Mod+13,+History+Of+Atom+Part+A

History of the Atom

How did we figure out atoms exist?

Who figured it out?

“To understand the very large, we must understand the very small.”

Greek Model

Democritus’s model of atom

No protons, electrons, or neutrons

Solid and INDESTRUCTABLE

No experiments to support idea

Properties of atoms:

• indestructible but could change into different forms.

• an infinite number of kinds so there are an infinite number of elements.

• hard substances have rough, prickly atoms that stick together, liquids have round, smooth atoms

that slide over one another.

• smell is caused by atoms interacting with the nose – rough atoms hurt.

• sleep is caused by atoms escaping the brain, death – too many escaped or didn’t return.

DEMOCRITUS (400 BC) – First Atomic Hypothesis

DALTON’S ATOMIC THEORY

I spent my lifetime trying to figure out how things combine, and now every sixth grader in the universe knows this stuff.

John Dalton was the first to make the connection between atomism and recent discoveries in the lab. He used the theory that there were small, indivisible atoms to explain the law of definite proportions and the law of conservation of matter.

1. ALL ELEMENTS ARE COMPOSED OF ATOMS

2. ALL ATOMS OF THE SAME ELEMENT HAVE THE SAME MASS, ATOMS OF DIFFERENT ELEMENTS HAVE DIFFERENT MASSES.

3. COMPOUNDS CONTAIN ATOMS OF MORE THAN ONE ELEMENT

4. IN A PARTICULAR COMPOUND, ATOMS OF DIFFERENT ELEMENTS ALWAYS COMBINE IN THE SAME WAY.

5. IN CHEMICAL REACTIONS, ATOMS FORM ONE OR MORE COMPOUNDS OR ELEMENTS REDISTRIBUTE OR REARRANGE IN RELATION TO OTHER ATOMS TO FORM ONE OR MORE NEW COMPOUNDS. ATOMS THEMSELVES DO NOT UNDERGO A CHANGE OF IDENTITY IN CHEMICAL REACTIONS.

DALTON’S ATOMIC THEORY

This seems almost ridiculously simple now, but these scientists new nothing of atoms, protons, neutrons, etc… What Dalton did was to collect various works from many scientists and create this theory of matter. No easy task in a time without cell phones, cars, or even telegraphs.

Thomson Model of the Atom

J. J. Thomson - English physicist. 1897

Made a piece of equipment called a cathode ray tube.

It is a vacuum tube - all the air has been pumped out.

Thomson’s Experiment

Voltage source

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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

Thomson’s ExperimentThomson’s Experiment

Voltage source

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Voltage source

Thomson’s ExperimentThomson’s Experiment

By adding an electric fieldBy adding an electric field

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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 negativemoving pieces were negative

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- By adding an electric fieldBy adding an electric field

J.J. Thomson

• He proved that atoms of any element can be made to emit tiny negative particles.

• From this he concluded that ALL atoms must contain these negative particles.

• He knew that atoms did not have a net negative charge and so there must be a positive charge balancing the negative charge.

J.J. Thomson

Thomson’s Model

• Found the electron.

• Couldn’t find positive (for a while).

• Said the atom was like plum pudding.

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

Ernest Rutherford (1871-1937)

• Learned physics in J.J. Thomson’ lab.

• Noticed that ‘alpha’ particles were sometime deflected by something in the air.

• Gold-foil experiment

Rutherford

PAPER

Rutherford

PAPER

Lead block

Uranium

Gold Foil

Flourescent Screen

Rutherford “shot” alpha particles (a type of radiation that is made of a helium nucleus of 2 protons and 2 neutrons) at a thin piece of gold foil.

Rutherford expected…

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

• Because…• The positive charges were spread out

evenly. Alone, a single positive charge was not enough to stop the bigger alpha particle with its two protons and much heavier mass. The alpha particle was like a bull in a china shop

So, Rutherfod expected…

Because

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

What he got

How he explained it

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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.

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Bohr Model

After Rutherford’s discovery, Bohr proposed that electrons travel in definite orbits around the nucleus.

Planetary model

Neils Bohr

Bohr’s Model

Nucleus

Electron

Orbit

Energy Levels

Modern View or Quantum Mechanical Model

• The atom is mostly empty space.

• Two regions.

• Nucleus- protons and neutrons.

• Electron cloud- region where you might find an electron.

Development of Atomic Models

Rutherford modelIn the early twentieth century, Rutherfordshowed that most of an atom's mass isconcentrated in a small, positively chargedregion called the nucleus.

Bohr modelAfter Rutherford's discovery, Bohr proposedthat electrons travel in definite orbits aroundthe nucleus.

Thomson modelIn the nineteenth century, Thomson describedthe atom as a ball of positive charge containinga number of electrons.

Quantum mechanical modelModern atomic theory described theelectronic structure of the atom as theprobability of finding electrons withincertain regions of space.

Review Models of the Atom

Dalton proposes theindivisible unit of anelement is the atom.

Copyright © 2007 Pearson Benjamin Cummings. All rights reserved.

Thomson discoverselectrons, believed toreside within a sphere ofuniform positive charge(the “plum-pudding model).

Rutherford demonstrates the existence of a positivelycharged nucleus thatcontains nearly all themass of an atom.

Bohr proposes fixedcircular orbits aroundthe nucleus for electrons.

In the current model of the atom, electrons occupy regions of space (orbitals) around the nucleus determined by their energies.