The History of the Atom

1
The History of the Atom
2
Table of Contents

• Black Boxes
• Democritus
• Daltons Theory
• Thomsons Electron
• Rutherfords Nucleus
• Goldsteins Protons
• Chadwicks Neutron
• Atomic Symbols

Back to Table of Contents
3
Black Boxes
Black Boxes are anything that you cannot see
inside
Like a cell phone Or a locked box Or an atom
4
The Beginning of the Atom
5
The ancient Greeks (back around 500BC) believed
there only four elements Earth Air Water Fire
6
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

7
The Atomic Theory
8
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
9
Daltons 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.

10
Daltons Model of the Atom
He believed the atom was a solid sphere
An analogy to his atom would be
11
Thomsons Cathode Ray Tube Experiments and the
Discovery of the Electron
12
The Electron

• JJ Thomson (1856-1940) used the cathode ray tube
  to prove that the atom was made up of electrons

Click here to listen to him talk about it
13
(No Transcript)
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.

15
(No Transcript)
16

• 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!

17
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) Therefore,
because opposites attract, he concluded that the
cathode ray must be negative He called these
particles
Negative end
Positive end
Negative terminal
Positive terminal
18
ELECTRONS!

• The discovery would alter Daltons 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)
• Lets look at Thomsons model of the atom

19
Thomsons 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?
20
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 well still refer to it as the Plum Pudding
model in class
21

• Rutherford and the Nucleus

22
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.
23

• He expected all of the alpha particles to go
  straight through
• It would be like if you were shooting bullets at
  a cakeall of the bullets (or alpha particles)
  would go straight through the cake (or gold foil
  atoms)

24
Gold foil
Detector screen
Alpha particles source
Alpha particles
Thomsons Atom
Alpha particles
25
But, what he found was this
26
Or
27
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
28
To recap the Gold Foil Experiment
29

• Rutherford proved that an atom was
• mostly empty space
• with a very small, very dense, positively charged
  nucleus in it.

30
He believed the atom was made of a tiny
positively charged nucleus with negatively
charged particles orbiting it
Rutherfords Nuclear Model of the Atom
31
Or
If the atom were Paul Brown Stadium, the nucleus
would be a pea on the fifty-yard line
32
A Rutherford Atom Analogy
In which the peach pit is the nucleus and the
rest of the peach represents the electrons
buzzing around
33
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)

34
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

35
Reading symbols
36
Try it

• Find the
• Atomic number
• Mass number
• Charge
• of protons
• of neutrons
• of electrons

37
The answers

• Find the
• Atomic number
• Mass number
• Charge
• of protons (p)
• of neutrons (n0)
• of electrons (e-)

9 19 -1 9 10 10
38
How about this one
0
41
Ca
20

• Find the
• Atomic number
• Mass number
• Charge
• of protons (p)
• of neutrons (n0)
• of electrons (e-)

39
The answers
0
41
Ca
20

• Find the
• Atomic number
• Mass number
• Charge
• of protons (p)
• of neutrons (n0)
• of electrons (e-)

20 41 0 20 21 20