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~400 B. C. Ancient Greece. Democritus: . “All matter is made of tiny, unbreakable particles; atomos ”. Aristotle: “All matter is made of 4 elements; earth, air, fire and water”. Aristotelian viewpoint triumphs for 2000 years…. 2000 years later….

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Ancient greece

~400 B. C.

Ancient Greece

Democritus: .

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

Aristotle:

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

Aristotelian viewpoint triumphs for 2000 years…


2000 years later
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
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.


J j thompson english physicist 1856 1940 nobel prize in physics 1906

100 years later, another classy brit enters the scene:

J.J. Thompson:English Physicist (1856 – 1940)Nobel Prize in Physics 1906

Famous for:

  • Cathode Ray Experiments

  • “Discovery” of proton, neutron and electron

  • ‘Plum Pudding’ Model of the Atom


Ancient greece

Voltage source

Thomson’s Experiment

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


Ancient greece

Voltage source

Thomson’s Experiment

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


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

positive,

(protons)

negative,

(electrons)

andneutral(neutrons)

…pieces inside them.

(but where in the atom were they?)


Thompson suggested a plum pudding model of the atom

electrons had…

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charged

“pudding”

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conduction of electricity

Thompson suggested a“Plum Pudding” Model of the atom:

It explained conduction of electricity:


Thompson s model of the atom
Thompson’s Model of the Atom: had…

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)


Ernest rutherford new zealand physicist 1871 1937 nobel prize in physics 1908

Plum Pudding gets spilled by: had…

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

Famous for:

  • Gold Foil Experiment

  • “Discovery” of the nucleus

  • Nuclear Model of the Atom


Rutherford s experiment
Rutherford’s experiment had…

Took

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
How it worked: had…

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.


Ancient greece

What he expected had…

What he saw:


He expected
He Expected had…

  • 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…



Ancient greece

+ atom

How he explained what he saw:

  • 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
Result of Rutherford’s Gold Foil Experiment: atom “Discovery” of the Nucleus

Rutherford’s Scattering

Experiment

metal

foil

alpha

particles

Ernest Rutherford

1871-1937

film loop

what he expected:

ricocheting

alpha particles!

what he got:


Goodbye plum pudding

++ atom

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10-8 cm

10-13 cm

Goodbye, Plum Pudding;

  • Thomson’s Atom:

    • diffuse mass and charge

    • + alpha particles would have passed through

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


Niels bohr danish physicist 1885 1962 nobel prize in physics 1906
Niels Bohr: atomDanish Physicist (1885 – 1962)Nobel Prize in Physics 1906

Famous for:

  • Planetary Model of the Atom

  • “Father of” Quantum Theory / Mechanics

“Electrons have specific energy levels,

…like planets have specific orbits.”


Bohr said
Bohr said: atom

Electrons can only

be at certain, specific

energy

levels

(distances from the nucleus)

n = 1

n = 2

n = 3

n = the primary quantum number

(distance from nucleus


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

can hold 18 electrons

can hold 8 electrons

can hold 2 electrons


Valence shell
Valence Shell: atom

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
Bohr Model Diagrams atom…show energy levels (shells) and the electrons in them.nucleus is not shown.

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

Inner shells fill first;

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

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

…before 4th shell starts to fill