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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 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