Physics: models of the atom

The tutor reviews the progress of thought about atoms through the ages.

Democritus, a Greek philosopher living around 400 BC, is credited as the first promoter of the concept of the atom. He proposed that all matter consists of tiny, indivisible particles, which he called atoms.

The next published refinement of Democritus’s idea was Dalton’s atomic theory of 1808. He agreed with Democritus, elaborating the idea of an element – a type of atom. Every element has its unique type of atom.

After further study came the Thomson model, in common use from the 1890s to 1911. It was also called the “plum pudding model”, and proposed that an atom consists of positively charged “dough” with negatively charged “raisins” scattered throughout. Thomson, who discovered the electron in 1897, realized that the electrons should be moving.

Rutherford, in 1911, carried out an experiment whose results led to the idea of the nucleus. The Rutherford model claims that the center of an atom is heavy and positive, while electrons orbit distantly around it. Most of the “volume” of an atom is empty space between the nucleus and the electrons.

The Bohr model (1913) refined the Rutherford model, suggesting that the electrons orbit at only certain distances (levels) from the nucleus, and that they can jump from one level to another. The levels are called shells.

Rutherford’s model is still used until high school; in grades 11 and 12, Bohr’s is studied.

In a future post I’ll explain Rutherford’s experiment and why it led to such a breakthrough.


Giancoli, Douglas C. Physics. New Jersey: Prentice Hall, 1998.

Bullard, Jean et al. Science Probe 10. Scarborough: Nelson Canada, 1996.

Jack of Oracle Tutoring by Jack and Diane, Campbell River, BC.

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