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Chem 115 PRINCIPLES OF CHEMISTRY (Lecture)

DEVELOPMENT OF ATOMIC THEORY
Early Greek Theories
(400 B
...
He believed that
matter cannot divided permanently
...
He claimed
that atoms is a sphere with no electrons,
protons, and neutrons
...
C) Aristotle claimed that there are
only four elements: Earth, Air, Water, and
Fire
...
He
claimed that all of these four could be
reacted to create one another
...
Democritus’ representation of Atom
...


Earth

Wet

Cold
Water

Figure 2
...


The Early Greek Theories was disproved due to
lack of experimental evidence for the reason
that the idea of Democritus and Aristotle were
all based on reasoning and it has no scientific
value
...
It is stated that
regardless of the source or method of
preparation, a chemical compound will
always have its component elements in a
fixed mass ratio
...
It stated that matter
cannot be generated or destroyed, and the
total mass of chemical reaction will remain
constant
...
Law of Definite (or Constant)
Composition

Figure 3
...
It stated that in the
law of Multiple Proportion that two
elements could combine to form different
compounds, with the first element’s ratio
being fixed but the second element’s ratio
being variable
...

This the following explains how theory
explain the mass law: Mass Conservation,
Definite Composition, and Multiple
Proportions
...


(courses
...
com)
2 hydrogen atoms at 1
mass units each =

+

(1897) Cathode Rays

Figure 5
...
John Dalton’s Atomic Theory

(1897) J
...
Thompson discovered that atom
could be divided a negatively charged part
of atom called the electron
...

Negatively Charged (Electron)
Negatively Charged Electron

Positively Charged Matter
Negatively Charged Electron

Figure 7
...
J
...

This experiment is approved with the
discovery of electrons
...
Diagram of J
...
Thomson's Cathode
Ray Tube (https://www
...
org)

Chem 115 PRINCIPLES OF CHEMISTRY (Lecture)

1900’s
(1909) Robert Millikan discovered the mass
of the electron through his experiment of
introducing charged oil droplets into an
electrically charged fields
...
He created an atomic
model different from Thompson
...
It has a very small, densely positive
charged nucleus (which is full of protons),
and it is surrounded by electrons moving at
extremely high speed
...
Millikan’s Oil-drop Experiment
(en
...
org)

Figure 9
...
blogspot
...

Nucleus refers to the center of an atom, it
contains protons and neutrons, and around
which the electrons rotate
...
1 Rutherford’s Atomic Model

(1913) Neil Bohr developed the Bohr Model
or Rutherford-Bohr Model, similar to the
Solar System that is composed of a small,
dense nucleus surrounded by orbiting
electrons, but with the use of electrostatic
forces as the attraction instead of using
gravity
...
Bohr Model (science
...
com)

(ehs
...
org)

(1932) James Chadwick discovers the
Neutron
...
He calculated the velocity of the
protons in the beams using kinematics
...
He found
that it had same mass of the proton
...
James Chadwick’s discovery of
neutron (https://www
...
com)



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