Title: The History of the Atom
1The History of the Atom
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2Table of Contents
- Black Boxes
- Democritus
- Daltons Theory
- Thomsons Electron
- Rutherfords Nucleus
- Goldsteins Protons and Chadwicks Neutron
- Review all three models
- Atomic Symbols
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3Black Boxes
Black Boxes are anything that you cannot see
inside
Like a cell phone Or a locked box Or an atom
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4The Beginning of the Atom
Find out about it here
5The ancient Greeks (back around 500BC) believed
there were only four elements Earth Air Water Fire
But
6Democritus
- 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 any further - He called these particles
- Atoms
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7The Atomic Theory
Learn about it here
8Democritus idea of the atom was largely ignored
until an English schoolteacher did some
experiments over 2000 years later, he was
John Dalton (1766-1844)
Click here to learn about his atomic theory
9Daltons Atomic Theory (1803)
- All matter is made up of tiny indivisible and
indestructible particles called atoms - All atoms of one element are exactly alike, but
they are different from atoms of other elements - Atoms of different elements can physically mix
together or can chemically combine into compounds - Chemical reactions occur when atoms are
separated, joined, or rearranged.
Click here to see his model
10Daltons Model of the Atom
He believed the atom was a solid sphere
An analogy to his atom would be
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11Thomsons Cathode Ray Tube Experiments and the
Discovery of the Electron
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12The Electron
- JJ Thomson (1856-1940) used the CATHODE RAY TUBE
to show that the atom was made up of electrons in
1897
Click here to listen to him talk about the
electron
Click here to see a cathode ray tube
13Go on
14- 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. - Click here to see what happened
15Go on
16- So, the magnet caused the cathode ray to move.
What does that tell us? - Would a magnet affect a light from a flashlight?
- Not at all!
- Therefore, the cathode ray must be a particle!
What else did he find
17Thomson also noticed
That the cathode ray was coming out of the
negative end (cathode) of the tube and going
toward the positive end (anode) Therefore,
because opposites attract, he concluded that the
cathode ray must be negative Click here to find
out what he called these
particles
Negative end
Positive end
Negative terminal
Positive terminal
18ELECTRONS!!
- This discovery would alter Daltons model of the
atom because now we know there is something
inside it - But, in addition to these negatively charged
electrons, there must be something giving it a
positive charge because the overall charge of the
atom is neutral (not negative) - Lets look at Thomsons model of the atom
Actually, George Stoney came up with that name
in 1874 but he never proved their existence
19Thomsons Plum Pudding Model of the Atom
He believed the atom was made of positively
charged stuff with negatively charged particles
scattered throughout
Negative electrons
Why the plum pudding model? What is plum
pudding? Click here to find out
Positive Charge
Positive charge spread across sphere
20Plum Pudding is an English dish sort of like
bread pudding with raisins in it.
An American analogy to his atom would be
Is like
Even though we could call it the chocolate chip
cookie model, well still refer to it as the
Plum Pudding model in class
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21- Rutherford and the Nucleus
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22Ernest 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.
Go on
23- He expected all of the alpha particles to go
straight through because - It would be like you were shooting bullets at a
cakeall of the bullets (or alpha particles)
would easily go straight through the cake (or
gold foil atoms)
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24Alpha particles going straight through
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25But, what he found was this
Alpha particles were scattered by the gold foil
Click here to go back to previous slide if you
want to see what he expected again
Go on
26Or
Go on
27Did 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 see it a different way Or, to go on
28To recap the Gold Foil Experiment
Go on
29Through the Gold Foil Experiment
- Rutherford proved that an atom was
- mostly empty space (because most particles went
straight through) - with a very small (only a few particles were
deflected), very dense (it was hard enough to
deflect the alpha particles), positively charged
nucleus in it.
why would it have to be positive?
30- Do you remember what opposites do?
- They attract, right?
- Since the alpha particles are positive, what
would a negatively charged nucleus cause? - The alpha particles would be attracted and be
stuck, rather than deflected out. - But, a positive nucleus would repel (deflect),
which is what happened
Lets look at Rutherfords model of the atom
31Rutherfords Nuclear Model of the Atom
He believed the atom was made of a tiny
positively charged nucleus with negatively
charged particles orbiting it
Another viewmore like you will have to draw it
32Or
If the atom were Paul Brown Stadium, the nucleus
would be a pea on the fifty-yard line
Click here for an analogy
33A Rutherford Atom Analogy
In which the peach pit is the nucleus and the
rest of the peach represents the electrons
buzzing around
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34Other 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) - Irene and Joliet Curie also are given credit for
finding the neutron
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35To Review
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36Atomic 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
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37Reading symbols
Any atom that has gained or lost an electron (or
more than one) will have a charge they are
called ions
Try some problems
38Try it
- Find the
- Atomic number
- Mass number
- Charge
- of protons
- of neutrons
- of electrons
Click here for answers
39The answers
- Find the
- Atomic number
- Mass number
- Charge
- of protons (p)
- of neutrons (n0)
- of electrons (e-)
9 19 -1 9 10 10
Try another problem
40How about this one
0
41
Ca
20
- Find the
- Atomic number
- Mass number
- Charge
- of p
- of n0
- of e-
Click here for answers
41The answers
0
41
Ca
20
- Find the
- Atomic number
- Mass number
- Charge
- of p
- of n0
- of e-
20 41 0 20 21 20
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