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History
of
Atomic Theory
RWF Chapter 3 Notes
Old Dead Greek Guys
Democritus—
matter is
made up of indivisible
particles
Aristotle—matter is
continuous
The Beginning of
Modern Chemistry
or
Old Dead Guys from the 18th
and 19th Centuries
Mme. Et M. Lavoisier
Antoine Lavoisier
The
Father of Modern
Chemistry
Law of Conservation of Matter
By conducting careful
experimentation discovered 33
substances that could not be
broken down. He called these
elements.
Joseph Louis Proust
Law
of Constant
Composition
John Dalton
John Dalton
An Early Wildcat
4
Postulates:
Matter is made up of tiny
particles called atoms.
All atoms of an element are
identical.
Atoms are neither created
nor destroyed.
John Dalton
An Early Wildcat
A
given compound always
has the same relative
numbers and kinds of atoms.
J. J. Thomson
J. J. Thomson
Cathode
Ray—radiation
emitted from the cathode in a
cathode ray tube
Cathode Ray Tube—Tube
containing no or little gas,
electrodes at each end.
J. J. Thomson
Put
charged plates above and
below the tube. Found that the
beam of radiation bends
toward the positively charged
plate, and away from the
negatively charged plate.
J. J. Thomson
Conclusion:
Cathode Rays are
made of negatively charged
particles called electrons.
Measured the ratio of the
electron’s mass to charge
Plum Pudding Model
Robert Millikan
Measured
experimentally the
charge of electrons.
Calculated from Thomson’s
work the mass of an
electron.
Earnest Rutherford
Gold
foil experiment
Found that atoms are made
up of dense nucleus with
empty space around it where
electrons whiz around
Rutherford Model
Niels Bohr
Bohr Model
Subatomic Particles
Protons
Neutrons
Electrons
Protons
Positive
electric charge
Mass of 1 amu (atomic
mass unit)
Live in the nucleus
Neutron
Electrically
neutral
Mass of 1 amu {Actually,
its mass is slightly more
than a proton}
Lives in the nucleus
Electrons
Negative
electric charge
Mass of zero*
Lives outside the nucleus in
“Electron Clouds”
Atomic Number
The
number of protons in
an atom
What makes one element
different from another
In a neutral atom, the
number of protons = the
number of electrons
Ions
Atoms
are usually electrically
neutral—that is they usually
have equal numbers of protons
and electrons
An ion is an atom that has a
positive or negative charge
Ions are formed when atoms
lose or gain electrons
Mass Number
The
number of protons +
neutrons (nucleons)
Isotopes
Atoms
of the same element
that have different numbers
of neutrons, different
masses.
Isotopes
Carbon-12:
6 protons, 6
neutrons, 6 electrons
Carbon-14: 6 protons, 8
neutrons, 6 electrons
Atomic Mass
Atomic
mass is the average
mass of all the isotopes of the
element
Weighted by abundances of
isotopes
Based on Carbon-12 isotope:
def’n of atomic mass unit
Radioactivity
Henri
Becquerel Accidentally
Discovered Radioactivity
Marie
and Pierre Curie Studied
radioactivity and discovered
other radioactive elements.
Nuclear Stability
Smaller
atoms: ratio of
protons to neutrons = 1:1
Larger atoms stable with
ratio of 2:3
Earnest Rutherford
Discovered
3 types of
radiation:
Alpha particles: two protons
and two neutrons, mass of
4amu, positive 2 charge.
42He, 42a
Earnest Rutherford
Beta
particles: made up of
electrons, negative 1 charge,
0
0
no mass. 1-e 1-b
Gamma radiation: made up of
electromagnetic radiation (like
x-rays). No particles, no
mass, no charge.
Radioactivity
 Strong
Nuclear Force holds the
nucleus together.
 If the nucleus is unstable, it breaks
apart.
 It may emit an alpha particle (two
protons and two neutrons)
 It may emit a beta particle (a neutron
breaks apart into a proton which stays
in the nucleus and a electron which
zings out)
Gamma
radiation is emitted
as a release of the strong
nuclear force.
It may break in two, making
two or more smaller atoms
Nuclear changes
Fission:
a large unstable
atom breaks apart into 2 or
more pieces, releasing
energy
Fusion: small atoms crunch
together to form larger atoms
Half-life
The
half-life of a radioactive
substance is the time it takes
for half of the sample of the
substance to decay. Half-life
can be in microseconds or in
thousands of years, depending
on the substance.
Half-life
For
example, if the half-life
of a substance is 2 days,
how much of a 12.0 gram
sample would you have left
after 2 days? After 4 days?
After 8 days? After 1 day?
Mass number
51
Cr
24
Atomic number
2+
51
Cr
24
16 2O
8
51
Cr
24
Chromium-51
Da Rules
Protons = atomic number
Neutrons = mass – protons
Mass = protons + neutrons
Electrons = protons - charge
Z = ATOMIC NUMBER
A = MASS NUMBER
Atomic
number
P
Q
R
S
T
U
V
W
13
17
28
3
53
Mass
number
1
27
62
7
98
P = PROTONS
N = NEUTRONS
E = ELECTRONS
P
N
0
E
charge
0
3+
18
10
1+
66
0
55
0
144 93 0
Z = ATOMIC NUMBER
A = MASS NUMBER
P
Q
R
S
T
U
V
W
P = PROTONS
N = NEUTRONS
E = ELECTRONS
Atomic
number
Mass
number
P
N
E
1
13
17
28
3
53
55
93
1
27
35
62
7
119
98
237
1
13
17
28
3
53
55
93
0
14
18
34
4
66
43
144
1
10 3+
18 128
2 1+
53
55
93
charge
Z = ATOMC NUMBER
A = MASS NUMBER
Atomic
number
P
Q
R
S
T
U
V
W
93
92
8
28
4
Mass
number
32
237
40
61
23
P = PROTONS
N = NEUTRONS
E = ELECTRONS
P
N
16
E
charge
0
0
146
0
22+
5
0
12
0
31 28 0
Z = ATOMC NUMBER
A = MASS NUMBER
P
Q
R
S
T
U
V
W
P = PROTONS
N = NEUTRONS
E = ELECTRONS
Atomic
number
Mass
number
P
N
E
16
93
92
8
28
4
12
28
32
237
238
40
61
9
23
59
16
93
92
8
28
4
12
28
16
144
146
32
33
5
11
31
16
93
92
10 226 2+
4
12
28
charge
Quiz
1. State the law of
conservation of
matter.
2. What are Dalton’s
postulates?
3. What will a cathode
ray do if a positive
electrode is placed
near the beam?
4. Which particle has
a positive charge?
5. Which particle has
a negative charge?
6. Which particle has
a neutral charge?
7. Which particle(s)
has a mass of one
amu?
8. Which particle(s)
has a mass of zero?
9. What is the atomic
number of lithium?
10. What is the atomic
mass of chlorine?
11. How many
electrons will a neutral
atom of iron have?
12. How many
neutrons will an atom
of fluorine have?
13. How many
electrons will a
sodium ion have if it
has a positive one
charge?
14. What are the three
types of radioactive
decay?
15. Which experiment
showed that an atom
has a dense nucleus
where most of the
mass is located?
16. In the following
diagram label A, B, and
C as Alpha, Beta, or
Gamma radiation.