Where the mass sits
People fired tiny positive particles at a very thin sheet of gold.
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- Most particles go straight through the foil.
- Some bend sharply as they pass.
- A few bounce straight back.
The idea
In 1909, Geiger and Marsden fired alpha particles at thin gold foil. They worked with Rutherford. Alpha particles are positively charged. Thomson's model said they would pass straight or bend a little.
Most did pass straight through. Some bent sharply. A few bounced back. This is called scattering. In 1911, Rutherford explained it. Thomson's model could not.
Dense means a lot of mass in a very small space.
| What was seen | What it means |
|---|---|
| Most pass straight | Most of an atom is empty space |
| Some bend sharply | Positive charge is packed in a tiny centre |
| A few bounce back | The centre is dense and holds most of the mass |
That centre is the nucleus. Electrons revolve (go round) the nucleus, like planets around the Sun. This is the planetary model.
The nucleus is about 10⁵ times smaller than the atom. An atom is about 10⁻¹⁰ m across. A nucleus is about 10⁻¹⁵ m. Check: 10⁻¹⁰ ÷ 10⁻¹⁵ = 10⁵. Imagine an atom as a town 1 km wide. The nucleus is a marble at the centre, 1 cm across.
A grain of fine sand is about 10⁻³ m across. So 10⁻³ ÷ 10⁻¹⁰ = 10⁷ atoms fit in a row across it.