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The universe of modern atomic theory was created in 1803 when chemist John Dalton
published his paper, A New System of Chemical Philosophy. In this treatise
he proposed that everything is composed of many small pieces, called atoms.
| KEY POINTS OF |
A
New System of Chemical Philosopy |
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1) |
Matter is made of indivisible atoms
"Matter though divisible to an extreme degree is nevertheless infinitely
divisible. That is, there must be some point beyond which we cannot go..." |
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2) |
Each element consists of identical atoms
which "are perfectly alike in weight and figure, etc." |
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3) |
Atoms are unchangeable |
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~John
Dalton, 1808, 1810
Source: Rutherford et al., Project Physics Unit 5 Text, 1971
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Dalton thought of atoms of different elements as
different in size and in mass. For example, Dalton envisioned an oxygen atom to be
larger and made up of more matter than a smaller atom, such as hydrogen.
| Dalton also tried to give a
specific value to the mass of each atom as part of his research into meteorology
and the physical properties of gases. He accomplished this by comparing the mass of
each element to the mass of hydrogen. For example, an atom of oxygen ‘weighs’
16.0 times more than hydrogen, so Dalton assigned oxygen an atomic mass value of 16.0. |
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Common Atomic Masses |
| Hydrogen
(H) |
1.0 |
| Helium (He) |
4.0 |
| Nitrogen (N) |
14.0 |
| Oxygen
(O) |
16.0 |
Scientists began to accept Dalton’s theory that matter
is composed of atoms. However, they wondered if atoms were made of even smaller
building blocks and if changes would occur each time a specific building block was added
to an atom.
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