Transcript Chapter 3: The Atom

Chapter 3: The Atom
“The Building Blocks of Matter”
Early Definitions of the Atom
►The Greek philosopher Democritus (460
B.C. – 370 B.C.) was among the first to
suggest the existence of atoms (from
the Greek word “atomos”)
 He believed that atoms were indivisible and
indestructible
 His ideas did agree with later scientific
theory, but did not explain chemical
behavior, and was not based on the
scientific method – but just philosophy
Aristotle
► Aristotle
believed in 4
elements:
► Earth, Air, Fire, and
Water.
European Alchemists
►
1100 A.D.
► Their
work evolved
into what is now
modern chemistry.
► Trying
to change
ordinary materials into
gold.
The 1700’s
► Little
work was done on atomic theory until
the 1700’s
► Aristotle’s and Democritus’ theories stood
► However some Laws were attained:
 Law of Conservation of Mass
 Law of Definite Proportions
 Law of Multiple Proportions
Law of Conservation of Mass
► 1789:
by French Chemist Antoine Lavoisier
► Mass is neither created nor destroyed during
ordinary chemical reactions or physical changes
► Simply, the mass of what you end up with
(products), must equal the mass of what you
started with (reactants)
Law of Definite Proportions
► 1800:
French chemist Joseph Proust
► Chemical
compounds contain the same
elements in exactly the same proportions by
mass, regardless of t he size of the sample
► Ex.) Salt, NaCl: always consists of
 39.34% Sodium
 60.66% Chlorine
Law of Multiple Proproportions
► John
Dalton first expressed this observation
in 1803 and it is sometimes called Dalton's
Law
► When elements combine they do so in a
ratio of small whole numbers.
 Ex.) carbon and oxygen react to form CO or
CO2, but not CO1.8
Video About Dalton
Dalton’s Atomic Theory (experiment based!)
John Dalton
(1766 – 1844)
1) All matter is composed of
extremely small particles
called atoms.
2) Atoms of the same element are
identical. Atoms of any one
element are different from
those of any other element.
3) Atoms cannot be subdivided, created, or
destroyed.
4) Atoms of different elements combine in
simple whole-number ratios to form
chemical compounds
Dalton Continued…
5) In chemical reactions, atoms are
combined, separated, or rearranged –
but never changed into atoms of
another element.
Experiments determining the structure of the
atom
► Atom:
smallest particle
of an element that
retains the chemical
properties of that
element.
► 1897:
 J. J. Thomson- used
Cathode ray tubes
Thomson’s Experiment
Voltage source
-
+
Thomson’s Experiment
Voltage source
-
+
Thomson’s Experiment
Voltage source

+
Passing an electric current makes a beam
appear to move from the negative to the
positive end.
Thomson’s Experiment
Voltage source
► By
adding an electric field……
Thomson’s Experiment
Voltage source
+
 By adding an electric field, he found that
the moving pieces were negatively charged
1904: Thompson’s Model
► These
were concluded to
be electrons.
► Stability suggested that
were there is a negative,
there should be a
positive. He couldn’t
find the positive (for a
while).
► Described the atom as
plum pudding.
► A cloud of positive stuff,
with the electrons able
to be removed.
1909: Millikan’s Experiment
Atomizer
Oil spray
+
-
Oil
Microscope
Millikan’s Experiment
+
_
X-rays
X-rays charge the oil drops.
Millikan’s Experiment
Some drops would fall slower than others…
From the mass of the drop and the charges on the two plates,
he calculated the mass of and charge magnitude on an
electron 1 electron = 9.11 x 10 –31 kg
1911: Earnest Rutherford’s
Experiment
► Used
uranium which as it decays produces
alpha particles.
► Aimed alpha particles at gold foil by drilling
hole in a lead block.
► Since the mass is evenly distributed in gold
atoms, and if Thompson was correct in his
structural picture of the atom, Rutherford
believed the alpha particles should go straight
through the foil.
► Used gold foil because it could be made very
thin (only a few atoms thick).
Lead
block
Uranium
Florescent
Screen
Gold Foil
What he expected
Because
Because, he thought the mass was
evenly distributed in the atom.
What he got
Video About Rutherford
How he explained it
► Atom
is mostly empty
► Small dense, positive piece
at center.
► Alpha particles
(have a +2 charge)
are deflected by it
if they get close
+
enough.
So Rutherford discovered the NUCLEUS!!
+
Modern View
► The
atom is mostly
empty space.
► Two regions
► Nucleus- protons and
neutrons.
► Electron cloud- region
where you might find
an electron.
► More
on this later…
STOP
Discovery of the Nucleus
Ernest Rutherford
shot  particles at a
thin sheet of gold
foil and observed
the pattern of
scatter of the
particles.
© 2009, Prentice-Hall,
The Nuclear Atom
Since some particles
were deflected at
large angles,
Thompson’s model
could not be correct.
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The Nuclear Atom
► Rutherford
postulated a very small,
dense nucleus with the electrons
around the outside of the atom.
► Most of the volume of the atom is
empty space.
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Other Subatomic Particles
► Protons
were discovered by Rutherford in
1919.
► Neutrons were discovered by James
Chadwick in 1932.
© 2009, Prentice-Hall, Inc.
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.
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Symbols of Elements
Elements are symbolized by one or two
letters.
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Atomic Number
All atoms of the same element have the same
number of protons:
The atomic number (Z)
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Atomic Mass
The mass of an atom in atomic mass units
(amu) is the total number of protons and
neutrons in the atom.
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Isotopes
► Isotopes
are atoms of the same element with
different masses.
► Isotopes have different numbers of neutrons.
11
C
6
12
C
6
13
C
6
14
C
6
© 2009, Prentice-Hall, Inc.
Review Video
Assignment
► Project
1: Atomic Structure Timeline
Atomic Structure Timeline
Requirements:
► Must include a title.
► Must include a spot for each of the 11
items.
► Must include at least the Who, When, and
What.
► Must have a picture (color is preferred) for
each item.
Options:
1. Comic book format
2. Standard timeline
11 Important Figures/Discoveries
► Democritus
► Aristotle
► Antoine
Lavoisier – Law
► Joseph Proust – Law
► John Dalton – Law
► John Dalton – Theory
► J.J. Thomson
► Robert Millikan
► Ernest Rutherford – Gold Foil
► Rutherford – Discovery of proton
► Chadwick – Discovery of neutron