Democritus
400 BC
•First person to propose the "ultimate particle"
•believed that matter could only be divided a limited number of times, eventually there would be a
"smallest piece"
•Called it "atomos", meaning “not to be cut.”
•Thought that atoms were small particles that were made out of the same material but were all
different sizes and shapes
•believed that they were in constant motion
John Dalton
1803
• Recognized the existence of atoms and elements
• Assigned a relative atomic weight of one to hydrogen, from that developed a relative atomic weight
scale from assumed atomic ratios
• Law of multiple proportions
• Proposal of the atomic theory in 1803
JJ Thomson
1897
•Discovered electron
•"plum pudding" model
---the atom was positive and there were negative forces wandering around it
-discovered atoms of the same element but different atomic weight
Nov 1, 1909
Ernest Marsden
Ernest Marsden
What: He discovered the existence of the atomic nucleus. How: He discovered this by using the Gold Foil
Experiment or the Rutherford Experiment.
Rutherford
1909
(Thought Thomson was wrong)
•thought of an atom having a positive nucleus with negative electrons surrounding it
•Atom was mostly empty
- Came to this conclusion through the "Gold Foil Experiment"
-Had particles pass through gold foil with detective screen surrounding them
-The particles that didn't pass made him come to the conclusion that most of the atom is empty
space
-Flaws: thought electrons contained charges that are accelerating
Niels Bohr
1913
•decided to make a new model based off of Rutherford's model, but changed the orbit of the electron
•created energy levels in the atom, where only a certain amount of electrons could fit on one energy
level of the atom
•used Planck's ideas in order to create quantum mechanics
Oct 31, 1926
Erwin Schrödinger
What: He generated a mathematical model for the distribution of electrons in an atom. How: He
combined the equations for the behavior of waves with the Louis de Broglie equation
James Chadwick
1932
•discovered the neutron in 1932
•discovery of the neutron led directly to the discovery of fission because he found out that Sice
neutrons have no electrical charge they could spilt the nucleus without anything to get in their way.
and ultimately to the atomic bomb
460
Democritus
Democritus
What: He created the first atomic model. How: His model of the atom depended on analogies from
human senses.
First person to propose the "ultimate particle"
Called it "atomos", meaning “not to be cut.”
believed that they were in constant motion
Oct 29, 1897
J.J. Thomson
What: He discovered the Plum Pudding Model of the atom. How: In J.J.'s model, the atom is
composed of electrons surrounded by an area of positive charges to balance out the negative
charge
Oct 29, 1904
Hantaro Nagaoka
What: Nagaoka developed an early, incorrect "planetary model" of the atom. How: The model
was based on the stability of Saturn's rings.
Nov 1, 1909
Ernest Marsden
What: He discovered the existence of the atomic nucleus. How: He discovered this by using the
Gold Foil Experiment or the Rutherford Experiment.
Oct 31, 1911
Ernest Rutherford
What: Theorized that atoms have their positive charge concentrated in a very small nucleus.
How: He pioneered the Rutherford model of the atom, with his discovery and interpretation of
Rutherford scattering in his gold foil experiment.
Oct 31, 1913
Niels Bohr
What: Presented a picture of an atomic structure. How: He passed on to a study of the structure
of atoms based on Rutherford's discovery of the atomic nucleus. (This lead him to the picture of
his atom.)
Oct 31, 1926
Erwin Schrödinger
What: He generated a mathematical model for the distribution of electrons in an atom. How: He
combined the equations for the behavior of waves with the Louis de Broglie equation.
Oct 31, 1932
James Chadwick
What: He proved the existence of neutrons. How: He heard of a new method of detecting
particles emitted by radioactive elements developed by Irene Joliot-Curie.
