Atomic Structure
4-1 Studying Atoms
What was Dalton’s theory of the structure of matter?
What contributions did Thomson and Rutherford
make to the development of atomic theory?
Ancient Greek Models of Atoms
Democritus believed that all matter consisted of extremely small particles (Atoms)that could
not be divided.
 He thought atoms had specific sets of properties.
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Atoms in liquids were round and smooth
Atoms in solids were rough and prickly
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Aristotle believed that ALL SUBSTANCES WERE BUILT FROM FOUR ELEMNTS AND THAT
there was a limit to the number of times matter could be divided.
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By the 1800s, the atomic model of matter was the supported by data from experiments.
Dalton’s Atomic Theory
John Dalton was a teacher who spent his spare time
doing scientific experiments, especially ones on the
behavior of gases in air. He determined gas consists of
individual particles.
 Evidence for Atoms
 Burning magnesium combines with oxygen and
forms magnesium oxide
 The ratio of the masses is constant:
 1– gram sample of magnesium combines with
65.8 grams of oxygen. A 1- gram sample of
magnesium combines with 658 grams of
oxygen.
 The wooden spheres on the right were made by
Dalton to represent the atoms of different elements.
To review: What did Dalton notice that all compounds have in common?
Using logic: Why do you think there are holes in Dalton’s wooden spheres?
Dalton’s Atomic Theory Cont.
 Dalton’s Theory explains why all the elements in a compound
always join in the same way. Remember that a theory must
explain the data from many experiments.
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All matter is made up of individual particles (atoms) which can’t be
divided
Four main points:
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All elements are composed of atoms.
All atoms of the same element have the same mass and atoms of different
elements have different masses.
Compounds contain atoms of more than one element.
In a particular compound, atoms of different elements always combine in the
same way.
 Also widely accepted in Dalton’s time, scientists found that
not all of his ideas were correct.
 What did Dalton notice that all compounds have in common?
Thomson’s Model of the Atom
 Thomson’s Experiments
 Used an electric circuit to learn more about atoms
Hypothesized that the beam was a stream of charged particles
that interacted with the air in the tube and caused the air to
glow
Thomson’s Model of the Atom Continued
 Evidence for Subatomic Particles
 Experiments provided the first evidence that atoms are made
of smaller particles
 Thomson’s Model
 Like a scoop of chocolate chip ice cream
Chips are negatively charged particles
 Chips are spread through a mass of
positively charged mater

How do objects with the same charge act when they come close to each other?
Rutherford’s Atomic Theory
Ernest Rutherford (1871-1937) was actually surprised by the results of his own
experiments.
 Rutherford’s Hypothesis: the mass and charge at any location in the gold would be
too small to change the path of an alpha particle
 The Gold Foil Experiment: the equipment pictured was used to test his hypothesis
SOME OF THE LOCATIONS OF THE FLASHES ON THE
SCREEN DID NOT SUPPORT RUTHERFORD’S PREDICTION
SOME OF THE ALPHA PARTICLES BEHAVED AS THOUGHT
THEY HAD STRUCK AN OBJECT AND BOUNCED STRAIGHT
BACK
 Discovery of the Nucleus: dense, in the center of the atom, positively charged
4-2 The Structure of an Atom
What are three subatomic particles?
What properties can be used to compare
protons, electrons, and neutrons?
How are atoms of one element different from
atoms of other elements?
What is the difference between two isotopes of
the same element?
Properties of Subatomic Particles
Protons, electrons and neutrons are subatomic particles.
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Protons: positive charge varies among elements…each nucleus must
contain at least one particle with a positive charge (1-100)
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Electrons: negatively charged…outside the nucleus
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Neutrons: neutral charge…in the nucleus…mass almost equal to mass of
protons
Comparing Subatomic Particles
Protons, electrons, and neutrons can be distinguished
by mass, charge, and location in an atom.
Atomic Number and Mass Number
 Atomic Number: equal to the
number of protons in an atom of
that element
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Hydrogen only has one proton
Different elements have different
numbers of protons
S = sulfur = atomic number = 16
Fe= iron = atomic number = 26
 Mass Number: sum of the protons
and neutrons
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Al = 13 protons + 14 neutrons = 27
Ag = silver = atomic number = 47
Isotopes
Isotopes of an element have the same atomic number but different mass numbers because they have
different numbers of neutrons.
Isotopes are atoms of the same element that have different numbers of neutrons and different mass
numbers
Oxygen: 8 protons, 8 neutrons and mass number of 16
Oxygen-16
But some have 9 neutrons and a mass number of 17
Oxygen-17
And some have 10 neutrons and a mass number of 18
Oxygen-18
Water that contains hydrogen-2 atoms (instead of
hydrogen-1 atoms)is called heavy water
Compare physical properties of ordinary water and
heavy water.
Check your understanding:
How are the compositions of heavy water and ordinary water similar?
What type of hydrogen atoms does ordinary water contain?
What type of hydrogen atoms does heavy water contains?
4-3 Atomic Theory
 What can happen to electrons when atoms gain or
lose energy?
 What model do scientists use to describe how
electrons behave in atoms?
 What is the most stable configuration of electrons in
an atom?
Chronological Models of the Atom
Bohr’s Model of the Atom
-constant speeds and fixed orbits, like planets around the sun
Bohr’s models
correctly
introduced the
concept of energy
levels, but energy
levels cannot be
used to describe
the actual location
of an electron.
Electron Cloud Model
The electron cloud
model can be used
to model the
probability that an
electron is in a
certain location but
the exact speed and
location of a single
electron cannot be
determined.
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How is a moving airplane
propeller similar to an electron
cloud?
What other examples can you
think of hat could model the
concept of an electron cloud?
Class Participation Opportunities
 Build or draw models that represent the changes
over time in scientists’ understanding of the atomic
structure.
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A three dimensional version of the time line could be displayed
as a mobile or a diorama
Don’t forget to not the time scale on the time line.
 Research the life of one of the following scientists:
 Dalton, Thomson, Nagaoka, Rutherford, Bohr, de Broglie,
Schrodinger, or Chadwick
 Research what was known about planetary obits
during the same period of time that modern atomic
theory was evolving
Atomic Orbitals
 An electron cloud is a good approximation of how
electrons behave in their orbitals.
Electron Configurations
 The arrangement of electrons in the orbitals of the
atom
 The most stable electron configuration is the one in
which the electrons are in orbitals with the lowest
possible energies.
 When all electrons in the atom have the lowest
possible energies, the atom is said to be ground
stable. (lithium, atomic number of 3, 2 in first orbital
and 1 in second orbital)
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If lithium absorbs energy, one of it’s electrons on the first
orbital can move to the second orbital. This is lithium in an
excited state, not very stable.)
Analogy
 When is the
configuration of the
gymnast like an atom
in an excited state?
 When is the gymnast
most like an atom in its
ground state?