Download AtomicHistoryStructureCPFDONE

The Evolution of
the Atomic
Model
Democritus to Rutherford
Exploring the Black Box
 Imagine
you’ve received a present
that you are not allowed to open. You
REALLY want to know what’s inside!
 What might you do to figure out the
contents?
Exploring the Black Box
 In
your group, try to figure out the
contents and internal structure your
present WITHOUT opening it

Note: Do NOT shake the box too violently
as the contents may fall out!
 As
a group, be sure to answer the
questions on the hand-out!
So What is a Black Box?
A
black box refers to any system
that has inner workings that cannot
be directly observed because it is
not possible or feasible
 Can
you think of any other black box
examples?
 An
atom is an example of a black box!
The Atomic Model and the
Nature of Science
When
scientists wanted to
determine the structure of an
atom, they were not able to
actually see inside of the atom
 They
had to rely on empirical
evidence, indirect evidence,
observation, and inference when
developing their ideas-just as you did
when determining the contents of
your present!
The Atomic Model and the Nature
of Science
 Scientists
developed a model to express
their new ideas
 The understanding of the structure of the
atom has changed many times over the
course of centuries
As new evidence emerged, the ideas of
the atomic model changed
 Lecturing about the scientists of the
atomic model isn’t quite as fun and
interesting as understanding:


HOW was the atomic model developed?
WHY has the atomic model changed over the
years?
What is Today’s Atomic
Model?
Dense, Positively
Charged
Nucleus
Mostly
Empty
Space
Composed of Protons,
Neutrons, and Electrons
Negatively
Charged
Electron Cloud
Most Probable
Location of the
Electrons
Timeline of Development of Current
Atomic Model
Discovery
of the
Neutron
Discovery
of the
Proton
450 BC
Democritus
proposed
the idea of
atomos.
1802
Beginning
of
Modern
Atomic
Theory
1897
1911 1913 1930
Discovery
of the
Nucleus
Discovery
of the
Electron
Introduction
of the wave
mechanical
model
The Idea
of Energy
Levels for
Electrons
was
Proposed
Democritus

The Greek philosopher
Democritus began the search
for a description of matter
more than 2400 years ago
 He
described the material
world as made up of tiny,
indivisible, hard particles
called atoms

“Atomos” – not to be cut
 All
atoms were made of the
same material but were
different shapes and sizes
400 BC
Democritus vs. The Philosophers

The eminent
philosophers of the
time, Aristotle and
Plato, formulated the
notion that there can
be no ultimately
“atomic” view of
matter


This dominated
Western ideas for
many centuries
Aristotle and Plato
favored the earth, fire,
air and water
approach to the
nature of matter
Then Along Came Dalton’s
 The notion of an atomic theory of
Model
matter reemerged during the 17th
century as a result of the work of
John Dalton
 The English chemist performed a
number of experiments that
eventually led to the acceptance
of the idea of atoms
 His theory is comprised of 4
postulates and is commonly
referred to as the billiard ball
model
Dalton’s Postulate #1
Each
element is composed
of extremely small particles
called atoms
Dalton’s Postulate #2
All
atoms of a given element
are identical
H
H
Dalton’s Postulate #2
But,
the atoms of one element
are different from the atoms of
all other elements
O
H
Dalton’s Postulate #3
Atoms of one element
cannot be changed into
atoms of a different
element by chemical
reactions
In other words, atoms are
neither created nor
destroyed in chemical
reactions
They are indestructible!
Dalton’s Postulate #4
Compounds are formed when
atoms of more than one element
combine
A given compound always has the
same relative number and kinds of
atoms
The Discovery of Atomic
Structure
 The
ancient Greeks were the first to
postulate that matter was indivisible
 Even Dalton did not have any direct
evidence for the existence of atoms!
 Later, scientists realized that the atom
consisted of subatomic, charged particles
 What landmark discoveries led to this
conclusion?
Thomson’s Atomic Model
In
1897, the work
of an English
scientist J. J.
Thomson
provided the first
hint that atoms
were made up
of even smaller
particles
Thomson’s Experiment
Thomson
studied electrical discharge
through a glass tube pumped almost
empty of neutral air
 When
a high voltage was applied to the
electrodes, negatively-charged radiation,
called cathode rays, would travel from the
negative electrode to the positive
electrode
 The path of the cathode rays could be
altered by the presence of a magnetic
field
Cathode Ray Tube
Cathode-Ray Tubes – Ever Seen
One?
http://www.howstuffworks.com/tv4.htm
21
Thomson Model
 Thomson
observed
that cathode rays
behaved identically
regardless of the
identity of the metal
used as cathode
material
 He described cathode
rays as streams of
negatively-charged
particles which came
from within the atom
Thomson’s Conclusions
• “We have, in the cathode
rays, matter in a new
state...a state in which all
matter...is of one and the
same kind; this matter
being the substance from
which all the chemical
elements are built up."


