Download WS#7 Developing the Atomic Model

Scientist:
Per:
Date:
UNIT 1:
STRUCTURE OF MATTER
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#7
DEVELOPING THE ATOMIC MODEL
Objective: I can evaluate which model of the atom is supported by historical experiments by analyzing 4
different atomic models, considering current evidence and writing a TIEA paragraph.
Over time, scientists have proposed many different models (or ideas) about what the atom might
be like. The pictures and descriptions below show four of the most well-known models of the atom:
Plum Pudding Model
Solid Sphere Model
That atom can be divided into fluid (the “pudding”)
and electrons (the “plums”). Most of the atom is
made of fluid. The fluid spreads out over the whole
atom and is positive. The electrons are scattered
throughout and are negative.
Bohr Model (Planetary Model)
The atom is a tiny, solid sphere that cannot be
divided up into smaller particles or pieces.
The nucleus contains protons and neutrons and is
located in the center of the atom. The protons are
heavy and positively charged. The neutrons are also
heavy, but they are neutral. The electrons are tiny
and negatively charged and circle the nucleus in
fixed energy levels (shells)
The atom can be divided into a nucleus and electrons.
The nucleus occupies a small amount of space in the
center of the atom. The nucleus is dense and
positively charged. The electrons are tiny and
negatively charged. Most of the atom is empty space.
Nuclear Model
J.J. Thompson (1897): Cathode Ray Experiment
On each figure below, talk to the text:
Figure 1
J.J. Thompson passed an stream of electric current through many types of atoms in a vaccum tube.
When he did this, he observed a glowing ray called the “cathode ray” come out of the various atoms.
The cathode rays passed through thin sheets of foil, so J.J. Thompson knew that the cathode rays
coming from the atoms were made of particles much smaller and lighter than normal atoms.
Figure 2
When J.J. Thompson exposed the tiny particles in the
cathode rays to positive and negative charge, they
were attracted to positive charge (moved toward it)
and repelled by negative charge (moved away from it).
He reasoned that the small particles were negative
(since they were attracted to positive charge); he
called these negative particles ‘electrons’ inside of the
atom.
Question: Which atomic model can we eliminate based on Thomson’s experiment?
Claim:
Evidence:
Reasoning:
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Ernest Rutherford (1911): The Gold Foil Experiment
Figure 1
In the early 1900’s, it was discovered that certain types of rock emitted (shot out)
positive particles, called ‘alpha particles’. These positive particles contained two
positive charges (2+), and had the same mass as a helium atom.
Figure 2
Rutherford fired a stream of alpha particles at an
extremely thin piece of gold foil. Given the high
energy of the particles, Rutherford expected the
particles to shoot straight through the metal. To
his great surprise however, about 1% of the alpha
particles were deflected, or bounced, to the side or
Expected Results
Actual Results
backwards.
Figure 3
Rutherford reasoned that, since most of the alpha particles went straight
through the foil, most of the gold foil was empty space. But what was
causing the alpha particles to bounce to the sides and backwards? He
realized that some of the alpha particles were hitting a dense (small and
heavy), positive center. He called this center the ‘nucleus’.
Question: Which atomic model can we eliminate based on Rutherford’s experiment?
Claim:
Evidence:
Reasoning:
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What’s in the nucleus?
Ernest Rutherford (1918) and James Chadwick(1932):
Experiments by Rutherford in 1918 detected heavy positive particles that were emitted from atoms.
He called these positive particles ‘protons’, and reasoned that they must be in the positive nucleus.
However, the atomic model was still not complete. Experiments
by Chadwick in 1932 detected a yet another new particle.
Colliding atoms together at high speed, he detected uncharged
(neutral) particles which he called ‘neutrons’. When Chadwick
measured the mass of a neutron, he found that it was heavy, so
the neutrons must be located inside the small, heavy (dense)
nucleus of the atom with the protons.
Where are the electrons?
Niels Bohr (1913)
With the model of the nucleus emerging, one big question remained unanswered. Where were the
electrons? Scientists struggled with the question as they couldn’t imagine a stable configuration between
the positively charged nucleus and the negatively charged electrons. If protons and electrons held
opposite charges, shouldn’t the negatively charged electrons be attracted to (and fall into) positively
charged nucleus?
In examining electromagnetic radiation emissions of the hydrogen atom, Niels Bohr concluded that the
electrons must be located in specific “energy levels” (sometimes called “shells”). He imagined the
electrons orbiting the nucleus much like the planets orbit the Sun. It is a model that is not perfect, yet
remains useful for introducing students to basic atomic structure.
Bohr’s model proved groundbreaking in many respects and he won the 1922 Nobel Prize for his efforts.
But Bohr’s model was based on the hydrogen atom and failed to account for new experimental data
produced when using other elements, such as helium. His model was later superceded by the Quantum
Mechanical Model put forth by Heisenberg and Shroedinger. Nevertheless, the Bohr model of the atom
remains useful as an instructional tool and student model to this day.
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Atomic Model Conclusion: Which Model Best Explains the Evidence?
Now that you have eliminated all of the atomic models except for one, you need to present your
conclusions about the structure of the atom to other scientists. Scientists communicate to each other by
writing, so you must write a well-reasoned TIEA paragraph that explains your conclusions.
Write your paragraph in response to the following prompt:
Which model of the atom best explains the experiments you read in class? Use evidence from each
experiment (Thompson, Rutherford, and Bohr) to justify your answer.
T: According to the current model of atomic structure, the atom is made of ______________________________________________
____________________________________________________________________________________________________________________________________
_____________________________________________________________________________________(describe the basic structure of the atom).
I: Throughout scientific history, several famous scientists including _______________________, ____________________________,
and ______________________________ conducted experiments to discover the structure of the atom.
E1: For example, _____________________________ (name of scientist) found that ________________________________________________
____________________________________________________________________________________________________________________________________
___________________________________________________________________________________________________________________________________.
A1: This fits with the current model of atomic structure, because _________________________________________________________
____________________________________________________________________________________________________________________________________
___________________________________________________________________________________________________________________________________.
E2: Later, _____________________________ (name of scientist) found that ________________________________________________________
____________________________________________________________________________________________________________________________________
___________________________________________________________________________________________________________________________________.
A2: This fits with the current model of atomic structure, because _________________________________________________________
____________________________________________________________________________________________________________________________________
___________________________________________________________________________________________________________________________________.
E3: Finally, _____________________________ (name of scientist) found that ______________________________________________
____________________________________________________________________________________________________________________________________
___________________________________________________________________________________________________________________________________.
A3: This fits with the current model of atomic structure, because _________________________________________________________
____________________________________________________________________________________________________________________________________
___________________________________________________________________________________________________________________________________.
C(conclusion): These experiments led to the discovery of _________________________________________________________________
____________________________________________________________________________________________________________________________________
___________________________________________________________________, which are all included in the current model of the atom.
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