Thomson's and Rutherford's models

Thomson's model

Once the electron was known, scientists asked how the parts are arranged. In 1898, J. J. Thomson gave the first model. He pictured the atom as a sphere of positive charge, with the electrons embedded in it, like seeds in a watermelon, or like raisins in a pudding. The positive and negative charges are equal, so the atom is neutral.

This model was simple, but an experiment was soon to show that it was wrong.

Rutherford's experiment

In 1911, Ernest Rutherford and his helpers fired a beam of alpha particles at a very thin gold foil. An alpha particle is a helium atom without its electrons. It has a charge of $+2$ and a mass of $4\ \text{u}$, and it moves very fast. Gold can be hammered into a foil only a few hundred atoms thick. A screen around the foil showed where the particles went.

If Thomson was right, the positive charge is spread over the whole atom, and it is not strong at any one place. The fast alpha particles should pass almost straight through.

You should find that only a particle aimed almost straight at the nucleus is thrown back, and most aims barely bend the path.

What was seen

1. Most of the alpha particles passed straight through the foil.
2. A small number were deflected by small angles.
3. About one in every 12000 particles rebounded. It came almost straight back.

Rutherford said later that it was as strange as firing a large shell at a piece of tissue paper, and seeing it come back.

What Rutherford concluded

1. Most of the space inside an atom is empty. This is why most particles went straight through.
2. All the positive charge is in a very small region. This is why only a few particles were deflected, or rebounded. A positive alpha particle was pushed back by a positive centre.
3. This centre is the nucleus. It has nearly all the mass of the atom, because a heavy gold nucleus could turn back a fast alpha particle.
4. The electrons move around the nucleus.

How small is the nucleus?

The radius of the nucleus is about $10^{-15}\ \text{m}$. The radius of the atom is about $10^{-10}\ \text{m}$. So the nucleus is about $100000$ times smaller than the atom.

Practise with new numbers

Try it in Python

This program finds how much wider the atom is than its nucleus.

What was wrong with Rutherford's model?

Rutherford's model pictured the electrons moving in circles around the nucleus. But a charged particle that moves in a circle is always changing its direction. By the laws of physics, it should lose energy as light, and spiral into the nucleus. The atom would not be stable. Yet atoms are stable. Another model was needed.

Remember these

• Thomson: a positive sphere with electrons in it. It could not explain the rebound of alpha particles.
• Rutherford: most particles passed straight, a few were deflected, about $1$ in $12000$ rebounded.
• The atom is mostly empty space. The nucleus is small, positive and heavy.
• The electrons of Rutherford's model should spiral into the nucleus, so the model fails.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Why did most alpha particles pass straight through the gold foil?

2. What did the rare rebounding alpha particles show?

3. Why was Thomson's model rejected after the gold foil experiment?

4. What was wrong with Rutherford's model?