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When did the concept of an
“ATOM” first come about ?
Chapter 3: Atoms: The
Building Blocks of Matter
Objectives
 Explain
the laws of
conservation of mass, definite
proportions, and multiple
proportions
 Summarize the essential points
of Dalton’s atomic theory
What is the derivation of
the word “atom” ?
 “a”
– not
 “tomos” - cutting
 Matter
can only be subdivided
as small as an elemental
particle: THE ATOM
3-1:
The Atom: From Philosophical
Idea to Scientific Theory
Democritus


About 400 BC
World is made of 2 things
– 1) empty space
– 2) tiny particles called “atoms”
Aristotle


Matter is continuous and NOT MADE
OF ATOMS
few believers in atoms for nearly 2,000
yrs
 WHY?????????????????
1790s: chemistry revolutionized by
emphasis on quantitative analysis!
Led to discovery of 3 basic
chemical laws…..
Law of conservation of mass

Mass is neither destroyed nor
created during chemical reactions
or physical changes
Law of definite proportion

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.

H2O: always 11% H, 89% O
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
Compound
Mass of N
Mass of O
In sample(g) in sample
N2O
28.02
16.00
N2O3
28.02
48.00
N2O5
28.02
80.00
Ratio of O
masses
combined
with
constant
mass of N
Compound Mass of N
In sample(g)
N2O
28.02
Mass of O in Ratio of O
sample
masses
combined with
constant mass
of N in 3 cmpds
16.00
1
N2O3
28.02
48.00
3
N2O5
28.02
80.00
5
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 - early 1800s
 Studied
experiments of other
scientists
 Proposed explanation for the
previous 3 laws …became
known as Dalton’s Atomic
Theory
Dalton’s Atomic Theory
1.
All matter is composed of very small
particles called atoms
2.
*All atoms of an element are exactly alike
in size, mass and other properties. Atoms
of different elements differ in size, mass
and other properties
3.
*Atoms cannot be broken into smaller
particles
Dalton’s Atomic Theory (con’t)
4. Atoms of different elements combine
in simple whole-number ratios to
form chemical compounds
5. In chemical reactions, atoms are
combined, separated, rearranged

*later modified to explain new
observations
 Rapid
expansion of knowledge
about the atom began with
Dalton and continues today
Objectives
 Explain
the laws of
conservation of mass, definite
proportions, and multiple
proportions
 Summarize the essential points
of Dalton’s atomic theory