“I can see no escape from the conclusion that
[cathode rays] are charges of electricity carried by
particles of matter.”
but...
“What are these particles? Are they atoms, or
molecules, or matter in a still finer state of subdivision?
- J. J. Thomson
23
• Cathode rays
(electrons) are tiny,
negatively charged
“corpuscles”
• Since atoms are
neutral, Thomson
inferred that these
electrons must be
surrounded by a
“cloud” of positive
charge
J J Thomson’s Atomic Model
• Thomson believed that the electrons were like plums
embedded in a positively charged “pudding,” thus it was
called the “plum pudding” model
• Never had plum pudding? Think of a blueberry muffin
If Electrons Exist, How Big are They?
 Thomson
calculated the mass-to-charge ratio
for cathode ray particles: it was over 1000 times
smaller than for a charged hydrogen atom
 This
fact suggested:
- Either cathode rays carried a huge charge,
- or they had very small mass
 Robert
Millikan measured the charge of a
cathode ray particle in 1910
 From that he could calculate the mass: ~1800
times lighter than a hydrogen atom
26
More Pieces of the Atom
Ernest Rutherford
(1871-1937)
 Nuclear
physicist from
New Zealand
 Thomson’s student
 Taught at Cambridge
University in Great
Britain
27
Ernest Rutherford’s Atomic Model (18711937)
Where exactly are those electrons?
Thomson’s Theory: “Plum Pudding”
 Electrons embedded in a positive pudding
Rutherford’s idea:
 Shoot something at them to see where they are!
Rutherford Has an Idea…
What if I shoot alpha radiation
at gold atoms in gold foil?
Rutherford’s Experiments (1910-11)
(Done by undergrad Ernest Marsden/physicist Hans Geiger)
 Fired
beam of positively-charged alpha
particles at very thin gold foil
 Alpha particles caused flashes of light when
they hit the zinc sulfide screen
30
Rutherford’s Prediction Based
on Thomson’s Model
 The
alpha particles to pass through
without changing direction very much
 This is because the positive charges were
spread out evenly

Alone, they were not enough to stop the
alpha particles
What Rutherford Expected
He thought the
mass was evenly
distributed in the
atom
The Results Rutherford Actually
Got…
Let’s See for Ourselves!
+
Rutherford’s Conclusion about the Atom
 Atom
is mostly empty
space
 It has a small, dense,
positive center called
the nucleus
 Alpha particles are
deflected by the
nucleus if they get
close enough
+
But Wait – There’s More!
James Chadwick
(1932)
Discovered a neutral
(uncharged) particle in the
nucleus. Called it the
“neutron”
Atom “split” by John Cockcroft
and Ernest Walton, using a
particle accelerator, in late
1932
37
38
Properties of Subatomic Particles
Electron
9.110 x 10-28
Relative
Charge
-1
Proton
1.673 x 10-24
+1
Neutron
1.675 x 10-24
0
Property
Mass (g)
Particle
Location
in Atom
Outside
nucleus
Inside
nucleus
Inside
nucleus
The Atomic Model Evolves…

Rutherford’s model became
known as the “planetary model”

The “sun” was the positivelycharged dense nucleus and the
negatively-charged
electrons
were the “planets”
The Planetary Model is
Doomed!
 By
all the rules of classical physics, an electron in
orbit around an atomic nucleus would be very
unstable


This is because the orbiting electrons would give off
energy as it is continually accelerating in a curved
path
Like a satellite orbiting Earth
 Resulting
loss of energy implies that the electron
would eventually crash into the nucleus and the
atom would collapse!
Why Doesn’t the Model Work?
WAIT…What?!
Electron crashes into the nucleus!?
Since this does not happen, the Rutherford
model could not be accepted!
Along Comes Neils Bohr…

Bohr worked with Rutherford
and made the assumption
that the charged electron
simply does not fall into the
nucleus

He stated that the nucleus is
at the center of the atom

Electrons can only exist at
specific energy levels called
orbits

Each new ‘orbit’ is a new layer
in the atom

This was the only way
electrons could exist for a long
time without giving off energy
Shortcomings of the Bohr
Model

Bohr’s model was too simple



Worked well with only hydrogen because H only has
one electron
Bohr also avoided the problem of why the
negatively-charged electron would not just fall
into the positively charged nucleus, by simply
assuming it does not happen
Furthermore, there is a problem with describing an
electron merely as a small particle moving in
circular orbits around the nucleus
So there is more to the atomic puzzle…
The Position of Electrons Can’t be
Predicted Precisely
Werner Heisenberg (1927)
The “Uncertainty Principle”
• There’s an upper limit to how precisely an electron’s
position and momentum can be known
• The more precisely one is known, the less precisely
the other can be known
45
Electrons Move in “Probability
Clouds”, not Circular Orbits
 The
exact path of
an electron can’t
be predicted
 If
we know the
electron is
somewhere in the
atom, it’s velocity
is uncertain by
~7,300 km/s (~ 16
million mph)!
46
Introducing the Quantum
Mechanical Model!

Modern atomic theory describes the
electronic structure of the atom as the
probability of finding electrons within certain
regions of space (orbitals)

NOT CIRCULAR ORBITS
In Other Words…
Newtonian Certainty Cannot be Obtained
in the Subatomic World
“I cannot believe that God
plays dice with the
universe.”
“Albert, stop telling God
what to do.”
48