Unit 8: The
Planetary Model of the Atom
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Unit Overview:
In
the last unit, you explored the historical development of the model of the
atom, from its philosophical to scientific conceptions and to the scientific
evidence for subatomic particles. You
examined the spherical, plum-pudding, and nuclear historic models of the atom.
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The
Spherical Model:
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What it’s based on: ● Philosophical Concept ● Law of conservation of matter ● Law of definite proportions ● Law of multiple proportions |
Why it explains the evidence: If matter cannot be created or destroyed and
can only combine in small, whole-number ratios, then whole atoms must react. |
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The
Plum-Pudding Model:
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What it’s based on: ● Thomson’s Cathode Ray Tube Experiment ● The discovery of the electron |
Why it explains the evidence: There is evidence of a negatively-charged
particle within the atom, but matter is also neutral, so the negative
particles are placed into a sea of positive charge. |
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The
Nuclear Model:
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What it’s based on: ● Rutherford’s Gold Foil Experiment ● The discovery of the nucleus |
Why it explains the evidence: Because so few alpha particles encountered a
positive charge within the foil, it cannot be a sea of positive charge, but
rather a dense center of positive charge. |
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In
this unit, you will further examine this development the model of the atom,
focusing on deepening our understanding of the structural components of the
atom.
What are the flaws of Rutherford’s
Nuclear Model of the Atom?
Examination
of the nuclear model left some unanswered questions:
1. How do we describe the
nucleus of the atom?
2. Why do the electrons stay
outside of the nucleus of an atom?
As
scientists answered these questions, the model of the atom continued to change
over time.
How do we describe the nucleus of the
atom?
Two
historical experiments that contributed to our understanding of the nucleus are
described below:
1.
Rutherford’s Gold Foil
Experiment, part 2:
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● Scientist: Ernest Rutherford, 1920
● Summary of experiment: Scattering patterns were examined
for various metal foils.
● Experimental evidence: The scattering patterns
were different for different elements.
● Explanation: The size of the nucleus is
different for different elements.
● Discovery: The proton is a positively charged particle that exists within the
nucleus of the atom.
● Model of the atom: This evidence adds to our understanding of
the nucleus, describing that the size of the nucleus is based on the numbers of
protons in the nucleus of the atom.
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2.
Chadwick’s Beryllium
Experiment:
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● Scientist: James Chadwick, 1920
● Summary of experiment: Alpha particles, highly
energetic, positively-charged particles were shot through a sheet of beryllium,
detecting what happened to them.
○ This experiment was very
similar to Rutherford’s gold foil experiment, except instead of shooting the
alpha particles at a thin sheet of metal foil, they were shot at a thicker
sheet of beryllium metal. This change in
thickness meant that the alpha particles were inside the foil for a longer
period of time.
○ Chadwick chose to extend
Rutherford’s experiment because he was building off of Rutherford’s findings,
adding more information about the nucleus.
● Experimental evidence: A new, uncharged particle
was detected on the other side of the sheet of metal, as well as protons.
● Explanation: This uncharged particle
must also exist inside of the nucleus of the atom.
● Discovery: The neutron is the uncharged particle that exists within the nucleus of
the atom.
● Model of the atom: This evidence adds to our understanding of
the nucleus, describing that the size of the nucleus is also based on the
existence of the neutrons in the nucleus of the atom.
Why do the electrons stay outside of
the nucleus of an atom?
Recall
that Thomson discovered the electron during the Cathode Ray Tube experiment, in
which he noticed that the light ray was attracted to the positive end of a
magnetic field, leading to the explanation that it was made up of negatively
charged particles. So, the question for
chemists looking at the nuclear model of the atom became: “Why aren’t the
electrons simply attracted to the nucleus?”
In
other words, if we know that electrons are negatively charged and we know that
the nucleus is positively charged, then it would make sense that the two parts
of the atom would be attracted to one another.
However, it is only the negatively-charged particles that made up the
cathode ray, indicating that they are outside of the nucleus. Chemists wanted to know what kept the
electrons outside of the nucleus.
Bohr’s Hydrogen Model:
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● Scientist: Neils Bohr, 1913
● Summary of experiment: The light given off by
hydrogen when an electric current is passed through it was separated into its
component colors. These component colors
make up the emission spectrum of the element.
The wavelengths of emitted light were used to calculate the energy
associated with it.
○ This experiment was very
similar to Thomson’s Cathode Ray Tube experiment, except instead of placing the
ray into a magnetic field, the ray of light was passed through a prism to
determine its component colors.
○ Bohr chose to extend Thomson’
experiment because he was building off of his findings, adding more information
about the electrons.
● Experimental evidence: Each band of light emitted
has a specific wavelength associated with it, which also corresponds with a
specific energy value.
● Explanation: The bands of light that are
emitted indicate the movement of an electron between specific energy states.
● Discovery: Energy levels are specific areas around the nucleus that have
different amounts of energy.
○ The ground state of an atom is the state of lowest energy in which all
electrons are in their lowest possible energy levels.
○ The excited state of an atom is a temporary state of the atom in which
electrons are in higher energy levels.
○ When atoms absorb energy (by
heat or electricity), electrons gain energy which enable them to move to an
excited state.
○ When the electrons fall back
to the ground state, energy is emitted in the form of light.
● Model of the atom: This evidence generally answers that energy
keeps the electrons outside of the nucleus of the atom.
What is the planetary model of the
atom?
Together,
the scientific evidence of Rutherford, Chadwick, and Bohr address the questions
of the nuclear model. From that
evidence, the planetary model was developed.
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What
it looks like:
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What
it’s based on: An
extension of Rutherford’s Gold Foil Experiment → protons. Chadwick’s
Beryllium Experiment → neutrons. Bohr’s
Hydrogen Model → energy levels. |
Why
it explains the evidence: The
e |
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Practice: Revisit this online quiz. (But now, you can also
complete last 3 questions.)
BrainPOP Activity
Overview:
In
this activity, you will have another opportunity to examine the historical
development of the atom over time.
Directions:
1. Go to BrainPOP and watch the
video on Atomic Model. If a login is required, please
enter the following:
a. username: jcesc
b. password: qfaf9361
2. Choose between the Graphic
Organizer and the Worksheet. Complete at
least one of them. Be sure to save your document, so that you will be able to upload it to
your log.
3.
Choose between the Make-a-Map and Related Reading. Complete at least one of them. Be sure
to save your document, so that you will be able to upload it to your log.
a. If you choose Make-a-Map, use
the words and/or images to relate the ideas of this unit in a way that makes
sense to you.
b. If you choose Related
Reading, choose at least 1 of the readings to write a 3-sentence summary of.
4. To check your understanding,
complete the quiz.
5.
IMPORTANT: This is considered an off-line activity, be
sure to keep track of the time that you spend on BrainPOP.
ChemLab 1: Bohr Model: Introduction
Overview:
Bohr’s
examination of the emission spectra of gases, specifically hydrogen caused
changes to the model of the atom. In
this lab, you will explore some of these spectra, considering the concept of
energy levels in the atom.
Directions:
1. Download the Student
Exploration and Vocabulary
sheets for the Bohr Model: Introduction.
2. Familiarize yourself with the
words on the vocabulary sheet.
3. Log-in to your Explore
Learning account.
4. Click on “Bohr Model:
Introduction” and launch the gizmo.
5. Answer the Prior Knowledge
Question.
6. Practice using the Gizmo,
using the Gizmo warm-up instructions.
7.
After you are comfortable using the Gizmo, begin the
activity. Use the lab sheet as a guide to complete the 2 activities:
a. Activity A: Absorption
Spectra