Thomson's Plum Pudding Model and the Discovery of the Electron

For most of the nineteenth century, scientists pictured atoms as Dalton had: tiny, solid, indivisible balls. Then, in 1897, a physicist in Cambridge showed that atoms contain something much smaller than themselves. Joseph John Thomson, known as J.J., had discovered the electron, the first subatomic particle. To explain where these electrons sat, he proposed a new picture of the atom, nicknamed the plum pudding model. It turned out to be wrong, but it was a crucial stepping stone, and the story of how it rose and fell shows science working at its best.

The mystery of cathode rays

In the second half of the 1800s, physicists experimented with sealed glass tubes from which most of the air had been pumped out. When a high voltage was connected across two metal electrodes inside, a strange glow appeared. Something travelled from the negative electrode (the cathode) to the positive one, making the glass glow where it struck. These were called cathode rays.

But what were they? Scientists were split:

• Many German physicists thought they were a kind of wave, like light, travelling through an invisible medium.
• Many British physicists suspected they were streams of charged particles.

The debate ran for about twenty years.

Thomson’s experiments (1897)

J.J. Thomson was only 28 when he became head of the Cavendish Laboratory at the University of Cambridge in 1884. In 1897, he designed a series of experiments to settle the question.

1. The rays carry negative charge

Thomson directed cathode rays into a metal cup connected to an electrometer and showed that it collected negative charge. When the rays were bent away by a magnet, the charge disappeared. The charge and the rays were inseparable.

2. Electric fields bend them

Earlier experimenters had failed to deflect cathode rays with electric fields. Thomson realised that remaining gas in their tubes was conducting and cancelling the field. With a much better vacuum, he showed that the rays bent towards the positive plate, as negatively charged particles should.

3. Measuring the charge-to-mass ratio

By passing the rays through both electric and magnetic fields, and adjusting them to balance, Thomson could calculate the ratio of charge to mass (e/m) of the particles.

The result was astonishing: the charge-to-mass ratio was about 1,000 times larger than that of the hydrogen ion, the lightest charged particle known. (The modern figure is about 1,836 times.) Either these particles carried a huge charge, or they had a tiny mass. Thomson argued for the tiny mass.

4. They’re universal

Crucially, Thomson got the same e/m whatever metal he used for the cathode and whatever gas was in the tube. These particles weren’t specific to one element: they were a component of all atoms.

Thomson called them “corpuscles”. The name electron, suggested earlier by George Johnstone Stoney for a unit of electric charge, soon took over. For the modern properties of the electron, see the electron.

It was a revolution. Atoms, supposedly indivisible, contained smaller particles. Thomson received the Nobel Prize in Physics in 1906.

The plum pudding model (1904)

If atoms contain negative electrons, but atoms overall are neutral, there must be positive charge somewhere to balance them. Where?

In 1904, Thomson proposed that an atom is a sphere of uniformly distributed positive charge, with the negatively charged electrons embedded throughout it, like plums (or raisins) in a traditional British Christmas pudding. Hence the nickname plum pudding model. (Some teachers compare it to a chocolate-chip cookie or a watermelon with seeds.)

Key features:

• The positive charge is spread out evenly through the whole volume of the atom.
• The electrons sit within this positive “pudding”, arranged in rings or shells so that the forces balance.
• The positive charge and the electrons’ negative charge cancel, so the atom is neutral.
• Thomson thought most of the atom’s mass came from the electrons, so he imagined atoms containing thousands of them. (Later measurements showed this was wrong: electrons contribute very little mass.)

Why it made sense

At the time, the model was entirely reasonable:

• It explained why atoms are neutral.
• It accounted for electrons being removable (forming positive ions).
• Thomson even tried to explain chemical periodicity by showing that electrons arranged in stable rings would repeat patterns as their number increased, an early attempt to connect atomic structure with the periodic table.

The fall of the pudding

The model’s great weakness was untested: nobody knew what the inside of an atom was really like.

Between 1908 and 1913, Hans Geiger and Ernest Marsden, working under Ernest Rutherford (a former student of Thomson’s) at the University of Manchester, fired alpha particles at thin gold foil.

If the plum pudding model were right, the positive charge would be spread thinly, and the fast, heavy alpha particles should pass through with only tiny deflections. Most did. But about 1 in 8,000 bounced back at large angles, something the plum pudding model simply couldn’t explain.

In 1911, Rutherford concluded that the positive charge and most of the mass are concentrated in a tiny, dense nucleus at the centre, with electrons outside it. The plum pudding had been replaced by the nuclear model. See Rutherford’s gold foil experiment.

Thomson’s legacy

• The electron: the discovery of the first subatomic particle opened the door to atomic physics, electronics and much of modern chemistry.
• Isotopes: in 1913, Thomson, with Francis Aston, passed beams of positive neon ions through electric and magnetic fields and found two separate spots, from neon-20 and neon-22. This was the first evidence that a stable element can have atoms of different masses, and it led to the invention of the mass spectrometer. See mass spectrometry and isotopes.
• A school of scientists: under Thomson, the Cavendish Laboratory became one of the world’s great research centres. Seven of his research assistants and his own son, George Paget Thomson, went on to win Nobel Prizes. In a neat twist, G.P. Thomson showed in 1927 that electrons behave as waves, while his father had shown they were particles. See wave-particle duality.

What the story teaches

The plum pudding model is a perfect example of how science works:

1. It explained the evidence available at the time.
2. It made predictions that could be tested.
3. When new evidence (the gold foil experiment) contradicted it, it was replaced by a better model.

A model being replaced doesn’t mean it was worthless. Thomson’s model was the first to include subatomic particles, and it set up the very experiment that overturned it. See the history of atomic models.

Key takeaways

• J.J. Thomson discovered the electron in 1897 by showing cathode rays are negatively charged particles with a very high charge-to-mass ratio.
• The same particles came from every element, so electrons are part of all atoms.
• His 1904 plum pudding model pictured electrons embedded in a sphere of spread-out positive charge.
• Rutherford’s gold foil experiment showed the positive charge is concentrated in a tiny nucleus, replacing the model.
• Thomson also found the first evidence for isotopes, leading to the mass spectrometer.