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History – Democritus to Dalton
Mr. Shields
Regents Chemistry
U01 L01
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History of the Atom
The earliest ideas in the development of atomic
structure can be traced back over 2,000 years to
the time of the Greek civilization
And the whole story wasn’t completed until 1932
when the neutron was finally discovered!
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Two key players in the early history of the atom
Democritus vs. Aristotle
source
460? – 370? B.C.
source
384-322 B.C
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The Greeks
Early Greek philosophers like Aristotle thought matter was
made up of 4 different elements.
Earth, wind, fire, water
These 4 things having various
Properties combined
To form everything else in the world
Matter could be divided forever
Into smaller and smaller pieces
And there was no such thing as
Empty space.
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Democritus Had a Different Idea
Democritus –
Proposed all matter is made up of indivisible particles
He called them “Atomos”. He proposed …
 Atoms moved though empty space
 There are different kinds of atoms with different sizes &
shapes
 The different properties of matter were a result of these
differences
 Changes in matter were a result of changes in groupings
of atoms not changes in the atoms themselves
Not Bad for 400 B.C!
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Democritus
But … The idea of atoms were rejected by other
Greek Philosophers like Aristotle
“If everything was made up of particles with empty
space between the particles with wouldn’t things
like rocks or even people not fall apart?”
The idea of the “ATOM” wasn’t re-investigated for almost
2 thousand years! During that time people went back to
believing all things were made of Earth, Air, Fire and Water.
Now let’s jump ahead 2000 years !
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JOHN DALTON (1766 – 1844)
1803 –After studying the experimental
observations of Lavoisier and
others Dalton concluded matter
could in fact be explained matter
in terms of ATOMS
A Revival of Democritus’ atomic theory
Dalton’s Atomic Theory of Matter was based on the following
5 statements…
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Dalton’s Atomic Theory of Matter
1. Each Element is composed of tiny indivisible particles called
ATOMS
2. ALL atoms of a given element are identical; atoms of
a specific element are different from other element
3. Atoms are neither created nor destroyed during
Chemical reactions (law of conservation of mass as
originally defined by Antoine Lavoisier in 1785)
4. In a chemical reaction atoms are separated, combined
or rearranged
And Lastly
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Dalton’s Atomic Theory of Matter
5. Different atoms combine in simple whole number ratios
to form compounds (as originally stated by Joseph
Proust in 1799)
This is called the LAW OF MULTIPLE PROPORTIONS
EXAMPLE: H2 + Cl2  2HCL
(1 atom of H always combines with one atom of Cl)
If I find water in nature it’s H20 and …
it’s H20 even when I make it in the Lab
2 H2 + O2 -> 2 H2O
2 atoms of H always combine with 1 atom of O to form
1 molecule of water
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Dalton’s Atomic Theory of Matter
Dalton’s Postulate’s formed the basis for our understanding
of Chemistry today
But Dalton and his contemporaries thought of Atom’s as
“HARD AND ROUND”
- in other words Atoms were like marbles
But they had NO Concept WHY atoms of one element
Behaved differently from the atoms of another element!
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