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Chpater Review
• Atomic Theory
• Atomic structure
• Valence electrons
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Topic 3
The development of the Atomic theory
 Democritus concluded that matter could
not be divided into smaller and smaller
pieces forever.
 Democritus named this small, indivisible
piece, an atom; atom comes from the Greek
word atomos, which means “not to be cut”
or “indivisible.”
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Topic 3
The development of the Atomic theory
Philosophical idea
 Aristotle was part of the generation that succeeded
Democritus.
 Aristotle did not believe in atoms, instead, he thought
that all matter was composed of 4 basic elements,
water, fire, earth, and air. His opinion was accepted
for nearly 2000 years.
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Topic 3
The development of the Atomic theory
The first atomic theory
 In the early 1800’s, the English
chemist John Dalton performed a
number of experiments that eventually
led to the acceptance of the idea of
atoms.
 In 1803, Dalton combined the results of his experiments
with other observations about matter and proposed an
atomic theory.
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Topic 3
The development of the Atomic theory
Foundations of Atomic Theory
• The transformation of a substance or substances
into one or more new substances is known as a
chemical reaction.
• Law of conservation of mass: mass is neither
created nor destroyed during ordinary chemical
reactions or physical changes.
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Topic 3
The development of the Atomic theory
Foundations of Atomic Theory, continued
• 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.
• 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.
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Topic 3
The development of the Atomic theory
Law of Conservation of Mass
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Topic 3
The development of the Atomic theory
Law of Multiple Proportions
The ratio of the masses of one element combined with a certain
mass of a second element is always a ratio of small whole
numbers.
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Topic 3
The development of the Atomic theory
Dalton’s Atomic Theory
1. All matter is composed of extremely small
particles called atoms.
2. Atoms of a given element are identical in
size, mass, and other properties; atoms of
different elements differ in size, mass, and
other properties.
3. Atoms cannot be subdivided, created, or
destroyed.
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Topic 3
The development of the Atomic theory
Dalton’s Atomic Theory, continued
4. Atoms of different elements combine in simple
whole-number ratios to form chemical
compounds.
5. In chemical reaction, atoms are combined,
separated, or rearranged.
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Topic 3
The development of the Atomic theory
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 (by
a nuclear change)
• A given element can have atoms with different
masses (isotopes)
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Topic 3
The development of the Atomic theory
Modern Atomic Theory
BUT, Some
unchanged.
important
concepts
remain
• All matter is composed of atoms.
• Atoms of any one element differ in properties from
atoms of another element.
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Chapter 3
The structure of the atom
Table of Contents
Section 1 Developing the atomic theory
Section 2 The structure of the atom
Section 3 Counting atoms
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Structure of the atom
• Scientific investigations showed that atoms are
actually composed of several basic smaller particles.
• Atom: Defined as the smallest particle of an element
that retains the chemical properties of that element.
• All atoms consist of two regions.
- The nucleus is a very small region located at the
center of an atom (protons + neutrons)
- Surrounding the nucleus, is a region occupied by
negatively charged particles called electrons.
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Discovery of the electron
In 1897, J.J. Thomson provided the first
hint that an atom is made of smaller
particles.
When current is passed through a
cathode ray tube, a stream of
particles travel from the cathode(-) to
the anode (+), producing a beam or
ray.
When a magnetic or an
electric field was placed
around the tube, the rays
were deflected away from
the negatively charged end.
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Discovery of the electron
The mass of
the electron
9.10·10-28 g
Cathode Ray Tube
experiment
Millikan’s Experiment (1909)
Charge to
mass ratio of
the electron
-1.76·10 8 C/g
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Discovery of the electron
• Thomson concluded: All cathode rays are
composed of identical negatively charged particles,
electrons.
• He also found the charge to mass ratio of the
electrons.
• Millikan measured the charge of the electrons.
• Scientists used the charge of electrons and the
charge to mass ratio of the electron to determine
the mass of an electron.
• The mass of an electron was found to be about
1/2000! the mass of a hydrogen atom.
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Thomson’s plum pudding model
• Based on what was learned about electrons:
1. Because atoms are electrically neutral, they must
contain a positive charge to balance the negative
electrons.
2. Because electrons have so much less mass than
atoms, atoms must contain other particles that
account for most of their mass.
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Thomson’s plum pudding model
• Thomson proposed a model for the atom that is called
the plum pudding model,
• The negative electrons were spread evenly throughout
the positive charge of the rest of the atom.
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Discovery of the atomic nucleus
• After Thomson proposed his plum
pudding model of the atom, new
experiments disproved this model.
• In 1911, Ernest Rutherford and his associates Hans
Geiger and Ernest Marsden bombarded a thin piece
of gold foil with fast positively charged alpha particles.
• Most of the alpha particles passed through the foil.
However, a some particles were deflected and some
were deflected back to the source.
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Chapter 3
Section 2 The Structure of the Atom
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Rutherford's experiment
• particles experienced some
powerful force within the atom.
•This force occupies a very
small amount of space (few 
particles had been deflected.
•He concluded: The force must
be caused by a very densely
packed matter with a positive
charge (The nucleus)
EUREKA!!!!!!! Rutherford discovered that size of the
nucleus was very small compared to the size of the atom.
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Rutherford’s atomic Model
• Dense, positive nucleus
at the center of the
atom.
• Most of atom is empty
space.
• But, can electrons just
float around in empty
space?
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The Bohr Model
• Rutherford’s model proposed that electrons
were held in an atom by the attraction
between them and the positive nucleus.
• In 1913, scientist Niels Bohr proposed a model in which
he placed each electron in a specific energy level.
• Electrons move in definite orbits around the nucleus,
like planets circle the sun. These orbits, or energy
levels, are located at certain distances from the
nucleus.
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Bohr’s atom
• Electrons don’t just float around, they have a distinct orbit.
• There is a certain amount of energy in these levels.
(Energy to pull the electron away)
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The Wave Model
Louis De Broglie - 1924
• Bohr’s model just worked well in explaining
the simplest hydrogen atom.
• Today’s atomic model is
based on the principles of
wave mechanics.
• Electrons do not move in a
definite orbit. It is impossible
to determine its exact
location.
Scientists can only predict where an electron is most
likely to be found.
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Modern atomic model
Schrödinger - 1926
An atom has a small positively
charged nucleus surrounded by a large
region in which there are enough
electrons to make the atom neutral.
The region of space around the
nucleus where an electron is most
likely to be found is called an
orbital.
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Orbitals
Orbitals have different shapes and maximum numbers at
any level.
s (sharp) - spherical (max = 1)
p (principal) - dumb-bell shaped (max = 3)
d (diffuse) - four-lobe-shaped (max = 5)
f (fundamental) - six-lobe shaped (max = 7)
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Exit Slip
Determine whether each of the following
statements is true or false. If false, correct the
statement.
(a) The nucleus has most of the mass and
comprises most of the volume of an atom.
(b) Every atom of a given element has the same
number of protons.
(c) The number of electrons in an atom equals the
number of neutrons in the atom.
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