Reading Exercise: The History of the Atomic Model

Democritus’ atoms

Around 440 BC, Greek philosopher Democritus was the first to suggest the existence of atoms, tiny particles that make up all matter. The word atom comes from the Greek ”atomos” meaning indivisible.

However, most of his colleagues, especially Aristotle, did not agree with Democritus. Instead, they thought that matter was made up of the four ”elements” fire, water, wind and earth.

Dalton’s spheres

It took over 2000 years until another scientist would challenge Aristotle’s ”element” theory.

In 1803, John Dalton, an English teacher from Manchester, carried out experiments proving that all matter is made up of tiny particles. He chose to use Democritus’ name and called them atoms.

In Dalton’s model atoms were tiny, hard spheres that vary in size and mass, but cannot be split into smaller pieces.

Thomson’s plum pudding

It took a much shorter time to reach the next step in the discovery of the atomic model.

J.J. Thomson, another English scientist, discovered the electron in 1897 and developed the plum pudding model of the atom for which he received the Nobel prize in 1906.

The plum pudding model said that the tiny negative electrons were distributed in a positive mass inside the atom. The electrons were like negative raising in a positive plum pudding dough.

Rutherford’s gold foil experiment

The next experiment to develop the atomic model even further were carried out by one of Thomson’s former students, Ernest Rutherford, who was originally from New Zealand.

In 1909, Rutherford and his team conducted one of the most important experiments in the history of science. They used a gold foil which was bombarded with alpha particles (= Helium nuclei which have 2 protons and 2 neutrons).

If Thomson’s plum pudding model were true, you would expect all alpha particles to punch holes through the positive ”dough” and pass straight through the foil.

The results looked somewhat different. Most alpha particles did pass straight through the gold foil. However, some particles did not pass through and bounced back. This suggested that there were parts in the foil where mass was very concentrated, while the rest seemed to be empty space.

As a consequence, Rutherford introduced the modern planetary model of the atom where the electrons circulate around a nucleus. The nucleus is small, but contains most of the atom’s mass and is where the alpha particles bounced back in his experiment. The major part of the atom is empty space where the alpha particles could pass through.

By the way, Ernest Rutherford received a Nobel prize as well. However, it was for discovering the concept of half-life for radioactive substances rather than his work on the atomic model.

Tasks:

  1. Draw a timeline including the four stages of the atomic model’s development.
  2. Describe what the word ”atom” means.
  3. Describe what atoms were like in Dalton’s atomic model.
  4. State what Thomson discovered.
  5. Describe what atoms were like in Thomson’s atomic model.
  6. Describe the gold foil experiment Rutherford conducted.
  7. Explain how the gold foil experiment showed that Thomson’s theory was wrong.
  8. Describe what atoms are like in Rutherford’s model.
  9. Challenge: Find out how the atomic model developed further. You could look at the work of Niels Bohr, Werner Heisenberg and James Chadwick.

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