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Transcript
The Atom: From Philosophical
Idea to Scientific Theory

Democritus was one of first to propose the
idea of a basic particle of matter (400 B.C.)
 Called it the atom (means indivisible in Greek).

Aristotle lived after Democritus (300 B.C.),
but believed in all matter being continuous.
 Aristotle’s view was accepted until the late 1700s.
 No solid evidence existed to support Aristotle’s
views.
Foundations of Atomic Theory

As technology improved, so did scientific
theories.

In the 1790s, improvements in balances
enabled scientists to accurately measure
masses of substances they were studying.
This led to three new laws:
○ The law of conservation of mass.
○ The law of definite proportions.
○ The law of multiple proportions.
The Law of Conservation of Mass

Mass is neither created nor destroyed
during ordinary chemical reactions or
physical changes.
The Law of Definite Proportions

A chemical compound contains the
same elements in exactly the same
proportions by mass regardless of the
size of the sample or source of the
compound.
11.9% hydrogen
88.1% oxygen
11.9% hydrogen
88.1% oxygen
The Law of Multiple Proportions

If two or more different compounds are
composed of the same two elements,
then the ratio of the masses of the
second element combined with a certain
mass of the first element is always a
ratio of small whole numbers.
Dalton’s Atomic Theory

In 1808, John Dalton proposed an explanation for the
laws.
 All matter is made up of small particles (atoms).
 Every element has atoms that are unique for that element and
identical.
 Atoms cannot be subdivided, created, or destroyed.
 Atoms of different elements combine in simple whole-number
ratios to form chemical compounds.
 In chemical reactions, atoms are combined, separated, or
rearranged.
Modern Atomic Theory

Changes to Dalton’s Theory
 Atoms ARE divisible (nuclear reactions).
 Atoms of a given element CAN have
different masses (isotopes).
Putting it all together…

Why were Dalton’s ideas accepted and
not Aristotle or Democritus’?

What laws did Dalton explain using his
theory?