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Chapter 3
Section 2 The Structure of the Atom
Objectives
• Summarize the observed properties of cathode rays
that led to the discovery of the electron.
• Summarize the experiment carried out by
Rutherford and his co-workers that led to the
discovery of the nucleus.
• List the properties of protons, neutrons, and
electrons.
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Chapter 3
Section 2 The Structure of the Atom
Discovery of the Electron
Cathode Rays and Electrons
• Experiments in the late 1800s showed that cathode
rays were composed of negatively charged
particles.
• These particles were named electrons.
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Chapter 3
Section 2 The Structure of the Atom
Discovery of the Electron, continued
Charge and Mass of the Electron
• Joseph John Thomson’s cathode-ray tube
experiments measured the charge-to-mass ratio of
an electron.
• Robert A. Millikan’s oil drop experiment measured
the charge of an electron.
• With this information, scientists were able to
determine the mass of an electron.
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Chapter 3
Section 2 The Structure of the Atom
Discovery of the Electron, continued
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Expanding the Modern Atomic
Theory
J.J. Thompson
J.J. Thompson is the person who is credited for
discovering the electron. Thompson created a tube
that had a positively charged anode on one side and a
negatively charged cathode on the other side.
Thompson then applied a magnet to the middle of the
tube and discovered that negatively charged particles
were emanating towards the positive magnetic field.
From this, Thompson concluded that negatively
charged particles, called electrons, were present in
atoms.
Thompson then created the Plum Pudding model,
which suggested that electrons and protons were
randomly placed throughout the atom.
The “Plum Pudding”
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Chapter 3
Section 2 The Structure of the Atom
Discovery of the Atomic Nucleus
• More detail of the atom’s structure was provided in
1911 by Ernest Rutherford and his associates Hans
Geiger and Ernest Marsden.
• The results of their gold foil experiment led to the
discovery of a very densely packed bundle of matter
with a positive electric charge.
• Rutherford called this positive bundle of matter the
nucleus.
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Chapter 3
Section 2 The Structure of the Atom
Gold Foil Experiment
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Rutherford's Experiment continued…
•
When the data from this
experiment was gathered the
scientists were shocked.
•
Alpha particles were known
to be dense and at that time
atoms where thought to be
far less dense.
•
One experiment likened
having an alpha particle
bounce back to firing an
artillery shell into a roll of
toilet paper and having it
bounce back.
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The nucleus is very small and
VERY dense
Relative scale model of an atom and the solar system
Do you perceive a gold ring to contain a larger fraction of
solid matter than the solar system?
On this scale, the nearest star would be a little over 10,000
miles away.
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Rutherford's Experiment
In 1911 British scientist Ernest Rutherford
set out to test Thomson’s proposal by
firing a beam of charged particles at
atoms.
Alpha particles are heavy particles with
twice the positive charge of a proton.
Alpha particles are now known to be the
nuclei of helium atoms, which contain two
protons and two neutrons.
Ernest Rutherford
Ernest Rutherford's experiment was to
emit alpha particles towards a thin gold
sheet. Rutherford would then determine
where the deflections of the alpha particles
would go, and therefore be able to theorize
what kind of placement protons and
electrons had.
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Rutherford's Experiment continued…
Rutherford observed that most of the alpha particles went strait
through the foil. However a large proportion were deflected through
small angles and some (though very few) deflected straight back.
Rutherford then theorized that there was something called a nucleus,
which contained a high density of positively charged particles.
Rutherford was able to say there was a nucleus because alpha
particles that deflected right back must have hit something more
massive and with a strong positive charge. This led Rutherford to
propose a very different model for the atom.
Instead of supposing that the positive charge and
mass were spread throughout the volume of the
atom, he theorized that it was concentrated in the
center of the atom. Rutherford called this
concentrated region of electric charge the nucleus
of the atom.
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Chapter 3
Section 2 The Structure of the Atom
Composition of the Atomic Nucleus
• Except for the nucleus of the simplest type of
hydrogen atom, all atomic nuclei are made of
protons and neutrons.
• A proton has a positive charge equal in magnitude
to the negative charge of an electron.
• Atoms are electrically neutral because they contain
equal numbers of protons and electrons.
• A neutron is electrically neutral.
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Chapter 3
Section 2 The Structure of the Atom
Composition of the Atomic Nucleus,
continued
• The nuclei of atoms of different elements differ in their
number of protons and therefore in the amount of
positive charge they possess.
• Thus, the number of protons determines that atom’s
identity.
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Chapter 3
Section 2 The Structure of the Atom
Composition of the Atomic Nucleus,
continued
Forces in the Nucleus
• When two protons are extremely close to each other,
there is a strong attraction between them.
• A similar attraction exists when neutrons are very close
to each other or when protons and neutrons are very
close together.
• The short-range proton-neutron, proton-proton, and
neutron-neutron force that holds nuclear particles
together is referred to as strong nuclear force.
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Chapter 3
Section 2 The Structure of the Atom
The Sizes of Atoms
• The radius of an atom is the distance from the center
of the nucleus to the outer portion of its electron
cloud.
• Because atomic radii are so small, they are
expressed using a unit that is more convenient for the
sizes of atoms.
• This unit is the picometer, pm.
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