HISTORYHISTORY OFOF THE ATOMIC MODELTHE ATOMIC MODEL

ATOMIC THEORYATOMIC THEORY

Mrs. Gibson

Physical Science

Democritus 400BCDemocritus 400BC

Proposed all objects have a smallest Proposed all objects have a smallest part/piecepart/piece

Named this smallest piece – Named this smallest piece – atomatom Atomos means indivisible, not to be cutAtomos means indivisible, not to be cut Atom – small hard particle, made of Atom – small hard particle, made of

same material, but in different sizes, same material, but in different sizes, shapes, and colors, indefinite in shapes, and colors, indefinite in number, always moving, join together.number, always moving, join together.

Uses observations to prove ideasUses observations to prove ideas

DALTON’SFour Points on the

Atomic Theory

All matter is All matter is made of atoms. made of atoms.

Atoms are Atoms are indivisible indivisible

and and indestructibleindestructible..

Atomsof 2 or morekinds make

up acompound

Atomsrearrangeduring a chemicalreaction

Atoms with thesame mass and

propertiesmake up an

element

John Dalton 1803John Dalton 1803

Uses scientific reasoning to prove ideasUses scientific reasoning to prove ideas

J. J. Thomson 1897J. J. Thomson 1897 Discovers that the atom is Discovers that the atom is divisibledivisible.. Theory is called Theory is called Plum Pudding ModelPlum Pudding Model

Atom is positive material with negative Atom is positive material with negative particles mixed in.particles mixed in.

Discovers Discovers ElectronElectron called it a corpuscle called it a corpuscle Discovers electron by passing electric current Discovers electron by passing electric current

through a gasthrough a gas Gas is neutral, but gives off negative charges.Gas is neutral, but gives off negative charges. Reasoned: If there is a negative charge, then Reasoned: If there is a negative charge, then

there must be a positive substance to make there must be a positive substance to make the gas neutralthe gas neutral..

J.J. with his Cathode Ray TubeJ.J. with his Cathode Ray Tube

Plum Pudding Model

Ernest Rutherford 1908Ernest Rutherford 1908

Gold Foil ExperimentGold Foil ExperimentDiscovers center of atom, Discovers center of atom,

Nucleus,Nucleus, contains positive contains positive ProtonsProtons

Atom has positive center Atom has positive center with negative charges with negative charges (electrons) scattered outside (electrons) scattered outside in in electron cloud.electron cloud.

Most of atom is empty spaceMost of atom is empty space

GOLD FOIL EXPERIMENTGOLD FOIL EXPERIMENT

Gold atoms and alpha

particles (path) up close

Most of the particles pass through the gold foil-some particles bounce back- must be a “solid” middle and it must be positive since it repels the other alpha particles-negative particles must be floating in the empty space

Rutherford’s ModelRutherford’s Model

Positive Nucleus

Empty Space

ElectronsProtons

Two atoms were walking down the street Two atoms were walking down the street when one stops and shoutswhen one stops and shouts

“ “Wait! Stop! I lost an electron.”Wait! Stop! I lost an electron.”

““You did? Are you sure?” says the second You did? Are you sure?” says the second atom frantically.atom frantically.

““Yes,” says the first “I’m POSITIVE!”Yes,” says the first “I’m POSITIVE!”

Wait for it...

Laughter and Applause

Niels Bohr 1913Niels Bohr 1913Changed Rutherford’s model Changed Rutherford’s model

Show that electrons are not Show that electrons are not just scattered, but travel just scattered, but travel around nucleus in around nucleus in orbitalsorbitals called called energy levelsenergy levels..

The Bohr Model is used most The Bohr Model is used most to represent atomic structureto represent atomic structure

I wonder if they know

that MY model is better than their model.

James Chadwick 1932James Chadwick 1932

Discovers NeutronDiscovers Neutron

Chadwick Finds Neutron by…Chadwick Finds Neutron by…

Chadwick concluded that the energy of the protons could be explained by the presence of a neutral particle comparable in mass to that of a proton.

Wave ModelWave Model Current Model Theory, based on wave Current Model Theory, based on wave

mechanicsmechanicsCannot determine exact location of Cannot determine exact location of

electrons, but can determine probable electrons, but can determine probable locations based on calculations.locations based on calculations.

Energy Levels fillEnergy Levels fill2 - 1st2 - 1st8- 2nd8- 2nd8-18 in 3rd 8-18 in 3rd

Let’s Explore Electron Let’s Explore Electron Configuration…Configuration…

Electron AppletElectron Applet

http://www.colorado.edu/physics/2000/applets/a3.htmlhttp://www.colorado.edu/physics/2000/applets/a3.html

For more information try the tutorial: For more information try the tutorial: Elements as AtomsElements as Atoms

http://www.colorado.edu/physics/2000/elements_as_atoms/index.htmlhttp://www.colorado.edu/physics/2000/elements_as_atoms/index.html

SummarySummary

What was the What was the Theory of…Theory of… Democritus?Democritus? Dalton?Dalton? Thomson?Thomson? Rutherford?Rutherford? Bohr?Bohr? Chadwick?Chadwick? Wave?Wave?

Who discovered the…Who discovered the… Electron?Electron? Proton?Proton? Nucleus?Nucleus? Neutron?Neutron?

What allows the What allows the scientists to scientists to develop/discover each develop/discover each new piece of the new piece of the atomic model?atomic model?