History of the Atomic Model

Seeing the Invisible

A Big Debate

Can matter be divided into smaller and smaller pieces

forever?

YES!

NO!

Atomic Models: Greek

Democritus (460-370 B.C.) Matter can not be

divided forever

• Smallest piece = “atom”(Gk “atomos” = “not to be cut”)

• He had no way of knowing what atoms looked like!

Gold Go ld

The word “atom” comes from a Greek word that means “unable to be cut”

Imagine you had a piece of gold that you then cut in half…

…and then you cut one of these smaller pieces in half…

… and you kept on cutting the leftover piece in half…

The word “atom” comes from a Greek word that means “unable to be cut”

…and kept going… …and kept

going…

…and kept going…

Eventually you would have 1 piece of gold left. If you cut it in half, you wouldn’t

have gold any more – you’d have something

else. This tiny, tiny single piece of gold is called an atom of gold. An atom is the smallest particle of an element that acts like the

element.

An atom of gold

Democritus

He hypothesized that atoms were:– Small & Hard

– Different in shape & size– Infinite

– Always moving– Capable of joining

Time Goes By…

• 1600s-1700s: Key experiments occur which support Democritus’s ideas.

• Robert Boyle (1627-1691)

• Antoine Lavoisier (1743-1794)

Dalton (1766-1844)

• English chemist and school teacher

• Did many experiments, studying gasses and proposed atomic theory

Atomic Models: Dalton

1. Elements composed of atoms; atoms are indestructible

2. Atoms of the same element are exactly alike

3. Atoms of different elements are different

4. Compounds formed by joining 2 atoms

Atomic Models: J.J. Thompson

• Passed electricity through an uncharged gas– The gas gave off rays to

show it was NEGATIVELY charged

– How?– Negative charges must

come from inside the atom!

=> Electrons!

But wait…

• How can an atom be NEUTRAL if it is full of

negatively charged particles (electrons?)

Atomic Models: J.J. Thompson

• The atoms are neutral… How?

• + charges must be present to balance - charges

• + & - lumped in a cluster he said looked like “plum pudding”

Thompson’s Model

Atomic Models: Rutherford

• Passed + Charged Particles through gold foil– Most passed right through

• Atom is mostly empty space– Some bounced off at odd angles

• Nucleus must be +• Calculate size of nucleus

Rutherford’s Gold Foil Experiment

Rutherford’s Work

Rutherford’s Model

Okay…

• So the atom is made up of positive and negative

particles.

• Where are the electrons found in the atom?

Atomic Models: Bohr

• Electrons are found in specific energy levels– Like planets

around the sun

Bohr’s Model

Atomic Models: Wave

• Electrons move so fast that it is impossible to determine their location

• Move in all directions around the nucleus