Chapter 4: Atomic
Structure
History of the Atom
-Atomic History song
Size of Atom Video
 Atoms and elements

400 B.C.- Democritus
Believed matter was made of empty
space and tiny particles he called
“atoms”
 He said that different things were
made of diff. types of atoms
 Leuccippus & Epicurus agreed w/
him
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335 B.C. - Aristotle
Said matter was continuous, what
you see is what you get
 Called it “hyle”
 Identified 4 elements: earth, wind,
fire, water

Late 1500’s - Galileo
1st person to make extensive use of
the experimental method to study
natural phenomena
 1st to disagree w/ Aristotle
 Agreed w/ Copernicus (sun center of
universe)

Late 1600’s – Sir Issac
Newton
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Brilliant English scientist
In his 23-24th year he was stuck at home
during the Bubonic plague
During this time he:
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Invented calculus
Did 100’s studies on light
Developed laws of motion
Developed laws of gravity
Atom idea resurfaced- said there were
small particles (offered no proof)
1789- Antoine Lavoisier

French scientist who developed the
Law of Conservation of Mass
1799- Joseph Proust

French scientist who developed the
Law of Definite Proportions
1803- John Dalton
English scientist
 Published series of 3 papers, based
on the findings of Proust and
Lavoisier
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1.
Dalton’s Atomic Theory
A. All matter is composed of small
particles called atoms that can’t be
broken down
B. Atoms of the same element have
identical properties (same mass,etc)
C. Atoms of different elements have
different properties
D. Atoms unite in simple ratios to form
compounds
2.
3.
Developed Law of Multiple
Proportions
Dalton’s Model of the Atom

Used solid wooden balls to show
matter was made of indivisible
particles
1876- Eugen Goldstein
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German scientist who introduced the
name “cathode ray” and its positive
counterpart “canal rays”
1895 – Wilhelm Roentgen
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German scientist who discovered Xrays
1896 – Antoine Henri
Becquerel
French scientist who discovered
radioactivity
 Radioactivity: the phenomenon in
which the unstable nuclei of certain
elements spontaneously emit
particles or energies

Forms of Radiation
1.
Alpha particle ( ) [Rutherford]
-helium nucleus (2 neutrons & 2
protons)
- can be stopped by sheet of paper
2. Beta Particle (ß) [Rutherford]
- high speed electron formed at the
moment of decay
-can be stopped by sheet of
aluminum or layers of clothing
3.
Gamma Rays ( ) [Villard]
-high energy x-ray
-stopped by few feet of concrete or few
inches of lead
1897- J.J. Thomson
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Did research on cathode rays, built a CRT
(cathode ray tube), now found in TV’s and
computer screens (originally built by
William Crookes)
CRT is a glass tube filled w/ a gas and it
has 2 electrodes (anode=positive
electrode, cathode=negative electrode)
Passed a voltage between the
electrodes, saw a light beam called a
cathode ray
 Subjected the beam to electric and
magnetic field, found the beam to
be negatively charged
 Called those negative particles electrons

Thomson’s Model of the
Atom
Called the “plum pudding model”
 Disproves Dalton’s theory (atom
can’t be broken down)
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1911- Robert Millikan
American physicist
 Did the “oil drop experiment”
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In this experiment he had oil drops that
were charged pass thru charged plates,
he offset the voltage on the plates to get
the drop to suspend briefly, wrote down
the voltage
He found the exact charge on an
electron(e-) to be (-1)
Also using Thomson’s work he found the
mass of the e- to be 1/1897th of a
Hydrogen atom
1911-1912 –Lord Ernest
Rutherford
New Zealand physicist
 Worked w/ Bohr, Geiger and
Marsden on the “Gold Foil
Experiment”
 Credited w/ the discovery of the
atomic nucleus
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Gold Foil Experiment
Set-Up
Gold Foil Movie
Gold Foil Experiment
Set-Up
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Lead box w/ pin hole
In box- piece of radioactive material
which gave off alpha particles (positive
particles)
Used a very thin sheet of gold foil as a
target (few atoms thick)
Surrounded the foil w/ a screen coated in
zinc sulfide that sparked when alpha
particles hit it
Observations
Observations
A.
B.
C.
99% of particles went straight thru
the foil like it wasn’t there
.9% went thru foil but passed thru
at a slight angle
.1 % hit the gold foil like a brick
wall & bounced right back
Conclusions
A.
B.
C.
Atoms made of mostly empty
space
Some sort of (+) charge is
concentrated in the nucleus to
repel the alpha particles (must
have a charge of (+1) to offset
the electron-because atoms are
neutral)
All the mass of the atom is
concentrated in one place
Rutherford’s Model of the
Atom
Atoms consist of a small, dense,
positively charged center (the
nucleus)-surrounded by empty
space
 Electrons travel in that empty space
orbiting around the positive nucleus
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1914- Discovery of Proton
Thomson predicted the existence of
a positive particle w/ CRT
experiments
 Rutherford coined term “proton” in
1919 (he is credited w/ the
discovery w/ Gold Foil experiment)
 Found positive particle, that is larger
and the exact opposite charge of the
electron, 1800 times heavier than
the electron
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1914- Atomic Number
Henry Mosley
 Studied x-rays produced by different
metals (used the metals as anodes)
 Saw that each metal produced a
different wavelength- because each
had a different number of protons

Atomic Number = # of protons
# protons = # electrons
# of protons determines the identity
of the element
1922- Bohr’s Model of the
Atom
Niels Bohr worked on gold foil
 Had electrons in specific orbits or
shells
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1932 – Discovery of the
Neutron
James Chadwick found high energy
particles w/ no charge and a mass
equal to a proton
 Call neutrons

Nucleons
Particles in nucleus
 Also known as hadrons
 Consist of protons and neutrons
 Each proton is made of 3 quarks (2
ups and 1 down), each neutron is
made of 3 quarks (2 downs and 1
up)
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Proton
Electron
Neutron
Mass
1
1/1900th
1
Charge
+1
-1
0
(compared
to H)
Mass Number
=# of protons + # of neutrons
# of neutrons = mass # - atomic #
Ex. Carbon
Mass #= 12
Atomic # = 6
# protons =# e- = 6
# of neutrons = 6
Atomic notation
Mass
#
222Rn
86
Atomic#
(#p=#e-)
#protons =
86
# electrons =
86
#neutrons =
136
Isotopes
JJ Thomson discovered
 Atoms of the same element that
have a different mass due to the
fact that have different # of
neutrons
 Disproves Dalton’s theory that
atoms of the same element are the
same
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Average Atomic Mass
(# on the periodic table)
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Scientists found the masses of all the
isotopes and their percent abundance
with a mass spectrometer
A mass spectrometer compares each
element to Carbon-12 atom
Measured in atomic mass unit (amu)
The average atomic mass was calculated
using those numbers (get a decimal #,
not a whole #, because it’s an average)
Ex. Find average atomic
mass (from isotopes)
Cu-63 62.930 amu
69.17% in the sample
Cu-65 64.928 amu
30.83% in the sample
1.
Find mass contribution of each
isotope
(mass) x ( %)
Cu-63  62.930 x .6917 = 43.53
Cu–65  64.928 x .3083 = 20.02
2.
Find average mass by adding the
isotopes mass contributions
together
Average atomic mass =
43.53 + 20.02 = 63.55 amu
History of the atom- part 1
 History of the atom- part 2
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