History of the Atomic Model

Solid Sphere Model

• Proposed by John Dalton in the early 1800s.
• Atoms are small, hard, indivisible spheres.
• New substances form by adding or rearranging atoms.
• Still useful for describing particle arrangement in solids, liquids, and gases.

Plum Pudding Model

• Developed by J.J. Thomson in the early 1900s after discovering the electron.
• Modified the solid sphere model to a sphere of positive charge.
• Negatively charged electrons are embedded within the positive sphere.
• Named the "plum pudding" model, referencing currants in a Christmas pudding.

Nuclear Model

• Developed by Ernest Rutherford around 1909-1911.
• Based on the alpha scattering experiment where positively charged alpha particles were fired at thin gold foil.
• Most alpha particles passed straight through undeflected.
• A small number of alpha particles were scattered in different directions
• Conclusion: The mass of the atom is concentrated at the center in a nucleus.
• The nucleus has a positive charge.
• The nuclear model replaced the plum pudding model.
• Later experiments revealed the positive charge of the nucleus is due to protons.

Planetary Model

• Proposed by Niels Bohr in 1913.
• Electrons orbit the nucleus in shells at set distances.
• Based on theoretical calculations and observations of light emitted from heated atoms.

Atomic Model

• James Chadwick's experiments in 1932 provided evidence for neutrons in the nucleus.
• This model, the atomic model, is the currently accepted model.
• The nucleus contains protons and neutrons.
• Electrons orbit the nucleus.