It’s time for… THE HISTORY OF THE ATOM

Aristotle- ~500 B.C. philosopher

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What was the world made of?

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Water and Earth because it was everywhere

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Air because it was around and over us

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Fire because it was an agent of change

Democritus 500 B.C.

• Philosopher • Proposed that matter is made of about 100 basic kinds of tiny, indivisible particles he called “atomos.” • Nobody believed him.

His atoms would look like this:

Who won?

• Aristotle’s theories could be easily understood • • The four elements could be easily seen and understood

Democritus had no proof

Smoke =air fire ashes = earth

400 B.C. - 1500 A.D.

The Dark Ages of Chemistry Little research or experimentation done with atoms (or anything else.)

John Dalton - 1803

Experimental Observations studying gases.

• CO vs CO 2 • Ran electricity through water and…it

decomposed

!

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2 H 2 O --> 2 H 2 + O 2

John Dalton

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All matter is made of indivisible particles called atoms.

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All atoms of a given element are identical in mass & properties.

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Atoms are not created or destroyed - just rearranged in reactions.

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Different atoms can combine in simple ratios to make compounds.

Atoms are all solid with uniform density Atoms, according to (like a gum ball) Dalton:

Cathode ray tube gaseous element running electricity through a *beam of light” in a was really made of negative particles!

Discovered the electron!

Realized that electrons were smaller than atoms. This information discredited part of Dalton’s atomic theory - Atoms were divisible after all. The atom is made of smaller particles (subatomic particles.)

Thomson’s Model of the Atom: Plum (electrons) Pudding (positive stuff) Today, it would probably be called the “Chocolate Chip Cookie” Model

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Ernest Rutherford 1911

Performed the famous gold foil experiment • Hypothosis: – If Thomson was correct and atoms contained only diffuse positive charge then… – Most of these heavy positive particles should go right through, right?

Experiment

Uranium Positive particles Gold foil Detection screen

Rutherford shoots alpha particles at a thin sheet of gold foil. He wants to see how tightly the protons and electrons are packed.

He used gold because it can be stretched thinnest of any metal – only a few hundred atoms thick!

He expects the alpha particles to be deflected a little by the gold atoms in the foil.

He expects the particles to be deflected a LITTLE if the particles are loosely packed, or a LOT if they are packed tightly.

This is not what happens.

Instead, 99% of the alpha particles go right through the gold as if nothing was there.

This meant that the atom was mostly empty space.

Alpha particles, like bullets, were going straight through the gold foil.

But once in a while, an alpha particle would NOT go straight through the gold.

It would deflect and bounce off at an angle.

This meant that there was something small but solid inside the atom.

The vast majority of alpha particles would go straight through, but . . .

. . .but occasionally one would bounce off, sometimes even straight back!

1% are deflected.

99% go straight through.

Rutherford realized that the only way both these things could happen was if all the heavy parts of the atom . . .

1% are deflected.

99% go straight through.

. . . were all concentrated in a small spot in the middle of the atom which he named the nucleus . . .

1% are deflected.

99% go straight through.

. . . while all the light weight electrons circled far away, leaving most of the atom completely empty.

1% are deflected.

99% go straight through.

Rutherford calculated that the nucleus was so small compared to the whole atom . . .

1% are deflected.

99% go straight through.

. . . that it was like a pea in a football stadium. But that pea would contain ALL the weight of the entire stadium.

1% are deflected.

99% go straight through.

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Dense, positively charged nucleus.

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Electrons surround the nucleus like bees around a hive.

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Atoms are mostly empty space.

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Problem: Why didn’t electrons get pulled into the nucleus?

Niels Bohr - 1913

• Used the Atomic Emission Spectrum to explain electrons in atoms.

What a handsome fellow!

Atomic Line Spectra

Expected to see a nice progression of color just like normal BUT…..

Gas such as Neon or Hydrogen Prism or diffraction grating

Atomic Line Spectra

Instead he got just a few discrete lines of color!

Gas such as Neon or Hydrogen Prism or diffraction grating

Nucleus

For Electrons

3  1 2  1 3  2

Conclusions:

• Electrons can only exist in certain orbits at a specific distance from the nucleus.

• Electrons could “jump” to higher orbits (absorbing energy). When they “fell” back down (released energy), they would give off light!

• If an electron stayed in its orbit, it didn’t lose any energy and would not fall into the nucleus.

Bohr’s Atomic Model:

• Compares electrons orbiting the nucleus of an atom to planets orbiting around the sun.

• Electrons occupy distinct orbitals.

• Orbitals have certain energies and distances from the nucleus.

• Bohr’s theory explains how light is given off – a huge success!

Erwin Schrodinger 1926

• Used mathematical equations to study electrons.

• Equation gives regions around the nucleus where electrons are likely to be found.

• Electrons are located in regions called electron clouds. These regions tell us where electrons are most likely to be.

Schrodinger’s Model of the Atom:

2 Parts of the Atom…..

• Nucleus • Electron Cloud Different types of orbitals have different shapes and different levels of energy.