History of the Atomic Model
Seeing the Invisible
A Big Debate
Can matter be divided into
smaller and smaller pieces
forever?
YES!
NO!
Aristotle’s Claim
• The universe consists of the following
– Earth
– Water
– Fire
– Wind
– Void
Atomic Models: Greek
Democritus (460-370 B.C.)
 Matter can not be
divided forever
• Smallest piece = “atom”
(Greek “atomos” = “not to be
cut”)
• He had no way of knowing
what atoms looked like!
The word “atom” comes from a Greek
word that means “unable to be cut”
… and you kept
Imagine
you
on cutting
the
had
a piece
leftover
piece in
of
gold that
half…
you then cut
in half…
…and then you
cut one of these
smaller pieces in
half…
GoGold
ld
The word “atom” comes from a Greek
word that means “unable to be cut”
…and kept
…and
going…kept
going…
An atom of gold
Eventually you would have 1
keptcut it
piece of gold left.…and
If you
going… have
in half, you wouldn’t
gold any more – you’d have
something else. This tiny, tiny
single piece of gold is called
an atom of gold. An atom is
the smallest particle of an
element that acts like the
element.
Democritus
He hypothesized that atoms were:
– Small & Hard
– Diff in shape & size
– Infinite
– Always moving
– Capable of joining
Who Was Right?
• Greek society was slave based
• No actual experiments
– It was all a thought game
• Settled disagreements by verbal debate
• Aristotle was more famous so he usually
won
• His ideas carried through to the middle
ages. (2000 years later)…until the
Enlightenment
John Dalton (Late 1700’s)
•
•
•
School teacher in England
Based his conclusions on
experimentation and observations
(Law of Conservation).
Combined ideas of elements & matter
with that of atoms
Dalton’s Atomic Theory
1. All elements are composed of
submicroscopic indivisible parts called
atoms.
2. Atoms of the same element are identical,
those of different atoms are different.
3. Atoms of different elements combine in
whole number ratios to form compounds.
4. Chemical reactions involve the
rearrangement of atoms. No new atoms
are created or destroyed.
Dalton’s Theory
• Most of Dalton’s theory is accepted today.
• Except the part about atoms being
indivisible…
J.J. Thomson
and the Cathode Ray Tube
1897
English physicist
Provided the first evidence that atoms are made
of even smaller particles
Description of a cathode ray tube and a short video of
how it works:
http://www.chem.uiuc.edu/clcwebsite/cathode.html
Thomson’s Experiment
• How it works…
• Cathode/Anode in a vacuum.
+
Thomson’s Experiment
-
+
• Passing an electric current makes a beam appear to
move from the negative to the positive end.
Thompson’s Experiment
• By adding an electric or magnetic field…
Thompson’s Experiment
• By changing the electric field he found the
moving particles were negative
• This is still how all CRT TV’s work.
• Don’t try this at home! MAGNET LINK
Thompson’s Model
• Found the electron
• Concluded that there must be
a small positive charge
everywhere since atom was
neutral
• Atom was like plum pudding
– A bunch of positive stuff, with
electrons able to be removed (as
in his experiment, static, etc.)
Ernest Rutherford
• Former student of J.J. Thomson
– Believed in plum pudding model
1871-1937
• Wanted to find out how big they
(electrons) are
• Fired positively charged radiation
called alpha particles at a piece
of gold foil, which can be made a
few atoms thick
Rutherford’s Experiment
• When alpha particles hit a fluorescent
screen it will glow.
• Here’s what the setup looked like …
• What he expected to see: No change in the
path..
• This would have confirmed his atomic model
– Rutherford thought alpha particles should
pass through without a change in direction
– Positive charges in this model are spread
out evenly. Alone they should not be
enough to stop an alpha particle
Assumed
Alpha
Particle Path
• He was wrong… Rutherford animation
http://micro.magnet.fsu.edu/electromag/java/rutherford/
• How he explained it
– Atom is mostly empty
– Small dense, positive piece at the center:
he found the nucleus!
– Alpha particles are deflected if they get
close enough to positive center
Niels Bohr (1885-1862)
• Electrons have orbits about the
nucleus (planetary theory)
• Electrons could only exist at given
energy levels
• An energy level is where an electron
is likely to be moving
• Energy levels were like steps on a
ladder
– An electron can only be at any given
step at any given time
Modern Atomic Theory
Bohr Model—shows Quantum-mechanical model—
electrons in orbit
doesn’t show exact location
around protons and of electrons, just probable
neutrons
place
‘Actual’ Structure of the Atom
• There are two regions
– The nucleus
• Protons and neutrons
• Positive charge
• Almost all of the mass
– Electron cloud
• Most of the volume of an atom
• Region where electron can be
found
Subatomic particles
Counting the pieces
• Atomic number = number of protons
– Same as the number of electrons in a
neutral atom
• Mass number = the number of protons
+ neutrons
Atomic Mass Unit AMU
• Mass of a proton = 1.67 x 10 -27g
– A pretty inconvenient number tough to do
calculations with (even with scientific
notation!)
– New unit referenced to mass of an isotope
of carbon: carbon -12
– Carbon-12 has 6 protons and 6 neutrons
• Has a mass of 12.00000 amu – an atomic
mass unit
– Therefore 1 proton and 1 neutron has a
mass of 1 amu.
So why not whole numbers for
atomic masses in periodic table?
Isotopes!
• Reported numbers actually the
average atomic mass units for all
masses, This reflects the relative
abundance of isotopes for any given
element.
• In nature almost all elements occur as
a mixture of two or more isotopes
Isotopes
• Atoms of the same element can have
different numbers of neutrons
• Different mass numbers called isotopes
• For example H1 H2 H3 (all have one
proton and are hydrogen with only one
proton but are also called protium
deuterium, tritium)
• Hydrogen was responsible for the
explosion in Japans Nuclear Power
Plant (not a nuclear explosion!). Still
issues today