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The Atom: A Philosophical Idea
• The idea of atoms is very old.
• Democritus (Greek, 400 BC) - said that things
were made up of tiny, indivisible particles
(atoms) – only an idea
• Aristotle - disagreed with Democritus’ idea –
his opinion more popular.
• No evidence to support either idea.
Three Fundamental Laws of Chemistry
• Law of Conservation of Mass (1789):
– In ordinary chemical reactions, mass is neither
created nor destroyed.
– Mass of the reactants equals the mass of the
• Law of Definite Proportions (1797):
– A chemical compound contains the same elements
in exactly the same proportions regardless of the
size of the sample or source of the compound.
– Example: water – hydrogen to oxygen
ratio is always 2 to 1.
• Law of Multiple Proportions (1803):
– if two or more different compounds are composed
of the same two elements, then the proportions
can be expressed as simple, whole-number ratios.
– Example:
• Carbon and Oxygen
– Carbon monoxide - CO
– Carbon dioxide – CO2
John Dalton (1808)
• English school teacher, wanted to explain the laws
of conservation of mass, definite proportions and
multiple proportions.
• His model is called the Atomic Theory.
Postulates of his theory
• All matter is composed of extremely small particles
called atoms.
• Atoms of a given element are identical in size, mass,
and other properties; atoms of different elements differ
in size, mass, and other properties.
• Atoms cannot be subdivided, created, or destroyed.
• Atoms of different elements combine in simple wholenumber ratios to form chemical compounds.
• In chemical reactions, atoms are combined,
separated, or rearranged.
• Some aspects of Dalton’s atomic theory have
been revised. We now know that:
– Atoms are divisible into even smaller particles.
– A given element can have atoms with different
One Minute Paper
• You have one minute to answer these two
questions concerning today’s lesson.
– What was the most important thing you
– What is still muddy?