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The nucleus

Radiation and the atom · level 4, lesson 1 of 6

A question to start

Around 1910, scientists fired tiny particles at a sheet of gold foil less than a thousandth of a millimetre thick. Most went straight through, as if the gold were not there. A very few bounced straight back. What could they have hit?

The idea

In 1909, Hans Geiger and Ernest Marsden, working with Ernest Rutherford in Manchester, fired alpha particles (tiny, fast, positively charged particles given off by radioactive elements such as radium) at thin gold foil.

  • Most went straight through. So the atom is mostly empty space.
  • A few were turned aside, and a very few, about one in many thousands, bounced back. Something tiny, heavy and positively charged had pushed them away.

In 1911 Rutherford explained it: nearly all of an atom's mass is packed into a tiny centre, the nucleus, with a positive charge. The electrons, which are very light, are spread around it.

The nucleus is about a hundred thousand times smaller across than the atom. If an atom were as wide as a sports stadium, the nucleus would be a pea on the centre spot.

alpha sourcegold foilmost go througha few bounce back

Worked examples

1. To scale. An atom is about \(10^{-10}\) m across and a small nucleus about \(10^{-15}\) m: \(10^{-10} \div 10^{-15} = 100{,}000\) times smaller.

2. A stadium. Suppose a stadium is 200 m across. Then the nucleus would be \(200 \div 100{,}000 = 0.002\) m, 2 mm: about the size of a small pea.

3. Where the mass is. In a hydrogen atom, the nucleus is a single proton, about 1,836 times heavier than the electron. So more than 99.9% of the atom's mass is in the nucleus.

Try it

  1. Why did most alpha particles go straight through the foil?
  2. What did the few that bounced back tell Rutherford?
  3. Is the nucleus positive, negative or neutral?
  4. If an atom were 50 m across, about how big would its nucleus be?
Answers
  1. The atom is mostly empty space.
  2. There is a tiny, heavy, positively charged centre.
  3. Positive.
  4. \(50 \div 100{,}000 = 0.0005\) m, half a millimetre.

Common mistake

"Atoms are solid little balls." An atom is almost all empty space. It feels solid because the electrons of neighbouring atoms push on each other.

Where it leads