History of the Atom

Ancient Beginnings

Democritus (500 BC)
- Proposed the idea of atomic theory.
- Suggested that everything is made up of tiny, indivisible particles (atoms) separated by empty space.

John Dalton (1800s)
- Described atoms as solid spheres.
- Proposed that different types of atoms correspond to different elements.

J.J. Thomson (1897)
- Developed the plum pudding model.
- Discovered electrons as negatively charged particles within the atom.
- Proposed that the atom is a sphere of positive charge with electrons embedded in it.
Ernest Rutherford (1909)
- Conducted the gold foil experiment, firing alpha particles at gold.
- Found that some alpha particles were deflected, suggesting a dense core.
- Proposed the nuclear model of the atom:
  - Posited a central nucleus containing positive charge.
  - Introduced the concept of a cloud of electrons around the nucleus but had a flaw regarding electron collapse.

Niels Bohr (1913)
- Solved the Rutherford model's flaw by suggesting electrons orbit the nucleus like planets around the sun.
- Introduced the concept of electrons occupying shells.
- Bohr's model explains why atoms do not collapse.
Further Developments
- Rutherford's Later Discoveries
  - Positive charge in the nucleus is made up of protons.
- James Chadwick
  - Discovered neutrons, neutral particles in the nucleus.

The modern atomic model has evolved through various experiments and refinements by multiple scientists.
Current understanding of the atom is largely based on Bohr's model with additional discoveries regarding protons and neutrons.

These key developments in atomic theory have shaped our current understanding of atomic structure.