Evolution of the ATOM Project

• Create a foldable discussing the evolution of the atom starting with Democritis and ending with the Electron Cloud Model

Evolution of the ATOM Project

• Your Foldable should have a comic book like quality on the inside with pictures and comments to help describe the models and the experiments that led to them

Atomos: Not to Be Cut

The History of the Atomic Theory

The Battle of the two Philosophers…

460 BC Democritus

DEVELOPS THE IDEA OF THE ATOM.

He pounds materials in his mortar and

pestle until he reduces them to smaller

and smaller particles which he ultimately

calls…

ATOMOSATOMOS

(greek for indivisible)

Democritus’s Ideas

Atoms are solid & homogeneous all atoms made of the same

material.

Different types of atoms have different shapes and sizes.

The different shapes and sizes of the atoms determine the different properties of the substances

Atoms are infinite in number.

Aristotle

• Earth, Fire, Air and Water approach to the

nature of matter.

• All substances made of these four elements

• Blend these in different lend these in different proportions to get all proportions to get all substancessubstances

• Transmute Transmute LeadLead into into GOLDGOLD

ARISTOTLE WINS!!!

Democritus’ idea of “atoms” was ignored and forgotten for more

than 2000 years!

HISTORY OF THE ATOMHISTORY OF THE ATOM

1808 John Dalton (England)

•All matter is made up of tiny spheres called

…

ATOMSATOMS

Dalton combines the idea of elements with that of Dalton combines the idea of elements with that of atoms!atoms!

Dalton’s Atomic TheoryBilliard Ball Model

1. All elements are composed of atoms.

– indivisible and indestructible

2. Atoms of the same element are alike

3. Atoms of different elements are different

4. Chemical reactions involve the rearrangement of atoms.

Evidence #1 It is possible to remove a negatively charged particle from an atom using electrical forces (Thomson 1897)

Evidence #2: If a tiny particle is shot into the middle of an atom, it hits something dense in the center and bounces back in the direction from which it came. If a tiny particle is shot into the edges of the atom, it goes through. Most tiny particles shot at an atom will go through. (Rutherford, 1911)

Evidence #3: The farther from the center of an atom the negatively charged particles are, the easier they are to remove (Bohr, 1913)

Using the 5 models of the atom, answer discussion questions

Here comes the Electron…

HISTORY OF THE ATOMHISTORY OF THE ATOM

1898 Joseph John Thompson

found that atoms could sometimes eject a

far smaller negative particle which he

called an

ELECTRONELECTRON

Thomson’s Experiment

Voltage source

+-

Metal Disks

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

These rays were a stream of negatively These rays were a stream of negatively charged particlescharged particles

Thomson’s ExperimentThomson’s Experiment

Voltage source

+-

Voltage source

By adding an electric field he found that the By adding an electric field he found that the moving pieces were negative moving pieces were negative

+

-

Thomson’s ExperimentThomson’s Experiment

What’s the New Model Now?...

PLUM PUDDING MODELPLUM PUDDING MODEL

Thompson develops the idea that an atom was made up of

electrons scattered unevenly within an elastic sphere

surrounded by a soup of positive charge to balance the

electron's charge

1904

like plums surrounded by pudding.

Atoms are now considered divisible

Here comes the Proton…

HISTORY OF THE ATOMHISTORY OF THE ATOM

1910 Ernest Rutherford

Believed the plum pudding model of the Believed the plum pudding model of the atom was correct.atom was correct.

Wanted to see how positively charged Wanted to see how positively charged alpha particles (radioactive) interacted alpha particles (radioactive) interacted with solid matter.with solid matter.

Shot them at gold foil which was only a Shot them at gold foil which was only a few atoms thick.few atoms thick.

Rutherford’s experiment

• Here’s what it looked like

Lead block

Uranium

Gold Foil

Flourescent Screen

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.

What he expected…

Because…

Because he thought the positive charge was evenly distributed in the atom

Because, he thought the positive charge was evenly distributed in the atom

What he got…

How he explained it…

+

• Atom is mostly empty

• Small dense, positive piece at center

• Alpha particles are deflected by it

if they get close enough.

+

Density and the Atom

• Since most of the particles went through, the atom was mostly empty

• Because the alpha particles deflected so much, the positive pieces in the core of the atom had to be very heavy

• Positive core had a small volume, big mass, big density– This small dense positive area is the nucleus

HISTORY OF THE ATOMHISTORY OF THE ATOM

1913 Niels Bohr

Studied under Rutherford.

•Bohr refined Rutherford's idea

•electrons were in orbits (like

planets orbiting the sun)

•each orbit could contain only a set

number of electrons.

HELIUM ATOM

+N

N

+-

-

proton

electron

neutron

Shell

Nucleus

Bohr Model of Atom

The Bohr model of the atom, like many ideas in the history of science, was at first prompted by and later partially disproved by experimentation.

http://en.wikipedia.org/wiki/Category:Chemistry

Increasing energyof orbits

n = 1

n = 2

n = 3

A photon is emittedwith energy E = hf

e-e-

e-

e-

e-

e-

e-

e-

e-

e-

e-

The Electron Cloud Model • Developed between the

1920’s and 1930’s by Werner Heisenberg and Erwin Schrodinger

• States that the atom is mostly empty space

• Two regions– Nucleus- protons and neutrons

– Electron cloud- region where you have a high probability of finging an electron

Models of the Atom

Dalton’s model (1803)

Thomson’s plum-pudding model (1897)

Rutherford’s model (1909)

Bohr’s model (1913)

Charge-cloud model (present)

Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 125

Greek model(400 B.C.)

1800 1805 ..................... 1895 1900 1905 1910 1915 1920 1925 1930 1935 1940 1945

1803 John Dalton pictures atoms astiny, indestructible particles, with no internal structure.

1897 J.J. Thomson, a Britishscientist, discovers the electron,leading to his "plum-pudding" model. He pictures electronsembedded in a sphere ofpositive electric charge.

1904 Hantaro Nagaoka, aJapanese physicist, suggests that an atom has a centralnucleus. Electrons move in orbits like the rings around Saturn.

1911 New Zealander Ernest Rutherford statesthat an atom has a dense,positively charged nucleus. Electrons move randomly in the space around the nucleus.

1913 In Niels Bohr'smodel, the electrons move in spherical orbits at fixed distances from the nucleus.

1924 Frenchman Louis de Broglie proposes thatmoving particles like electronshave some properties of waves. Within a few years evidence is collected to support his idea.

1926 Erwin Schrodinger develops mathematicalequations to describe the motion of electrons in atoms. His work leads to the electron cloud model.

1932 James Chadwick, a British physicist, confirms the existence of neutrons, which have no charge. Atomic nuclei contain neutrons and positively charged protons.

What the inside of your foldable should look like….

What the inside of your foldable should look like….