Devel. of Atomic Theory

8/8/2019 Devel. of Atomic Theory

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The Development of Modern

Atomic Theory



In the beginning

Democritus- In the 4th century BC, heproposed that the universe was made of tiny

indivisible units. He called these atoms, from the Greek word

atomos, meaning unable to be divided or cut.

He did not have any evidence to support his

theory, but nonetheless, some peoplebelieved.



Daltons Atomic Theory In 1808, he proposed a revised atomic

theory.

According to Dalton, all atoms of a givenelement were exactly alike, and atoms of different elements could join to formcompounds.

Theory was based on experimental evidence. However, some parts of his work turned out

to be incorrect.



Thomsons Model of the Atom

1897- Experiments with electricity led to thediscovery that atoms were NOT indivisible!

He conducted a famous cathode ray tubeexperiment which suggested that these rayswere made of negatively charged particles

that came from inside of atoms.



Results from the Cathode Ray Tube

Experiment Thomson knew that a magnet deflected

charges, and he reasoned that because the

tube was a vacuum the beam must havecome from the cathode (positively chargedplate) or anode (negatively charged plate).

The direction of deflection indicated anegatively charged beam, and electrons werediscovered!!!!



Thomsons Plum Pudding Model

Based on the outcome of his experiment, heproposed a new model of the atom.

He reasoned that the electrons were spreadthroughout the atom, just like in plumpudding, a dessert in his time.

We might use the chocolate chip cookiemodel to better visualize it today.



Rutherfords Model of the Atom

Ernest Rutherford proposed his model shortlyafter Thomson.

He found that most of the mass of an atomwas concentrated at the atoms center, or thenucleus.

To understand this conclusion, you must learn about his famous gold foil experiment.



Gold Foil Experiment A beam of positively charged alpha particles

were aimed at a thin sheet of gold foil.

Because of Thomsons model, Rutherfordpredicted that there would not be a largeenough mass of positive charge to cause

many particles to bounce back. He thought they would travel in a straight path throughthe foil.



Gold Foil Experiment But, the observations did not match his

hypothesis. Most passed straight through,

but a few were deflected by a large amount. A few even bounced back!

He said, It was quite the most incredibleevent that has ever happened to me in my

life. It was almost as incredible as if youfired a 15- inch shell at a piece of tissuepaper and it came back and hit you.



Gold Foil Experiment



R

esults of R

utherfords experiment An atoms positive

charge in concentrated

in the center of theatom, called thenucleus.

He is also credited with

the discovery of theproton.

Here is his atomicmodel:

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Devel. of Atomic Theory