Studying Atoms 4.1Seeing the Invisible

The Big Debate

Can matter be divided into smaller and smaller pieces

forever?

YES!NO!

Aristotle Democritus

Atomic Models: GreekDemocritus (460-370 B.C.)

“Matter can not be divided forever”

• Smallest piece = “atom”

(Greek “atomos” = “uncuttable”)

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

• Strictly philosophy – no scientific evidence

• Most followed Aristotle

Gold Go ld

The word “atom” comes from a Greek word that means “uncuttable” or

“indivisible”

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…

…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. 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 & Solid

– Different in shape & size– Infinite

– Always moving– Capable of joining

4 Greek Elements 7:29

Time Goes By…

Dalton –1803• English chemist and school teacher• Studied gasses and how they exert

pressure in air.• Concluded they are made of tiny

particles• proposed his atomic theory

Dalton’s Atomic Theory:1. All elements composed of atoms

2. Atoms of the same element are exactly alike3. Compounds contain atoms of more than one

element4. In a given compound, atoms of different elements always combine in the same way(i.e. Calcium Chloride CaCl2 – always has 1 Calcium atom for every 2 Chlorine atoms)

Atomic Models: J.J. Thomson 1897

• Passed electricity through vacuum tube– Glowing beam attracted to positively charged plate – Beam was NEGATIVELY charged– Negative charges came from inside atom!

– Thomson discovered the ElectronElectron– Evidence: different metal plates produced same result– First experiment to prove atoms made of smaller

particles! JJ Thomson’s Cathode Ray 2:49

But wait…

•We know atoms are neutral, or all matter would be repelling

all other matter.•How can atoms be NEUTRAL if

they are full of negatively charged electrons?

Atomic Models: J.J. Thomson

• + charges must be present to balance charge of electron

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

Dalton’s ModelDalton’s ModelThomson’s ModelThomson’s Model

Atomic Models: Rutherford 1911

• Passed + Charged alpha particles thru gold foil– Most passed right

through– Some bounced off

at odd angles– Very few bounced

straight backRutherford’s Gold Foil Experiment 4:06

Rutherford’s Gold Foil Experiment

Rutherford’s Work

Rutherford’s Conclusion• the results showed that

something + was repelling +alpha particles

• More direct alpha particles hit mysterious + charge, greater

deflection• + charge concentrated in small

central area - Rutherford called nucleus

• Atom is mostly empty space!The atom 1:15

Ok, so here’s what we know so far…..

•The atom is made up of + particles in nucleus, and

negative electrons.•Where are these electrons

found in the atom?

The Atom Song 3:28

Atomic Models: Bohr

• Electrons are found in specific energy levels– Like planets

around the sun

Atomic Models: Wave

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

• Move in all directions around nucleus

Atoms song - Mark Rosengarten