1. A Pictorial History of Atomic Theory
From Ancient Greece to Modern Quantum Physics

2. Democritus BCE
Democritus was a Greek philosopher and not an experimental scientist.
He thought that there would be a particle that would be indivisible.

3. Ancient Greek Model He called this particle “atomos” or uncuttable.
It is not a detailed or useful model.
He thought some were smooth and some were sharp.

4. John Dalton 1766-1844 Dalton was a British experimental scientist.
Dalton’s model of an atom was not that much advanced from the ideas of Democritus.
He contributed important ideas about atoms.

5. Solid Sphere Model Atoms cannot be broken into smaller pieces.
In any element, all the atoms are exactly alike.
Atoms of 2 or more elements can combine to form compounds.

6. Solid Sphere Model Atoms of each element have a unique mass.
The masses of the elements in a compound are always in a constant ratio.

7. J.J. Thomson
Thomson experimented with with particles and found that they did not behave as predicted by Dalton’s idea of an atom.
He discovered the existence of the electron.

8. Plum Pudding Model 1900
An atom that was composed of a positively charged sphere and negatively charged particles fit his observations.

9. Hantaro Nagaoka
Hantaro Nagaoka was a Japanese physicist. He was educated in Japan and Europe. He taught at the University of Tokyo.

10. Saturnian Model 1904
He proposed an atom with a large positively charged nucleus and negatively charged particles orbiting it.
His idea was like Saturn with a huge planet holding orbiting rings.

11. Ernest Rutherford
Rutherford thought that the atom was mostly empty space – not a huge nucleus.

12. Planetary Model 1911
Rutherford still pictured orbiting electrons but randomly in all dimensions around a smaller nucleus.

13. Niels Bohr 1885-1962 Bohr was a Danish physicist.
He found that these models did not explain what he was observing.
They did not explain how energy was absorbed or released in chemical reactions.

14. Planetary with Energy Shells 1913
He proposed that electrons moved from one energy shell to another rather than randomly and that this explained how the energy is absorbed or released.

15. Planetary with Energy Shells 1913
He also proposed that the chemical properties of an element are determined by the electrons in the outermost orbit.

16. Many Scientists
Contributions of many scientists have made advancements in our concept of the atom.

17. Electron Cloud Model
We do not think in terms of an orbital path anymore but in orbital regions where there is a probability of the electron being.

18. Sir James Chadwick
Chadwick discovered a missing piece of the puzzle in the structure of the atom.
The weight of the protons and electrons did not add up to the total weight of the atom.

19. Neutrons in Nucleus
Chadwick discovered that there was another particle in the nucleus (as massive as the proton but without any charge).
There are usually an equal number of protons and neutrons.

20. Neutrons in Nucleus
Sometimes there are different numbers of neutrons than protons. This changes the atomic mass of the atom. We call these isotopes.

21. Modern Model of Atom
Our current understanding is of a complicated atomic structure.
There are many sub atomic particles and forces.
Electrons travel in orbitals.