Transcript 4.1 History of Atomic Model - Collinsville Public Schools

Studying Atoms 4.1
Seeing the Invisible
The Big Debate
Can matter be divided into
smaller and smaller pieces
forever?
Aristotle
Democritus
YES!
NO!
Atomic Models: Greek
Democritus (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
The word “atom” comes from a Greek
word that means “uncuttable” or
“indivisible”
… and you
Imagine
kept on cutting
you
had a
the leftover
piece
piece of
in half…
gold that
you then
cut in
half…
Go
ld
Gold
…and then you
cut one of
these smaller
pieces in half…
…and kept
…and
kept
going…
going…
An atom of gold
Eventually you would have
1 piece of gold left. If you
…and
kept
cut it in half, you
wouldn’t
have gold anygoing…
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.
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 alike
3. Compounds contain atoms of more than one
element
4. 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 Electron
– 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
Dalton’s Model
Thomson’s Model
• + charges must be present to
balance charge of electron
• + & - lumped in a cluster he said
looked like “plum pudding”
Atomic Models: Rutherford 1911
Rutherford’s Gold Foil Experiment 4:06
• Passed + Charged
alpha particles thru
gold foil
– Most passed right
through
– Some bounced off
at odd angles
– Very few bounced
straight back
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