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Unit 3 - Atomic Structure
Chapter 5
Addison/Wesley Textbook
Earliest Model
BC – Democritis, a
Greek philosopher, first
uses the word “atomos”
which means indivisible
 Definition of atom today
– Smallest particle of an
element that still retains
properties of that
element
 450
Late 1700’s
 Lavoisier
 Proust
– Law of Conservation of Matter
– Law of Constant Composition
This says that the same compound from
any source always contains the same
elements in the same proportion by mass
First Atomic Theory





John Dalton
Proposed in 1803
Compilation of other people’s
work and a little of his own
Still true except for one part
Good biography:
http://www.slcc.edu/schools/h
um_sci/physics/whatis/biograp
hy/dalton.html
Dalton’s Atomic Theory
 Each
element is composed of tiny,
indivisible atoms
 Each element’s atoms are the same and
unique
 Atoms are only rearranged in any chemical
reaction
 A compound has the same number and
kind of atom.
The Atom Today
 Since
1981 we have “seen” atoms with
a scanning tunneling microscope.
 Uses a fine tip and a stable
environment to trace the electronic field
and image it on a computer
 Lots of galleries on the Web:
 http://physics.nist.gov/GenInt/STM/fig1.
html
A Good Example
Discovery Atomic Structure
 Early
research comes from physicists’
work on electricity
 “Electricity” is property of “electron”, which
is amber
 In ancient Greece, pieces of amber were
rubbed and static electricity discharged
 Ben Franklin did early research in late
1700’s
Great Experimenter
His work
 Discovered
two kinds of charges, positive
and negative
 Opposite charges attract
 Like charges repel
 Objects pick up charges
 They discharge when touched to ground
Lightning
 New
research all the time
 Great photography – Check YouTube
 http://www.pbs.org/wgbh/nova/sciencenow
/3214/02.html
 http://www.teachersdomain.org/resource/p
hy03.sci.phys.mfw.lightning/
Electricity Research after Franklin
 Physicists
liked to zap things in the mid
1800’s
 Cathode ray tube was device used by
many (diagram)
 Same device used as TV screen
Cathode Ray Tube
How it Works
 Metal
is electrified in an evacuated tube
 All metals gave a greenish ray going to the
positive electrode
 Ray could be attracted by a positive
charge, repelled by a negative charge.
 It could actually make a paddle wheel
move - particle
Discovery of the Electron


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

JJ Thomson – Cavendish Lab - 1896
Used cathode ray tube to determine amount of
deflection
Determined that particle has a negative charge
Determined the charge to mass ratio of the
particle
Animation: http://highered.mcgrawhill.com/sites/0072512644/student_view0/chapte
r2/animations_center.html#
Finding the Charge of an Electron
physicist – Robert Millikan
 Famous Oil Drop Experiment (handout)
 See animation
 American
Explanation

Drops of oil are sprayed into a chamber

X-rays cause electrons to be formed

they cling to oil (in varying numbers)

Drops pass through a set of electric plates which have a charge

Millikan adjusted charge to balance the charge on each drop

Found the greatest common factor
Conclusion
on an electron is 1.60 X 10-19
Coulombs
 Mass of an electron is 9.11 X 10-19 grams
 Virtually without mass
 Charge
Discovery of Radiation
 Henri
Becquerel accidentally discovered
radiation in 1896
 Photographic plate wrapped and put in
drawer for weekend gets exposed
 Rock was “radiating” something
 Rock was pitchblende which contains
radium
Characteristics of radiation
 Spontaneously
emitted by some elements
 Studied by Marie and Pierre Curie
 They discovered several elements,
including uranium and polonium
 Atom emits radiation and then changes
 This gave clues to what atom is actually
made of
Marie and Pierre Curie

Good site
 http://www.aip.org/hist
ory/curie/
Further Research on Radiation
 Ernst
Rutherford is brought to Cavendish
Lab in early 1900’s
 Studied radioactivity
 Analyzed nature of radiation
 Handout
Magic Bullet
 Alpha
Particle chosen
 Right size
 Could be detected afterwards
 Helium nucleus – 2 protons and 2
neutrons
 +2 charge
Gold Foil Experiment

Rutherford got grad
students to design set up
 Geiger and Marsden
 Wanted to confirm
Thomson’s “Plum
Pudding” model of the
atom – electrons stuck in
positive pudding
 Handout
Explanation
 Find
a source of alpha particles
 Aim them at a piece of gold foil
 Check to see where they come out by
counting fluorescent spots
Results
 Most
went through
 Very small number were deflected almost
straight back
 Only explanation was that all matter was
concentrated into a dense nucleus
 Nucleus had a positive charge
 Electrons traveled in empty space around
the nucleus
 Movie: Empty Space
Results
 Most
went through
 Very small number were deflected almost
straight back
 Only explanation was that all matter was
concentrated into a dense nucleus
 Nucleus had a positive charge
 Electrons traveled in empty space around
the nucleus
 Movie: Empty Space – next slide
Atom is Empty Space
 From
NOVA
Modern Atomic Theory
There are 3 major subatomic particles
(protons, neutrons, and electrons).
There are basic particles that make these up
but we will not discuss them
The proton also came from the cathode ray
tube
The neutron was discovered by Chadwick, a
student of Rutherford in 1935.
Summary of Particles
PROTON
NEUTRON
ELECTRON
+1
NONE
-1
LARGE
LARGE
VERY SMALL
NUCLEUS
NUCLEUS
OUTSIDE
NUCLEUS
Planetary Model
 Proposed
by Rutherford
 Electrons orbit nucleus like planets around sun
 Atoms are neutral so
#protons = #electrons
 Charge on electron: 1.602 X 10-19 C or “1”
 Mass of proton: 1.67 X 10-24 g or 1 amu
(atomic mass unit)
Atomic Number
 Defined
by Henry Mosely (1887-1915)
 Student of Rutherford
 Unique for each element
 Number of protons in the nucleus
 What is atomic number of
nitrogen? Uranium?
Isotopes
 Means
“type or form”
 All atoms of the same element have the
same number of protons
 There may be different types of the same
elements, called isotopes
 Vary in number of neutrons, mass
 Try Carbon-12 and Carbon-14
Characteristics of Isotopes
 Varying
masses
 Same chemical and
physical properties
 Some may be
unstable, and
therefore radioactive
Symbol
 Carbon-12
 12
is mass number,
# protons + # neutrons
 Also written
12 C
6
 Mass
# - Atomic # = # of neutrons
Atomic Mass
 Mass
of an isotope in amu’s is simply the
Mass number
 Most elements have several common
isotopes
 Mass on periodic table must reflect this,
that is why there are decimals
 Weighted average calculation (like grades)
Calculation
 Multiply
the mass of each isotope by its
abundance as a decimal
 Add each of these to get weighted
average
 Try one
Mass Spectrometer
 Inject
gaseous form of element
 Strip electrons (positive charge)
 Sort by size with a magnetic field
 Computer counts the isotope and gives
a readout
Animation
 http://www.colby.edu/chemistry/OChem/D
EMOS/MassSpec.html