

Column B

Most particles went through the foil because
the nucleus is very small
A few particles bounced back becuase
they hit a nucleus
The number bouncing back was small becuase
they did not hit a nucleus

Which of these were arguments against Rutherford's model of the atom?

Atoms only gave off certain colours of light

Rutherford's model disagreed with the plum pudding model

Electrons would be pulled into the nucleus

Atoms only absorbed certain colours of light

Rutherford's model wasn't modern

How did Owen Chadwick change the model of the atom from what went before?

Say that there were electrons in the nucleus

Say that some of the particles in the nucleus were protons

Say that the plum pudding model was right, after all

Say that some of the particles in the nucleus were neutrons

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

 

electrons

neutrons

nucleus

seen

shells

Say that there were electrons in the nucleus

Say that some of the particles in the nucleus were protons

Say that the plum pudding model was right, after all

Say that some of the particles in the nucleus were neutrons

  • 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

First
Ancient Greek Philosophers
Second
John Dalton
Third
J.J. Thompson
Fourth
Ernest Rutherford
Fifth
Niels Bohr
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.
  • Question 2

Which part of the Ancient Greek model of the atom was right?

CORRECT ANSWER
Atoms are very tiny
EDDIE SAYS
Remember that most of the things that Ancient Greeks guessed about atoms turned out to be wrong. They didn't have information at that time to make better guesses- which shows how important evidence is.
  • Question 3

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. It's a phrase people use; the plums were like the electrons (and could be pulled out), and the stodgy pudding held the fruits together- like positive charge in an atom.
  • Question 4

Underline the sentence which contains a piece of evidence for the Plum Pudding model of the atom.

CORRECT ANSWER
J. J. Thompson was doing experiments with cathode rays; nowadays we know these are beams of electrons. He saw that certain gases could be made to give off electrons. This showed that atoms were not indivisible, and were made of smaller parts.
EDDIE SAYS
If you need to find evidence in a scientific story, look for things that people "saw", "noticed", "observed", "measured", or other words like that.
  • Question 5

What was the main flaw with the plum pudding model?

CORRECT ANSWER
The positive charge in the atom is not spread out
EDDIE SAYS
Scientific models are not 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 6

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

CORRECT ANSWER

Column A

Column B

What Rutherford did was
to fire alpha particles at a shee...
He expected
to see the particles go through t...
What 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 7

Match up these part sentences to explain why the nuclear model of the atom explained Rutherford's experiment. 

CORRECT ANSWER

Column A

Column B

Most particles went through the f...
they did not hit a nucleus
A few particles bounced back becu...
they hit a nucleus
The number bouncing back was smal...
the nucleus is very small
EDDIE SAYS
Have another look at the diagram of the scattering experiment if you're not sure about these.
  • Question 8

Which of these were arguments against Rutherford's model of the atom?

CORRECT ANSWER
Atoms only gave off certain colours of light
Electrons would be pulled into the nucleus
Atoms only absorbed certain colours of light
EDDIE SAYS
The important reasons in science are the ones which come from things people can see or measure, not who came up with the idea or when.
  • Question 9

How did Owen Chadwick change the model of the atom from what went before?

CORRECT ANSWER
Say that some of the particles in the nucleus were neutrons
EDDIE SAYS
If you think about it, it's not surprising that neutrons were the last particle to be discovered in the atom. They are in the nucleus, and they don't have any electrical charge- this makes them hard to find.
  • Question 10

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.
---- OR ----

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