Name: ______Date: ______Period: ______Page: ______

The History of the Atom

What is an atom? ______

Directions: As you watch the video, draw the models of the atom (except for Aristotle) and highlight key points.

History of Atomic Structure Timeline
Date / Scientist’s Name / Picture of Scientist / Model of the Atom / Scientist’s Contributions
460-370 BC / Democritus / / •  Was the first to discuss the Atomic Theory
o  All things were made of tiny, invisible, indestructible particles, called atoms
•  Thought atoms varied in size, shape and weight
384-332 BC / Aristotle / / •  Opposed the Atomic Theory
•  Thought matter was made of air, water, fire, and Earth
1808 / John Dalton / / •  Atomic Theory
ü  All matter is made up of atoms
ü  All atoms of a given element are identical in mass and properties
ü  Compounds are formed by a combination of 2+ different kinds of atoms
ü  A chemical reaction is the rearrangement of atoms
X Atoms are indivisible and indestructible
1897 / J.J. Thomson / / ·  Cathode Ray Experiments
·  Discovered the Electron
o  Negatively charged subatomic particle
·  Plum Pudding Model
o  The electrons are distributed throughout the atom with their charge balanced by the presence of a uniform sea of positive charge
1911 / Ernest Rutherford / / ·  Gold Foil Experiment
o  Observation: Some alpha particles deflected to the side and back to their origin
§  Conclusion: The atom contains a small, dense, positively charged nucleus
o  Observation: Most alpha particles went straight through the gold foil
§  Conclusion: The atom is mainly empty space
·  Planetary Model
o  The atom is mainly empty space and contains a dense, positively charged nucleus
·  Named Protons
o  Positively charged subatomic particles
1913 / Niels Bohr / / ·  Bohr Model
o  The atom is mainly empty space and contains a dense, positively charged nucleus
o  The nucleus is surrounded by electrons that travel in circular orbits
1925-present / Cloud Model /
/ •  Electrons move around in clouds around the nucleus
•  Only probability can be used to estimate the location of an electron
1932 / James Chadwick / / ·  Discovered the neutron
o  Neutrally charged subatomic particle