The gold foil experiment

In 1911, Geiger and Marsden, working with Ernest Rutherford, fired a narrow beam of alpha (α) particles at a sheet of gold foil. Alpha particles are small, positive particles given off by radioactive elements; in fact, they are helium nuclei. Gold was chosen because it can be beaten extremely thin, like the silver varq on sweets.

If Thomson's watermelon model were right, the alpha particles should pass straight through, or bend only a little. What actually happened:

• most passed straight through;
• some were bent sharply away;
• a few bounced straight back.

This bending is called scattering.

"What must be inside the atom?" Think it through:

• Most went straight through, so the atom is mostly empty space.
• A positive particle is pushed away only by something positive. A few bounced back, so there must be something very small, very heavy and positive in the middle.

Rutherford called it the nucleus. It holds all the positive charge and most of the mass. Electrons go round it like planets round the Sun: the planetary model.

How small is the nucleus? An atom is about 10⁻¹⁰ m across; its nucleus about 10⁻¹⁵ m: about one lakh (10⁵) times smaller. If an atom were a cricket ground 100 m across, the nucleus would be a grain of black pepper in the middle. A sheet of paper 0.1 mm thick is about a million (10⁶) atoms thick.

Rutherford also found that the positive charge comes from protons: heavier than electrons, with a charge of +1. A neutral atom has equal numbers of protons and electrons: helium has 2 and 2; sodium 11 and 11. He won the Nobel Prize in Chemistry in 1908.

The model's problem. Something going round in a circle is accelerating (you saw this in Chapter 4). An accelerating electron should lose energy, spiral inwards, and crash into the nucleus. Then atoms would collapse. But atoms are stable. A new model was needed.