Atoms: The Building
Blocks of Matter
The Atom: From Philosophical
Idea to Scientific Theory
Opinion or Theory?
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Smoking at an early age may make it more
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Early Views of Matter
• Democritus (450 B.C.) – proposed that matter
was not infinitely divisible; that matter could be
broken down into tiny particles, which were not
divisible.
• These particles were referred to as atomos.
• Aristotle (~400 B.C.) – believed that matter was
continuous.
Foundations of Atomic Theory
• Antoine Lavoisier – (late 1700’s) used a balance
to show that matter was not created or
destroyed during chemical reactions – Law of
Conservation of Matter (Mass).
• Joseph Proust (1799)- showed that compounds
always contain exactly the same proportion of
elements by mass– Law of constant composition
a.k.a. Law of definite proportion
For example: The mass of water (H2O) always
contains exactly 88.9 % oxygen and 11.1 %
hydrogen.
Dalton’s Atomic Theory
• John Dalton – (early 1800’s) proposed the
atomic theory of matter
Compound 1
Compound 2
Mass of oxygen that combines
with 1 gram of Carbon
1.33 g
2.66 g
• Compound 2 has exactly twice as much oxygen
as Compound 1
Dalton’s Atomic Theory
This can be explained in terms of atoms.
Compound 1: CO (Carbon monoxide)
Compound 2: CO2 (Carbon dioxide)
Law of Multiple Proportions
If 2 or more elements form a series of compounds,
the ratios of the masses of elements can be
reduced to small whole #’s
Dalton’s Atomic Theory
1. Matter is composed of tiny particles called atoms.
2. Atoms of a given element are identical in size,
mass, and properties. Atoms of different elements
are fundamentally different.
3. Atoms cannot be divided, created, or destroyed.
4. Atoms of different elements combine in simple
whole-number ratios to form chemical compounds.
5. In chemical reactions, atoms are combined,
separated, or rearranged.