The Development of the Atomic Model

This lesson traces the historical development of the atomic model from Dalton through to Chadwick, as required by the Edexcel GCSE Combined Science specification (1SC0). You need to understand how each model improved upon the last and be able to describe the key experiments that led to changes in our understanding of atomic structure.

---

Why Models Change Over Time

Scientific models are not fixed — they change as new experimental evidence becomes available. Each new discovery about the atom led scientists to revise or replace the existing model. This is how science progresses: models are tested, and when evidence no longer fits, a better model is proposed.

---

Dalton's Atomic Model (Early 1800s)

John Dalton proposed that:

• All matter is made of tiny, solid spheres called atoms.
• Atoms cannot be divided or broken down further.
• Each element is made of a unique type of atom.
• Atoms of different elements have different masses.

Dalton's model was the first to treat atoms as indivisible particles. It explained many observations about chemical reactions, such as the law of conservation of mass.