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Objectives:
◦ Explain
 The law of conservation of mass
 The law of definite proportions
 The law of multiple proportions
◦ Summarize the 5 points of Dalton’s atomic theory
◦ Explain the relationship between Dalton’s theory
and the law of conservation of mass, the law of
definite proportions, and the law of multiple
proportions.

Democritus (400 BC) –
particle theory of
matter
◦ Called basic particles
atomos (from Greek
indivisible)


Aristotle – believed
matter was continuous,
not particulate – this
opinion was accepted
for almost 2000 years
Neither view supported
by evidence until 18th
century
Thinkers,
not
doers!

Late 17th century – generally accepted that:
◦ Elements can’t be broken down further by ordinary
means
◦ Elements combine to form compounds, which have
different physical & chemical properties than the
elements that form them

Controversy – whether elements always
combine in the same ratio when forming a
particular compound

1790s – emphasis on quantitative analysis
of chemical reactions led to discovery of
several basic laws
◦ Law of conservation of mass
◦ Law of definite proportions – Whether you have 2 crystals
of NaCl or 200 grams, it is always 39.34% Na and 60.66% Cl by mass.
◦ Law of multiple proportions – when two or more
compounds are made up of the same two elements, the
second element always appears in a ratio of small whole
numbers when compared from the first compound to the
next
 Ex. CO & CO2 – say both contain 1.0g of C – in CO 1.33g of oxygen
combine with the 1.0g of C; in CO2 2.66g of oxygen combine with
the 1.0 g of C.
 Basically, this law just says you will never find CO1.5

1808 – John Dalton proposed an
explanation for each of those
three laws
1. All matter is composed of atoms
2. Atoms of a given element are
identical in size, mass, and other
properties; atoms of different
elements differ in size, mass, and
other properties.
3. Atoms cannot be subdivided,
created, or destroyed.
4. Atoms of different elements combine
in simple whole-number ratios to
form chemical compounds.
5. In chemical rxns, atoms are
combined, separated, or rearranged.
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

Law of conservation of mass explained by
point #3 and #5.
Law of multiple proportions explained by #4.
Law of definite proportions explained also by
#4. A given compound is always made up of
the same combination of atoms.

Dalton turned Democritus’ idea into a theory that
could be, and was, tested by experiment

Not all aspects of Dalton’s theory have proven to
be correct
◦ Atoms can be divided into smaller particles (the law of
conservation of mass still holds true in reactions though)
◦ A given element can have atoms with different masses
(we’ll talk more about this in section 3)

Has been modified, but the ideas 1) that matter is
composed of atoms and 2) atoms differ between
elements - remain unchanged.

Contributors to atomic theory
◦ Democritus
◦ Dalton

Points of Dalton’s theory
◦ Which ones have been modified?

Laws explained based on Dalton