Ancient Greece

Aristotle:

“All matter is made of 4 elements; earth, air, fire and water”

Democritus: .

“All matter is made of tiny, unbreakable particles; atomos”

Aristotelian viewpoint triumphs for 2000 years…

~400 B. C.

2000 years later…

John Dalton, a British Teacher in the late 1700’s, combined ideas of elements with that of atoms and developed

Dalton’s Atomic Theory

Dalton’s Atomic Theory: All matter is made of tiny indivisible particles called

atoms.

Atoms of the same element are identical.

Atoms combine to form compounds

in simple whole number ratios. (example: electrolysis)

Chemical reactions involve the rearrangement of atoms; no matter is created or destroyed.

Famous for:

• Cathode Ray Experiments

• “Discovery” of proton, neutron and electron

• ‘Plum Pudding’ Model of the Atom

J.J. Thompson:

English Physicist (1856 – 1940)Nobel Prize in Physics 1906

100 years later, another classy brit enters the scene:

Passing an electric current makes a beam Passing an electric current makes a beam appear to move from the negative to the appear to move from the negative to the positive endpositive end

Thomson’s ExperimentThomson’s Experiment

Voltage source

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Voltage source

Thomson’s ExperimentThomson’s Experiment

By adding an electric field he found that the By adding an electric field he found that the moving pieces were negative (electrons!)moving pieces were negative (electrons!)

+

-

This, and Thompson’s other experiments showed that atoms had…

positive,(protons)

negative,(electrons)

and neutral(neutrons)

…pieces inside them.

(but where in the atom were they?)

Thompson suggested a

“Plum Pudding” Model of the atom:

It explained conduction of electricity:

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+ +

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+ +

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+ +

conduction of electricity

electrons

positively charged“pudding”

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+

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Thompson’s Model of the Atom:

A bunch of positive stuff, with the electrons able to be removed

“Plum Pudding” Model;

a bunch of positive ‘pudding’,

with removable negative ‘plums’ (electrons)

Famous for:

• Gold Foil Experiment

• “Discovery” of the nucleus

• Nuclear Model of the Atom

Ernest Rutherford:

New Zealand Physicist (1871-1937) Nobel Prize in Physics 1908

Plum Pudding gets spilled by:

Rutherford’s experimentTook

positive charges(alpha particles; radioactive particles that are emitted by

Uranium) and

fired them

at Gold Atoms

His surprise:

Some of them bounced back.

How it worked:

When the alpha particles hit a florescent screen, the screen glows. So, he could keep track of where the positive charges ended up after he fired them at some gold foil.

What he expected

What he saw:

He Expected

• The alpha particles to pass through without changing direction very much

• Because

• The positive charges were spread out evenly. Alone they were not enough to stop the alpha particles…

Because, he thought the mass was evenly distributed in the atom

How he explained what he saw:

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• Atom is mostly empty

• Small dense, positive piece at center

• Alpha particles are deflected by it if they get close enough

Result of Rutherford’s Gold Foil Experiment:

“Discovery” of the Nucleus

Ernest Rutherford1871-1937

metalfoil

alphaparticles

film loop

what he expected:

what he got:

Rutherford’s ScatteringExperiment

ricochetingalpha particles!

Goodbye, Plum Pudding;

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+ +

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• Thomson’s Atom:– diffuse mass and charge

– + alpha particles would have passed through

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10-8 cm

10-13 cm

Hello, Nuclear Model•Rutherford’s Atom:

–concentrated mass and positive charge at the nucleus

–electrons roam empty space around the nucleus

– some + alpha particles repelled

Famous for:

• Planetary Model of the Atom

• “Father of” Quantum Theory / Mechanics

Niels Bohr:

Danish Physicist (1885 – 1962)Nobel Prize in Physics 1906

“Electrons have specific energy levels, …like planets have specific orbits.”

Bohr said:

Electrons can only be at certain, specific

energy levels

(distances from the nucleus)

n = the primary quantum number(distance from nucleus

n = 1

n = 2

n = 3

Electrons are found in ‘shells’ = ‘energy levels’

can hold 18 electrons

can hold 8 electrons

can hold 2 electrons

Valence Shell:The outermost shell that contains electrons.

Valence Electron:

An electron in an atom’s valence shell

Bohr Model Diagrams…show energy levels (shells) and the electrons in them.

nucleus is not shown.

1st shell (n = 1) gets 2 e-

2nd shell (n = 2) gets 8 e-

Inner shells fill first;

3rd shell (n = 3) gets 8 e-

…before 4th shell starts to fill