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Transcript
The History of the
Atom
Table of Contents
Black Boxes
Democritus
Dalton’s Theory
Thomson’s Electron
Rutherford’s Nucleus
Goldstein’s Protons
Chadwick’s Neutron
Atomic Symbols
Back to Table
of Contents
Black Boxes
Black Boxes are
anything that you
cannot see inside
Like a cell phone…
Or a locked box…
Or an atom…
The Beginning
of the Atom
The ancient Greeks (back around
500BC) believed there only four
elements
Earth
Air
Water
Fire
Democritus
• He also lived in ancient Greece but he
believed differently
• He said that all matter was made up of
particles that could not be divided into
smaller particles
• He called these particles –
– Atoms
The Atomic
Theory
Democritus’ idea of the atom was largely
ignored until an English schoolteacher did
some experiments over 2000 years later,
he was…
John Dalton
(1766-1804)
Leading to his atomic theory…
Dalton’s Atomic Theory
• All matter is made up of atoms
• Atoms are indestructible and cannot be divided
into smaller particles (Atoms are indivisible)
• All atoms of one element are exactly alike, but
they are different from atoms of other elements
• A given compound always has the same relative
numbers and kinds of atoms. Atoms join in
whole number ratios.
• Atoms are neither created nor destroyed in any
chemical reaction.
Dalton’s Model of the Atom
He believed the atom was a solid sphere
An analogy to his atom would be…
Thomson’s CaThode Ray Tube
Experiments and the
Discovery of the Electron
The Electron
JJ Thomson (18561940) used the
cathode ray tube to
prove that the atom
was made up of
electrons
Click here to listen
to him talk about it
•
•
•
•
But, what was that green light?
Was it a light?
Was it a particle?
To test this, he brought a magnet close to
the cathode ray tube to see what would
happen.
• So, the magnet caused the cathode ray to
move. What does that tell us?
• Would a magnet affect a light from a
flashlight? (you could try this at home)
• Probably not.
• Therefore, the cathode ray must be a
particle!
Thomson also noticed…
That the cathode ray was coming out of the negative
end (cathode) of the tube and going toward the
positive end (anode)
Negative
Negative
end
terminal
Positive
Positive
terminal
end
Therefore, because opposites attract, he concluded
that the cathode ray must be negative
He called these particles…
ELECTRONS!
The discovery would alter Dalton’s model of the
atom because now there is something inside it
But, in addition to the negatively charged
electrons, there must be something giving it a
positive charge because the overall charge of
the atom is neutral (not negative)
Let’s look at Thomson’s model of the atom…
Thomson’s Plum Pudding
Model of the Atom
He believed the atom was made of
positively charged stuff with negatively
charged particles scattered throughout
Why the “plum
pudding” model?
What is “plum
pudding”?
Plum Pudding is an English dish sort of like bread
pudding with raisins in it.
An American analogy to his atom
would be…
…Is like…
But we’ll still refer to it as the “Plum Pudding” model in class
Rutherford and the
Nucleus
Ernest Rutherford
In 1908, Rutherford performed
the Gold Foil Experiment.
(1871-1937)
In it, he shot alpha particles (very small,
very dense, very fast particles) at a thin
layer of gold foil.
• He expected all of the alpha particles to go
straight through
• It would be like if you were shooting bullets
at a cake…all of the bullets (or alpha
particles) would go straight through the
cake (or gold foil atoms)
Gold foil
Detector
screen
Alpha
particles
source
Alpha
particles
Thomson’s
Atom
Alpha
particles
But, what he found was this…
Or…
Did you notice how most of the alpha
particles went straight through but a very
tiny amount were deflected at odd
angles?
That could only happen if there was
something very tiny in the atom that was
dense enough to deflect the alpha
particles.
Like this…
To recap the Gold Foil Experiment
Rutherford proved that an atom was
mostly empty space
with a very small, very dense, positively
charged nucleus in it.
He believed the atom was made of a tiny
positively charged nucleus with
negatively charged particles orbiting it
Rutherford’s Nuclear Model
of the Atom
Or…
If the atom were Paul Brown
Stadium, the nucleus would be a
pea on the fifty-yard line
A Rutherford Atom Analogy
In which the peach pit is the nucleus
and the rest of the peach represents
the electrons buzzing around
Other important discoveries…
In 1886, Goldstein discovered the proton
(which is located in the nucleus)
In 1932, Chadwick discovered the neutron
(also located in the nucleus)
Atomic Symbols
• Now, we can determine the number of
each of these particles if we know the
atomic number and mass number.
• Atomic Number = # of protons
• Mass Number = # of protons + # of neutrons
• Charge = # of protons - # of electrons
Reading symbols
Symbols
Contain the symbol of the element, the
mass number and the atomic number
Mass
number
Atomic
number
X
Charge
Try it…
19
9
F
-1
Find the…
 Atomic number
 Mass number
 Charge
 # of protons
 # of neutrons
 # of electrons
The answers…
19
9
F
-1
Find the…
 Atomic number
= 9
 Mass number
= 19
 Charge
= -1
 # of protons (p+)
= 9
 # of neutrons (n0) = 10
 # of electrons (e-) = 10
How about this one…
41
20
Ca
0
Find the…
 Atomic number
 Mass number
 Charge
# of protons (p+)
# of neutrons (n0)
# of electrons (e-)
The answers…
41
20
Ca
0
Find the…
 Atomic number
= 20
 Mass number
= 41
 Charge
= 0
 # of protons (p+)
= 20
 # of neutrons (n0) = 21
 # of electrons (e-) = 20