TIMELINE OF THE MODELS*
?
460 BC
Democritus
“Atoms”
1800
John Dalton
“Billiard Ball”
1897
JJ
Thompson
“Plum
Pudding”
1911
Rutherford
“Planetary”
1912
Niels Bohr
“Bohr Model”
Today
Many
scientists
“Modern”
*There are many more models. These are the ones we’ll
cover in class.
THE ANCIENTS – B.C.
• Aristotle believed
everything was made
up of the fundamental
“elements”
•
•
•
•
Earth
Wind (air)
Fire
Water
DEMOCRITUS
• Indivisible = atomos
• atoms were distinct in size
and shape
• each element was therefore
unique
• substances of the world
were made from
combinations of different
atoms
EARLY SCIENTISTS
• The alchemists-those
wishing to turn lead into
gold – where some of the
first scientists. They
documented their
experiments.
THE LAW OF CONSERVATION OF MASS BY
ANTOINE LAVOISIER
Try the following problem to see if you understand this
law.
Suppose you have 15 grams of substance A and 13 grams
of substance B. How much substance C will be formed if
you also produce 9 grams of substance D as shown
below?
A
+
15 g
B
13g
Reactants
C
?g
+
D
9g
Products
JOHN DALTON --1800
(A.K.A “THE MARBLE GUY”)
• All matter is made of tiny
indestructible particles called
atoms.
• All atoms of the same element
are identical
• Atoms of different elements
combine to form compounds in
An atom
fixed ratios.
(Close-up view)
• Chemical reactions are
rearrangements of atoms.
Which of these statements are still considered correct?
JJ THOMSON (1897)
• Was used a Crooke’s
vacuum tube
• Found that running
electricity through a
gaseous element
produced a stream.
OBSERVATION
CONCLUSIONS
The beam can push a
paddle wheel so
Beam is made of
particles
The mass of the tube did
not change
particles came from
inside the atom, so
the atom is not
indivisible
The stream moved from
– to + so
Atoms are neutral, so,
atoms must also contain
charges that are...
Has a negative
charge
Positive
PLUM PUDDING MODEL
• Thomson figured out
that atoms contained
positive and negative
charges but had no
idea how they were
arranged.
• He guessed there was a
diffuse positive cloud
with negative charges
randomly distributed
-
Diffuse
positive
cloud
-
-
ERNEST RUTHERFORD (1910)
• Performed the
famous gold foil
experiment
• Won a Noble Prize
for Chemistry
GOLD FOIL EXPERIMENT
• Rutherford used
Radium as the source
of the alpha particles
and shot them at a
thin gold foil like
aluminum foil but
made of gold
• A fluorescent screen
sat behind the gold
foil on which he
could observe
the alpha particles’
impact.
GOLD FOIL EXPERIMENT
• When the particles
bounced back Rutherford
reasoned that it hit
something massive and
positive. This mass
became know as the
nucleus.
• When the alpha particles
went straight through it hit
nothing.
• This happened most often
so the atom is mostly
empty space
RUTHERFORD’S “PLANETARY MODEL
Rutherford’s said the negative
electrons orbited a positive
center (NUCLEUS)like our planets
orbit the sun.
• The nucleus has most of the
mass of the atom.
• The distance between the
positive center (nucleus) and the
electrons was huge-like a
marble in the center of a football
field.
• The atom was mostly empty
space!!!!
PLANETARY THEORY
• Nucleus of atom contains @99.9% of the mass of the
atom
• Nucleus is positively charged
• Electrons orbit the nucleus randomly and take up
@99.9% of the space
• Electrons are negatively charged
JAMES
CHADWICK
1932
DISCOVERED
THE NEUTRON
neutron
•Predicted by Rutherford in1920, but very
difficult to detect.
•New tool for probing and changing the
nucleus of an atom.
ONE LITTLE PROBLEM…
• Opposite charges attract
each other and the electrons
should gradually lose energy
and spiral inward toward the nucleus.
(BOOM! No more atom.)
SHREK MODEL
• Neils Bohr (1922) proved that electrons have
energy and don’t orbit randomly around
the nucleus
• They circle in well-defined energy levels like
the layers in an onion.
• Jumping to and from energy levels absorbs
or releases energy
BOHR’S MODEL
31
21
Nucleus
32
ERWIN SCHRODINGER
• Regions where electrons are most likely
found are called “electron clouds”.
• Different types of clouds have different
shapes and different amounts of energy.
• Results were spectacular!
• New model of the atom explained the
physical properties of atoms, and the
Periodic Table, perfectly. ( more later)
HEISENBERG'S UNCERTAINTY
PRINCIPLE
• The world of the atom is so small that just by us
looking for sub atomic particles changes their
location.
• Thus we can’t be sure certain about the size and
shape of energy levels but now….
1980’S
Atoms can be
“seen” with a
scanning
tunneling
microscope.
SCANNING TUNNELING MICROSCOPE
Fermi lab scientists in Batavia, Illinois study
and detect six types of quarks
Quarks are particles believed to make up
protons and neutrons
Cern Lab in Switzerland may have found
Higgs-Boson
More on this in your Physics class.