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History of Atomic Theories
Dr. Chin Chu
River Dell Regional High School
History of the Atomic Theory
Remember: a scientific theory explains
behaviors and the ‘nature’ of things
 Theories can be revised when new
discoveries are made
 The theory describing the composition of
matter has been revised many times

Ancient Times
Democritus
Aristotle
VS.
Democritus (460-370 BC)
1.Matter is made up of “atoms” that
are solid, indivisible and
indestructible
2.Atoms constantly move in space
3.Different atoms have different size
and shape
4.Changes in matter result from
changes in the grouping of atoms
5. Properties of matter result from
size, shape and movement
Aristotle (384-322 BC )
1. Four kinds of matter
a. Fire – Earth – Water – Air
2. One kind of matter can transform
into another
3. Rejected idea of the “atom” (idea then
ignored for almost 2000 years
4. This theory was more popular and
it was easier to accept
Aristotle’s Theory of Matter
Alchemists
Alchemy, a popular pursuit during the Middle
Ages, was the search for a way to transform
common metals into gold.
 Though not successful to achieve their original
goals, alchemists did generate vast amount of
data relating to physical and chemical properties
of matter, which in turn was instrumental in the
development of modern atomic theory later.
 Later on in the 17th century, some alchemists
began focusing on identifying new compounds
and reactions.

Near Modern Time

Important figures:
– Antoine Lavoisier (Law of Conservation of
Mass)
– Joseph Proust (Law of Definite Proportions)
– John Dalton (Law of Multiple Proportions)
Antoine Lavoisier (1770s)
Experiment:
2 Sn + O2

2 SnO
tin
oxygen
tin (II) oxide
mass before reaction = mass after reaction
Law of Conservation of Mass
Matter cannot be created or destroyed (in a
chemical or physical change)
1788-1799
Joseph Proust (1779)
Develops Law of Definite Composition- all
samples of a specific substance contain the
same mass ratio of the same elements
a. ex: all samples of CO2 contains 27.3%
carbon and 72.7% oxygen
b. therefore ‘elements’ are combining
in a whole number ratio
John Dalton (1803)
Dalton became a school teacher at the
age of 12 (he left school at age 11)
Loved meteorology - pioneer in this field
Studied works of Democritus, Boyle and
Proust
Wrote New System of Chemical Philosophy in
1808
Develops Law of Multiple Proportions
a. describes the ratio of elements by mass in
two different compounds composed of the
same elements
Example:
carbon monoxide vs. carbon dioxide
1 part oxygen : 2 parts oxygen
*when compared to the same amount of
carbon in each compound
Dalton collects data and develops
Atomic Theory in 1803
a. Matter is made of small particles-atoms
b. Atoms of a given element are identical in size,
mass, but differ from those of other
elements*.
c. Atoms cannot be subdivided or destroyed*.
d. Atoms combine in small whole number ratios to
form compounds.
e. Atoms combine, separate, or rearrange in
chemical reactions.
* Modified in Modern Atomic Theory
Subatomic Particles
A. J.J. Thomson (1887)
Experiments with cathode ray tubes
Voltage source
-
+
Vacuum tube
Metal Disks
Voltage source
-
+
Voltage source
-
+
Voltage source
-
+
Voltage source

+
Passing an electric current makes a beam
appear to move from the negative to the
positive end
Voltage source

+
Passing an electric current makes a beam
appear to move from the negative to the
positive end
Voltage source

+
Passing an electric current makes a beam
appear to move from the negative to the
positive end
Voltage source

+
Passing an electric current makes a beam
appear to move from the negative to the
positive end
Voltage source

By adding an electric field
Voltage source
+
 By adding an electric field
Voltage source
+
 By adding an electric field
Voltage source
+
 By adding an electric field
Voltage source
+
 By adding an electric field
Voltage source
+
 By adding an electric field
Voltage source
+
 By adding an electric field he found that the
moving pieces were negative
Thomson’s Model of the Atom
a. electrons present (-)
 b. atom is like plum
pudding - bunch of
positive stuff (pudding),
with the electrons
suspended (plums)
 Calculated the ratio
between the charge of the
electron and its mass:

e/m
“Chocolate Chip Cookie” or “Plum Pudding Model”
Millikan’s Oil Drop Experiments
Robert Milikan (1909)
– Oil Drop Experiment
– Measured the electrical charge on the
electron
– Mass can be calculated (Thomson
determined the e/m ratio)
– Mass is 1/1840 the mass of a hydrogen
atom
– electron has a mass of 9.11 x 10-28 g
Millikan’s Oil Drop Experiment (1909)
History of the Atomic Structure –
Summary thus far

So, at this point we know:
- Atoms are divisible particles
– Electrons are negatively charged
– The mass of an electron is very small
HOWEVER
– Atoms should have a (+) portion to balance
the negative part
- Electrons are so small that some other
particles must account for mass
Ernest Rutherford (1871-1937)
Experiment (1909)
Gold Foil Experiment (Expectations)
a. Shot alpha particles at atoms of gold
b. expected them to pass straight
through
Lead
block
Uranium
Florescent
Screen
Gold Foil
He thought this would happen:
According to Thomson Model
He thought the mass of the positive charge was
evenly distributed in the atom
Here is what he observed:
The positive region accounts for deflection
Gold Foil Experiment Results
a. Most positive alpha particles pass right
through
b. However, a few were deflected
c. Rutherford reasoned that the positive
alpha particle was deflected or repelled
by a concentration of positive charge
Gold Foil Experiment Conclusions
a. the atom is mostly empty space
b. the atom has a small, dense positive center
surrounded by electrons
History of the Atomic Structure

At this point in 1909, we know:
– p+ = 1.67 x 10-24 g
– e- = 9.11 x 10-28 g
– The charges balance!

But,
– How are the electrons arranged?
– There is still mass that is unaccounted for
Niels Bohr (1913)
Electrons orbit nucleus in
predictable paths
Chadwick (1891 – 1974)
In 1935
1. Discovers neutron in
nucleus
2. Neutron is neutral - does
not have a charge n0
3. Mass is 1.67 x 10-24 g
slightly greater than the
mass of a proton
History of the Atomic Theory
1803
1897
1909
1913
1935
Today
solid
particle
electron
proton
e- orbit
nucleus
neutron
Quantum
Atom
theory
Dalton
Thomson
Rutherford
Bohr
Chadwick
Schrodinger
and others
History of the Atomic Structure
Charges balanced
 Mass accounted for
 However –
what about the
behavior of the
electrons?

The Quantum Atom Theory
1. The atom is mostly empty
space
2. Two regions:
a. Nucleus- protons and neutrons
b. Electron cloud- region where
you have a 90% chance of
finding an electron
The Current Model for Atoms
Modern Atomic Theory
1. All matter is made up of small particles
called atoms.
2. Atoms of the same element have the same
chemical properties while atoms of different
elements have different properties
3. Not all atoms of an element have the same
mass, but they all have a definite average
mass which is characteristic. (isotopes)
Modern Atomic Theory (cont.)
4.
5.
Atoms of different elements combine to
form compounds and each element in
the compound loses its characteristic
properties.
Atoms cannot be subdivided by chemical
or physical changes – only by nuclear
changes