Download Structure of the atom

Chapter 3
Atoms: The Building Blocks of
Matter
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
• Although John Dalton thought atoms were indivisible,
investigators in the late 1800’s proved otherwise.
• Scientific investigations of matter showed that atoms
are actually composed of several basic types of smaller
particles.
• Today, an atom is 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
and neutrons are found in the nucleus. 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 (cathode rays) travel from the
cathode(- terminal) to the anode (+ terminal), 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
Cathode Ray Tube
Millikan’s Experiment
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Discovery of the electron
• Thomson concluded that all cathode rays are
composed of identical negatively charged particles,
which were named electrons.
• Thomson was also able to find the charge to mass ratio
of the electrons.
• In 1909, experiments conducted by Robert A. 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, two other
inferences were made about atomic structure:
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, where he believed 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 moving, 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.
Rutherford’s Experiment
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Chapter 3
Section 2 The Structure of the Atom
Gold Foil Experiment
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Discovery of the atomic nucleus
• Rutherford reasoned that the alpha particles must have
experienced some powerful force within the atom.
• The source of this force occupies a very small amount
of space, because so few of the total number of alpha
particles had been deflected.
• He concluded that the force must be caused by a very
densely packed bundle of matter with a positive electric
charge, which he called the nucleus.
• Rutherford discovered that size of the nucleus was very
small compared to the size of the atom.
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Rutherford’s atom
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The Bohr Model
• Rutherford’s model proposed that negatively charged
electrons were held in an atom by the attraction
between them and the positively charged nucleus.
• In 1913, scientist Niels Bohr proposed a model in which
he placed each electron in a specific energy level.
• According to Bohr’s atomic model, electrons move in
definite orbits around the nucleus, much 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
Bohr’s atom
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The Wave Model
• Bohr’s model worked well in explaining the structure
and behavior of simple atoms such as hydrogen.
However, it did not explain more complex atoms.
• Today’s atomic model is based on the principles of
wave mechanics, which involve complex mathematical
equations.
• According to this model, electrons do not move about
an atom in a definite path like planets around the sun.
In fact, it is impossible to determine the exact location
of an electron. Scientists can only predict where an
electron is most likely to be found.
• The probable location of an electron is based on how
much energy the electron has.
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The Wave Model
• According to the modern atomic model, 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.
• 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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Orbitals
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