10/8/2009
Early History
of Atomic
Theories
Ancient Greece
Empedocle (around 492-432 BC.)
Divided matter into four elements, that he also
called "roots": earth, air, fire and water
Leucippe of Milet in 420 BC
The word "atom" comes from the Greek "atomos"
and signifies "indivisible"
Democritus is credited with recording this theory
along with the experimentation that led to it
Chapter 3.1
The 18th Century
The Bible?!
Antoine Lavoisier (1743 – 1794)
“Father of modern chemistry”
66 AD - Simon Peter wrote:
“but the day of the Lord will come like
a thief, in which the heavens will pass
away with a roar and the elements will
be destroyed with intense heat, and
the earth and its works will be burned
up.”
2 Peter 3:10
Law of conservation of mass

Matter can be neither created nor destroyed.
Joseph Proust (1754 – 1826)
Law of definite proportions or constant composition

A given compound always contains exactly the same
proportion of elements by mass.

Early 19th Century
1.
Dalton’s Atomic Theory (1808)
John Dalton (1766 – 1844)
Law of multiple proportions

When two elements form a series of compounds, the
ratios of the masses of the second element that
combine with 1 gram of the first element can always be
reduced to a small whole number.
water
hydrogen peroxide
Ratio
H 2O
H2O2
For 1 g H
15.9 g O
31.8 g O
E.g., copper carbonate = 5.3 Cu : 4 O : 1 C by mass
2x
Elements must be made up of particles…
1. Each element is made up of tiny
particles called atoms.
2. The atoms of a given element are
identical

The atoms of different elements are
different in some fundamental way or
ways (e.g., mass).
ASIDE: Dalton compiled first table of atomic masses.
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Dalton’s Atomic Theory continued…
Late 19th Century
3. Chemical compounds are formed when
atoms combine with each other.

A given compound always has the same
relative numbers and types of atoms.
4. Chemical reactions involve reorganization
of atoms --- i.e, changes in the way they
are bound together.

2.
J. J. Thomson (1856 – 1940)

Discovered the electron
Experiments:
to learn structure of the atom
 Applied high voltage to a

The atoms themselves are not changed in a
chemical reaction.
Figure 1: A cathode-ray tube
• Metal electrodes sealed in glass tube containing gas
• Voltage applied between electrodes
Particles ejected from metal
Fast-moving particles transfer energy to gas
Excited gas glows (EMITS LIGHT)
Cambridge University
Nobel prize in Physics, 1906
cathode ray tube
Figure 2: How Thomson found the charge of particles
• Applied another voltage: across stream of particles
Particle stream bends towards POSITIVE
IMPLICATION: Particles must be negatively charged
Thomson’s Atomic Theory (1897)
Thomson’s findings:
Many different metals can eject electrons
Hypothesis: All atoms
contain electrons
Thomson’s
“Plum Pudding” or
“Raisin Bun” model
of the atom.
The 20th Century
3.
Ernest Rutherford (1871-1937)
Cambridge University
Nobel prize in Chemistry, 1908

Tested plum pudding model
Conclusion: atom has NUCLEUS of positive charge

Experiments:



Aimed high energy, massive, + charged particles
(a-particles) at thin metal foil
REASONING: should pass through…

if deflected
massive particles present
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Rutherford's Experiment:
-particle bombardment of metal foil
Atoms are electrically NEUTRAL
i.e., # protons = # electrons
 POSITIVE: charge on nucleus = # protons
 NEGATIVE: charge on electrons = # electrons
counter each other
Figure 6: Rutherford’s
metal foil(1911)
expt
Rutherford’s
Atomic Theory
Hypothesized results if Thomson's model
were correct.
Actual results – a
nucleus of positive
charge in the center!
Why do different atoms have different
properties?
Because of their electrons…
 All atoms of same ELEMENT have:
 unique # protons …THEREFORE… unique # electrons
1
1
 neutrons do not effect charge
H
16
8
hydrogen
23
11
Na sodium
ATOMIC SYMBOLS
Abbrev.
ep+
n0
continued…
Electrons determine chemical properties…
 ELECTRONS on “outside” of atom
 see the surroundings…
i.e., other atoms nearby
 can intermingle
atoms can combine
to form molecules !
O oxygen
Latin: natrium
e.g., “carbon twelve”
“A” Mass number
# p+ + # n0
“Z” Atomic number
12
6
C
Element symbol
# p+ (= # e-)
ISOTOPES of an element have nearly
identical chemical properties
 Atoms with same # protons (same element)
same # electrons
BUT different # neutrons (different mass)
Two isotopes of sodium
 Number of electrons in atom
 outermost electrons = valence electrons
 determine ability to interact with other
atoms
determine CHEMICAL PROPERTIES!
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