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The Atom
From Philosophical Idea to Scientific
Theory
The Atom
• Foundation of the Atomic Theory
• The transformation of a substance or substances into
one or more new substances is known as a chemical
reaction
The Atom
• Foundation of the Atomic Theory
• Democritus (460 - 370 BC)
» All matter is composed of tiny indestructible units,
called atoms
» Different kinds of matter
are made up of different
kinds of atoms
The Atom
• Foundation of the Atomic Theory
• Aristotle (384-322 BC)
» Disagreed with Democritus’ atomic theory
» Instead believed that matter was
made up of a combination of only
four elements water, fire, air, earth
» This was the belief for 2000 years
The Atom
Foundation of the Atomic Theory
Law of conservation of mass: mass is neither created
nor destroyed during ordinary chemical reactions or
physical changes
MASS reactants = MASS products
Atomic Theory
August 21
• How may significant figures would the product
of 2.467589 and 2.45 have?
• How many significant figures does 300.003
have?
• What group and period does Gold appear on?
Chapter 1 and 2 Test
• When you finish, turn in your test in the 1st
period shelf.
• Pick up a textbook and complete Chapter 3
vocabulary
• All PINK words on wall
• You may not use glossary
Atomic Theory
August 22
• How may significant figures would the sum of
2.4689 and 0.4500 have?
• How many significant figures does 00.40
have?
• What did the law of conservation of mass
state?
The Atom
Foundation of the Atomic Theory
Law of definite proportions: 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
Ex: oxygen = 8/9 of the mass of any
sample of pure water, while hydrogen =
remaining 1/9 of the mass
Joseph Proust, 1804
The Atom
Foundation of the Atomic Theory
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
CO and CO2
• C:O
• 1:1 ratio
• 12g to 16g
C:O
1: 2 ratio
12 g to 32 g
2 x the ratio oxygen to carbon
H2O
H2O2
• H:O
____:____ ratio
H:O
_____:_____ratio
____g : ____ g
_____g : _____ g
• file:///D:/student/ch03/sec01/vc04/hc603_01
_v04fs.htm
The Atom
• Dalton’s Atomic Theory
• John Dalton FRS (1766 - 1844)
» An English chemist, meteorologist and physicist
» He is best known for his pioneering
work in the development of modern
atomic theory, and his research into
color blindness
The Atom
• Foundation of the Atomic Theory
• Dalton’s Atomic 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
The Atom
• Foundation of the Atomic Theory
• Dalton’s Atomic Theory
» Atoms of different elements combine in simple wholenumber ratios to form chemical compounds
» In chemical reactions, atoms are combined, separated, or
rearranged
Modern Atomic Theory
Not all aspects of Dalton’s atomic theory have proven
to be correct.
 We now know that:
• Atoms are divisible into even smaller particles
• A given element can have atoms with different
masses
 Some important concepts remain unchanged
• All matter is composed of atoms
• Atoms of any one element differ in properties
from atoms of another element
2H2 + O2  2H2O
• The equation above shows that 2 molecules (made of 4
atoms) of hydrogen and 1 molecule (made of 2 atoms) of
oxygen produce 2 molecules of water. The total mass of the
product, water, is equal to the sum of the masses of each of
the reactants, hydrogen and oxygen.
• What parts of Dalton’s atomic theory are
illustrated by this reaction?
• What law does this reaction illustrate?
• If 3 g of element C combine with 8 g of
element D to form compound CD, how many
grams of D are needed to form compound
CD2?
Nitrogen and oxygen combine to form several compounds, as shown by
the following table. Calculate the ratio of the masses of nitrogen in each
of the following:
Compound
Mass of nitrogen that
combines with 1 g oxygen
NO
1.70
NO2
0.85
NO4
0.44
NO
NO 2
NO 2
NO 4
Which law do these data illustrate?
NO
NO 4
Compound
Mass of nitrogen that
combines with 1 g oxygen
NO
1.70
NO2
0.85
NO4
0.44
• NO
• N:O
• 1:1
1.7g :1 g
NO2
N:O
1:2
(8.5 x 2)= 1.7
1.7g : 2g
Compound
Mass of nitrogen that
combines with 1 g oxygen
NO
1.70
NO2
0.85
NO4
0.44
NO2
 N:O
 1:2
(2x.85) =1.7
1.7 g : 2 g

NO4
N:O
1:4
(.44 x 4) =1.76
1.76 to 4 g
August 23
WARM UP: Find the ratio of N :O
Compound
Mass of nitrogen that
combines with 1 g oxygen
NO
1.70
NO2
0.85
NO4
0.44
NO
NO 4
NO
N:O
NO4
N:O