Atomic Models

From the ancient Greeks to the

20th century

Democritus’s “model”

• Democritus only proposed the philosophical idea of atoms

Dalton’s Model• Dalton proposed the first Atomic Theory– All matter is composed of tiny particles – atoms– Atoms of a given element are all identical to each

other– Atoms cannot be divided, created or destroyed– Atoms combine in simple whole # ratios to form

compounds

JJ Thompson

• Proposed the “Plum Pudding” Model of atoms– The majority of an atom was a mass of positive

charge with negatives (electrons) imbedded throughout

Rutherford’s Nuclear Model

• Based on his famous gold foil experiment, Rutherford found evidence that atoms have a tiny region of positive charge (nucleus), surrounded by negative charge

Bohr’s Model

• Neils Bohr proposed that electrons are found in distinct energy levels that are a certain distance from the nucleus

The Modern Quantum Mechanical Model

• Based on work done by Louis deBroglie, Schroedinger proposed the wave mechanical (aka electron cloud) model of the atom, where electrons are likely to be found in “orbitals” of different shape.

Other key scientists involved…• Antoine Lavoisier – developed the law of

conservation of mass• Robert Millikan – experimentally determined the

mass and charge of an electron• Henry Moseley – determine the number of

protons in the nucleus of several atoms• James Chadwick – discovered neutrons• Max Planck – developed the idea that energy is

quantized (exists in bundles or packets)• Werner Heisenberg – developed the Uncertainty

Principle – one cannot know the exact position and speed of an electron simutaneously