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The Atom
Democritus
400 B.C.
Believed that matter was
composed of invisible
particles of matter he
called atoms
According to Democritus,
atoms could not be broken
into smaller particles.
Atomic Theory of Matter
• The theory that atoms are
the fundamental building
blocks of matter
reemerged in the early
19th century, championed
by John Dalton.
• Using two scientific laws
discovered in the late
1700’s, Dalton built his
atomic theory
Dalton’s Postulates (1803)
All matter is composed of extremely
small particles called atoms.
1. Law of Conservation of Mass
Antoine Lavoiser
• The total mass of substances
present at the end of a chemical
process is the same as the mass
of substances present before the
process took place.
2. Law of Constant Composition
Joseph Proust
•Also known as the law of definite
proportions.
•The elemental composition of a
pure substance never varies.
Dalton’s Postulates (1803)
All atoms of a given element are identical to
one another in mass and other properties,
but the atoms of one element are different
from the atoms of all other elements.
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Dalton’s Postulates (1803)
Atoms can not be subdivided, created or
destroyed.
Dalton’s Postulates (1803)
Compounds are formed when atoms of
more than one element combine; a given
compound always has the same relative
number and kind of atoms.
Size of an atom
Dalton’s Postulates (1803)
The radii of
most atoms fall
within the range of
5 x 10-11 m to 2 x 1010 m
Atoms in compounds combine, separate, or
rearrange in normal chemical reactions to
form new compounds. Atoms of an element
are not changed into atoms of a different
element by chemical reactions.
Experimentation on matter has
revealed to scientist that the atom is
divisible into smaller particles called
subatomic particles.
There are three subatomic particles:
J. J. Thomson (1897)
Using a cathode ray tube, he
determined the charge-to-mass ratio for
the electron as:
1.76 x 108 C/g
1. Electrons
2. Protons
3. Neutrons
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The Atom, circa 1900:
Milikan Oil Drop
Experiment (1909)
• “Plum pudding”
model, put forward by
Thompson.
• Positive sphere of
matter with negative
electrons imbedded in
it.
Radioactivity
• One of the pieces of evidence for
the fact that atoms are made of
smaller particles came from the
work of Marie Curie (18761934).
• She discovered radioactivity,
the spontaneous disintegration of
some elements into smaller
pieces.
Discovery of the Nucleus
Ernest Rutherford
shot  particles at
a thin sheet of
gold foil and
observed the
pattern of scatter
of the particles.
• Using voltage and change
in the rate of fall of
charged oil drops, he was
able to determine the
charge on each drop.
• From Thompson’s charge
to mass ratio, Milikan
determined the charge and
mass of an electron.
Particles of Radioactivity
• Three types of radiation were discovered by
Ernest Rutherford:
  particles
  particles
  rays
Discovery of the Nucleus
Since some particles
were deflected at large
angles, Thompson’s
model could not be
correct.
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The Nuclear Atom (1911)
• Rutherford postulated a very small, dense
nucleus of positive charge with the electrons
around the outside of the atom.
• Most of the volume of the atom is empty space.
Combination of Millikan’s
Findings and the Au Foil
Experiment Lead to
Rutherfords Model
Rutherford (~1911) Nuclear Model
• Protons were later discovered by Rutherford in 1919.
. .
heavy
central
(+) nucleus
James Chadwick (1932)
Further developed the
atomic model by theorizing that
alpha and beta radiation results
from the decomposition of a
neutral particle found in the
nucleus, the neutron
. .
. .
.
. .
.
e- “about”
nucleus
“sea of e-”
Subatomic Particles
• Protons and electrons are the only particles
that have a charge.
• Protons and neutrons have essentially the
same mass.
• The mass of an electron is so small we ignore
it.
• H atoms - 1 p; He atoms - 2 p
• mass He/mass H should = 2
• measured mass He/mass H = 4
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