Atomic Model History

Early Ideas

Greeks thought that there were 4 elements that made up ALL matter: – air, earth, fire, and water

Democritus – first to suggest that all matter was made up of some tiny, fundamental particle.– Called this particle “ATOMOS” –

meaning “indivisible”

John Dalton

• Very first atomic theory (what atoms were made of and how they behaved)

• Viewed atoms as tiny, solid spheres

“Billiard Ball” Model

Dalton’s Atomic Theory

1. Atoms are tiny, indivisible particles

2. All atoms of one element are the same.

3. Atoms of different elements are different.

4. Atoms of different elements combine to form compounds.

J.J. Thomson

• Cathode-ray tube experiments

• Discovered electrons (particles with a negative charge)

• Was the first to show that there are smaller pieces that make up an atom.

“Chocolate Chip Cookie” Model

(or “Plum Pudding”)

Ernest Rutherford

Gold Foil Experiment – positively charged alpha particles were passed through a thin sheet of gold foil.

– Most particles continued through on a straight path

– Discovery: the atom is made of mostly empty space

Ernest Rutherford

Gold Foil Experiment (cont.)– Some particles were deflected at

large angles– Discovery: atoms have a very

dense positive (+) center, called the nucleus

“Peach” Model(Positive nucleus and mostly

empty space)

Niels Bohr

• Electrons move around the nucleus in specific layers, or shells.

• Chemical properties of each element are determined mainly by the number of electrons in the outermost shell

James Chadwick

• Discovered neutrons

– Particles with no charge found in the nucleus

Modern Atomic Model

• Electrons move around the nucleus in a cloud• It is impossible to know the exact location of

an electron. (There are 3-D areas that they have a probability of being found in – more on this later… )

Schrödinger & Heisenberg

“Quantum Mechanical”

Model