1. The History of the
ATOM

2. Who are these men?
In this lesson, we’ll learn about the men whose quests for knowledge about the fundamental nature of the universe helped define our views.

3. (greek for indivisible or cannot be divided)
HISTORY OF THE ATOM
460 BC
Democritus develops the idea of atoms
He pounded up materials in his pestle and mortar until he had reduced them to smaller and smaller particles which he called…
ATOMA
(greek for indivisible or cannot be divided)

4. This theory was ignored and forgotten for more than 2000 years!

5. HISTORY OF THE ATOM ATOMS
1808
John Dalton
Suggested that all matter was made up of tiny spheres that were able to bounce around with perfect elasticity and called them…
ATOMS

6. Dalton’s Theory All elements are composed of atoms.
2. Atoms of the same element are exactly alike.
3. Atoms of different elements are different.
4. Compounds are formed by the joining of atoms of two or more elements.
Dalton’s Atom

7. HISTORY OF THE ATOM ELECTRON
1898
Joseph John Thompson
Found that atoms could sometimes eject a far smaller negative particle which he called an…
To dive deeper into electrons and light emission in a cathode ray, go to and download the animation showing how electrons emit light in a tube.
ELECTRON
To learn about J.J.Thomson’s cathode ray tube experiment, click here

8. HISTORY OF THE ATOM PLUM PUDDING MODEL
1904
Thomson develops the idea that an atom was made up of electrons scattered unevenly within an elastic sphere.
The electrons are surrounded by a soup of positive charge to balance the electron's negative charge like plums surrounded by pudding.
PLUM PUDDING
MODEL

9. HISTORY OF THE ATOM
1910
Ernest Rutherford
Oversaw Geiger and Marsden carrying out his famous experiment.
Fired Helium nuclei at a piece of gold foil which was only a few atoms thick.
Found that although most of them passed through. About 1 in 10,000 hit

10. HISTORY OF THE ATOM
gold foil
helium nuclei
helium nuclei
They found that while most of the helium nuclei passed through the foil, a small number were deflected and, to their surprise, some helium nuclei bounced straight back.

11. HISTORY OF THE ATOM
Rutherford’s new evidence allowed him to propose a more detailed model with a central nucleus.
He suggested that the positive charge was all in a central nucleus. With this holding the electrons in place by electrical attraction.
However, this was not the end of the story.

12. HISTORY OF THE ATOM
1913
Niels Bohr
Studied under Rutherford at the Victoria University in Manchester.
Refined Rutherford's idea by adding that the negatively charged electrons were in orbits, like planets orbiting the sun.
Each orbit is only able to contain a set number of electrons.
Calculated the exact energy levels for the hydrogen atom.

13. Bohr’s Atomic Theory
The Bohr model was a modification of the Rutherford model.
Bohr placed electrons in certain circular orbits around the nucleus called shells. +

14. HISTORY OF THE ATOM Proposed the electron cloud.
1926
Erwin Schrodinger
Proposed the electron cloud.
Stated that the electron do not travel in exact orbits but travel in clouds around the nucleus.

15. Electron cloud model
* Although this is an important model to recognize, we will be focusing on the Bohr Model in class.

16. HISTORY OF THE ATOM
1932
James Chadwick
Scientists could not figure out what made atoms so heavy.
Chadwick proposed the idea of neutrons—neutral particles that are located in the nucleus with the protons.
The new Bohr model includes these particles.
Neutrons

17. New Bohr Model of a HELIUM ATOM
Shell
proton N + - + N -
neutron
electron

18. Today’s Understanding of the Atom
With the help of new technology, such as particle accelerators, scientists now think protons and neutrons are made of smaller particles.

19. Evolution of the Atomic Model
Ask students to evaluate the advantages and limitation of each model.

20. The current and most accurate model of the atom shows electrons are in a cloud around the nucleus.
However, we will use the Bohr model in class to better understand the structure of the atom.