Uploaded August 2026 | Updated September 2026, 2 weeks ago
For centuries, atoms were mostly a philosophical idea. In the early 1800s, John Dalton helped turn that idea into a scientific theory by studying how elements combine.
Chemical compounds repeatedly formed in fixed proportions. Water, for example, always contained the same ratio of hydrogen to oxygen by mass. Dalton argued that this consistency made sense if matter came in discrete, countable units that combined in whole-number ratios.
His atoms were imagined as tiny, solid spheres, similar to microscopic billiard balls. That model helped explain chemical reactions, but it did not survive unchanged.
In 1897, J. J. Thomson used cathode-ray experiments to identify negatively charged particles that were far smaller than atoms. These particles became known as electrons. Their discovery showed that atoms were not solid, indivisible objects after all.
Thomson responded with a new model in which electrons were distributed through a region of positive charge, later nicknamed the plum pudding model.
The model was eventually replaced, but it marked an important turning point. Evidence had first established atoms as the building blocks of matter, then new evidence revealed that those building blocks had an internal structure of their own.
For centuries, atoms were mostly a philosophical idea. In the early 1800s, John Dalton helped turn that idea into a scientific theory by studying how elements combine.
Chemical compounds repeatedly formed in fixed proportions. Water, for example, always contained the same ratio of hydrogen to oxygen by mass. Dalton argued that this consistency made sense if matter came in discrete, countable units that combined in whole-number ratios.
His atoms were imagined as tiny, solid spheres, similar to microscopic billiard balls. That model helped explain chemical reactions, but it did not survive unchanged.
In 1897, J. J. Thomson used cathode-ray experiments to identify negatively charged particles that were far smaller than atoms. These particles became known as electrons. Their discovery showed that atoms were not solid, indivisible objects after all.
Thomson responded with a new model in which electrons were distributed through a region of positive charge, later nicknamed the plum pudding model.
The model was eventually replaced, but it marked an important turning point. Evidence had first established atoms as the building blocks of matter, then new evidence revealed that those building blocks had an internal structure of their own.
