9.04.6 History of the Atom

Syllabus

New experimental evidence may lead to a scientific model being changed or replaced:

Before the discovery of the electron, atoms were thought to be tiny spheres that could not be divided. The discovery of the electron led to the plum pudding model of the atom. The plum pudding model suggested that the atom is a ball of positive charge with negative electrons embedded in it.
The results from the alpha particle scattering experiment led to the conclusion that the mass of an atom was concentrated at the centre (nucleus) and that the nucleus was charged. This nuclear model replaced the plum pudding model.
Niels Bohr adapted the nuclear model by suggesting that electrons orbit the nucleus at specific distances. The theoretical calculations of Bohr agreed with experimental observations.
Later experiments led to the idea that the positive charge of any nucleus could be subdivided into a whole number of smaller particles, each particle having the same amount of positive charge. The name proton was given to these particles.
The experimental work of James Chadwick provided the evidence to show the existence of neutrons within the nucleus. This was about 20 years after the nucleus became an accepted scientific idea.

Students should be able to describe:

• why the new evidence from the scattering experiment led to a change in the atomic model

• the difference between the plum pudding model of the atom and the nuclear model of the atom.

{Details of experimental work supporting the Bohr and Chadwick are not required.}

What does this mean?

The Scientific Model

No one knows for sure what is inside an atom because it cannot be seen.

So we use a model - an idea that seems to satisfy what we know about atoms.

That model has changed over the years as we have learned more about atoms.

Changing our ideas when evidence shows our current idea isn't correct is how Science works.

Our current model of the atom may be wrong too.

If someone shows that it is then we will change our model.

Atom Models #1 Dalton

The idea of atoms was invented thousands of years ago.

But then it was ignored because there was no evidence.

The English Scientist, John Dalton, revived the idea and said that atoms were the smallest things that existed, and that they were indivisible - couldn't be broken into smaller parts.

And that Atoms had no charge.

And he provided evidence that convinced people.

(We don't need to know what it was)

Atom Models #2 The Plum Pudding

Eventually Scientists discovered the electron - a negative particle smaller than an atom that seemed to come from inside atoms.

So they changed their ideas to suggest that atoms were basically positive, with the positive charge cancelled out by electrons that would be scattered throughout the atom as far as possible apart because the negative charge would repel each other.

This was the most sensible model based on the evidence known at that time.

Unfortunately, it was wrong and it took a genius to show that it was wrong.

Atom models #3 Alpha Scattering and the Nuclear Model

A few years later scientists had discovered Alpha Particles.

These are positively charged particles smaller than atoms.

Eventually an experiment was attempted in which alpha particles were sent towards a very thin Gold foil.

Gold was chosen because it can be made into a sheet only a few atoms thick.

If all the atoms were all empty space then all the alpha particles would go through and cause the fluorescent screen to flash

If they were solid and basically positive then the alpha particles should bounce off the atoms.

Neither quite happened.

Most particles went straight through - suggesting that most of an atom was empty space.

But a few particles were deflected, or bounced off, suggesting that the positive charge in an atom was concentrated in a small space.

This is how the nucleus was discovered - the nuclear model.

Scientists assumed everything in the nucleus was positive, and used the number of flashes to work out how small the nucleus is

Atom models #4 Amending to the nuclear model

Eventually, scientists discovered that the nucleus of atoms were made of smaller particles.

(We don't need to know how. Or who?)

They called these protons - because they knew that they were positive.

Later, James Chadwick discovered that there were other particles in the nucleus that had no charge.

So, he called these neutrons.

(We don't need to know how he did this either - just his name)

James Chadwick - not actual colour

And a Danish Scientist, Niels Bohr, found evidence that electrons were arranged at different distances from the nucleus - 2 close, 8 further away, another eight further than this.

The model of the atom has become more complicated than this since the early Twentieth Century as experiments have given us more information.

But this model still works for almost all situations and so is the one we use at GCSE.