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The History of the Atom
Part 1 – The concept of the Atom
A long time ago, in a galaxy
country far, far away
There were two philosophers
One was Democritus
 Lived in Ancient Greece on
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the island of Sicily
First to use the term atom
(atomos meaning indivisible)
Described the atom by
mathematics of the day (His
peer was Pythagoras)
Atom was described as the
smallest part of a material
There were rock atoms, hair
atoms, etc.
Democritus’s atom would have
looked like this:
 Solid and
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Indestructible
No electrons
No nucleus
No protons
No neutrons
No Experiments to
support his ideas
The other was Aristotle
 He lived in the heart of
Greece (Athens)
 Defined matter as composed
of hot, cold, wet and dry
 These properties related to
fire, air, water, and earth
 Everything differed by the
percentages of each that
composed the object
And the winner was:
 Aristotle
 And set chemistry
back about 2000
years
 Chemistry didn’t make
a come back until the
Renaissance
Antoine Lavoisier
 Frenchman in the late 1700s
 Was working with gases
 He discovered that the amount
of mass that one started with
was equal to the amount of
mass that one ended with –
Law of conservation of mass.
 Also discovered that oxygen
caused things to burn
 Some call him the Father of
Chemistry
Joseph Proust
 Another Frenchman a few
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years after Lavoisier
Proposed Law of Definite
Proportions
Roughly stated: a
chemical could have only
one ratio of atoms within it.
In other words, a
compound will always
have the same elements in
the same ratio
Water has to be H2O and
not HO or HO2
John Dalton
 Took his own experiments
and those of others to
write a paper, which later
became known as the
Modern Atomic Theory
 It gave a compilation of the
information at the time and
allowed other scientist to
test his ideas.
Dalton's Atomic Theory
1) All matter is composed of extremely small
2)
3)
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5)
particles called atoms, which cannot be
subdivided, created, or destroyed.
Atoms of a given element are identical in their
physical and chemical properties.
Atoms of different elements differ in their
physical and chemical properties.
Atoms of different elements combine in simple,
whole-number ratios to form compounds
In chemical reactions, atoms are combined,
separated, or rearranged but are never
created, destroyed, or changed.
An Analogy for Dalton’s atom
An atom is small, spherical and the same
throughout.
A small atomic
fireball
Dalton also gave us the Law of
Multiple Proportions
 Elements could combine in different ratios
to produce different compounds
 For Example, nitrogen and oxygen can
form N2O, NO, NO2, N2O3, N2O4, and N2O5.
His ideas weren’t 100%
correct, but they gave
scientist a good start