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Atomic Theory – The History of the Atom
Student Notes
How are atoms studied?
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Atoms are too small in size to study easily
Size of Earth : soda can = soda can : atom
Sizing up the Atom
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into smaller and smaller particles – these are atoms.
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If you could line up 100,000,000 copper atoms in a single file, they would be approximately 1 cm long
The Size of the Atom
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Despite their small size, individual atoms are observable with instruments such as scanning electron microscopes
The Greek philosopher Democritus (460 B.C. – 370 B.C.) was among the first to suggest the existence of atoms
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Dalton’s Atomic Theory – The Ball Model: 1800s
Referred to the atom as a small, hard, indestructible sphere that cannot be subdivided
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An atom is the smallest particle of an element
an element is a substance made up of only 1 type of atom
J.J. Thomson
In 1897 J.J. Thomson demonstrated that there are negatively charged particles found within atoms using cathode ray tubes
After further investigation he concluded that the same negatively charged particles are found in all atoms
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Thomson’s Model
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Since his particles were negative, he knew that there must also be positively charged particles within the atom
Thomson’s Atomic Model
•Thomson believed that the electrons were like plums embedded in a positively charged “pudding”
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Ernest Rutherford’s Gold Foil Experiment - 1911
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“+” charged particles (alpha particles) were fired at a thin sheet of gold foil
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What he thought would happen given the “Plum Pudding” Model
Some alpha particles would be slightly diverted due to their avoidance of the other positive particles inside the atom
What he actually found
Most of the alpha particles slightly deflected and carried on but some bounced backwards
Rutherford’s Findings
 Most of the particles passed right through
 A few particles were deflected
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The Rutherford Atomic Model
Based on his experimental evidence he concluded that the atom:
Is mostly empty space
Contains a small, dense, positively charged nucleus at the center
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The Bohr-Rutherford Model
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Electrons move in orbits (energy levels) around the nucleus
Electrons more or less stay in their orbit unless an increase in energy causes them to move