The document discusses the history of the atomic theory. It begins by noting that the idea of atoms was first proposed by Democritus in ancient Greece, though there was no evidence to support the idea at the time. Later, Dalton used the laws of conservation of mass, definite proportions, and multiple proportions to develop his atomic theory, proposing that all matter is composed of atoms that combine in whole number ratios. While some aspects have been revised, Dalton's atomic theory explained the fundamental laws of chemistry and established the basic idea of atoms that is still accepted today.

1. 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.

2. 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
products.
• 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.

3. • 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

4. 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.

5. 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 whole-
number ratios to form chemical compounds.
• In chemical reactions, atoms are combined,
separated, or rearranged.

6. • 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
masses.

7. One Minute Paper
• You have one minute to answer these two
questions concerning today’s lesson.
– What was the most important thing you
learned?
– What is still muddy?