Download to the Lesson 26 Notes and Practice Booklet

Lesson 26: Models of the Atom !
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Part 1: Historical Models of the Atom!
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Model of the Atom
Discovered by
Description
Billiard Ball (1808)!
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Plum Pudding (1904)!
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Rutherford Model (1909)!
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Bohr Model (1913)!
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Problems?
Model of the Atom
Discovered by
Description
Problems?
de Broglie Model (1924)!
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Schrodinger Model (1926)!
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Part 2: The Discovery of the Electron!
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Dalton’s Billiard Ball model was the accepted atomic model for most of the 19th century. It was believed
that the atom is the indivisible, smallest unit of matter. !
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In 1897, J.J. Thomson _____________________________________ and showed that there were
smaller particles within atoms. The atom is not the smallest unit of matter!!
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The first cathode rays were experimented with in 1859. At the time,
scientists had no idea what these rays were. !
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Cathode rays appear when a cathode ray tube (CRT) is connected to a
strong voltage source. The beam of what looks like light flows from the
___________ to the _________ and can be produced by metal electrodes
of all kinds. !
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Thomson experimented with these cathode rays and found that
unlike light, these rays would bend in the the presence of a
strong magnetic field. !
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Orsted’s discovery in 1820:!
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Cathode Rays:!
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The Thomson Experiment:!
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Thomson created a special CRT with a adjustable magnetic and electric field inside. This allowed the
CRT to act like a special kind of mass spectrometer. The cathode rays were affected as they travelled
through the magnetic and electric field and the results could be measured. !
STEP 1: Velocity of the Cathode Rays!
When the electric field and the magnetic field were adjusted so the forces were balanced, Thomson
could calculate the speed of the cathode rays. This is also the principle behind a velocity selector on a
mass spectrometer. !
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Step 2: Charge to Mass Ratio!
When Thomson turned the electric field off, only the magnetic field remained. Using the velocity he
found in the first step, he used the radius of curvature for the cathode beam to determine the “charge to
mass ratio” for the cathode ray particles. !
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Thomson was able to show that the cathode ray beam contained particles with both mass and charge.
These particles had an identical Charge to Mass Ratio no matter what kind of metal was used for the
electrode. This indicated that there was the same small, negatively charged particle found in atoms of
all kinds of elements. !
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Later, in 1909, Millikan determined charge of the electron in his famous Oil Drop Experiment !
This allowed him to use Thomson’s Charge to Mass Ratio to calculate the mass of the electron. !
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Example: If a cathode ray beam exposed to a magnetic field of 0.040 T requires an electric field of !
1.2 x106 N/C to remain undeflected, how fast are the cathode rays? !
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Example: When the electric field is turned off, the same cathode ray beam curves with a radius of 4.26
mm in the 0.040 T magnetic field. Use the velocity from the previous example to calculate the charge to
mass ratio for the electrons. !
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Part 3: The Gold Foil Experiment!
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Ernest Rutherford discovered the Alpha Particle in year 1900 at McGill University. The alpha particle is
a very dense, doubly charged, positive ion (later shown to be a bare helium nucleus, containing two
protons and two neutrons). !
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In 1911, Alpha particles were used for the Gold Foil Experiment. In the Gold Foil Experiment, alpha
particles were shot at a very thin strip of gold foil. !
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2)
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Prediction:!
Actual Result!
“It was quite the most incredible event that has ever happened to me in my life. It was almost as
incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you.” Ernest Rutherford (1936)!
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3) Conclusion:!
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Practice Problems:!
!Thomson’s Experiment
!1. Derive the expression for the speed of a cathode ray particle passing straight through a magnetic
and an electric field.
!2. field
the expression to find the charge to mass ratio of a cathode ray particle.
!3. Derive
In Thomson’s experiment on cathode rays, the speed of the cathode ray particles was found to b
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4.0 x 107 m/s in a 1.0 x 10-4 T magnetic field. The corresponding electric field was provided by
charged parallel plates that had a separation of 2.0 cm. What was the potential difference across
the plates? (80 V)
If J.J. Thomson had used alpha particles traveling at 5.0 x 107 m/s through a magnetic field of 2.0
T, what would the radius of deflection be? (0.52 m)
If the charge to mass ratio of a particle is 1.5 x 105 C/kg and the mass is
2.0 x 10-15 kg, what is the charge of the particle? (3.0 x 10-10 C)
A proton travels through a magnetic field at a speed of 5.60 x 105 m/s perpendicular to the field.
If the radius of the arc of the deflected proton is 7.50 mm, what is the magnetic field strength?
(0.779 T)
Alpha particles travel undeflected through magnetic and electric fields that are perpendicular to
each another. The speed of the alpha particles is 7.50 x 105 m/s and the strength of the magnetic
field is 0.220 T. What is the electric field strength? (1.65 x 105 V/m)
Alpha particles travel through a magnetic field of 0.360 T and are deflected in an arc with a radius
of 8.20 cm. What is the energy of each alpha particle?
(6.71 x 10-15 J)
In a cathode ray tube an electron reaches a maximum speed of 4.75 x 107 m/s. If the electron
accelerated from rest, what is the potential difference across the tube? (6.42 kV)
In a cathode ray tube electrons are accelerated from rest by a potential difference of 1.40 kV. If
these electrons enter a magnetic field with a strength of 0.0220 T, what is the radius of arc for the
deflected electrons? (5.74 mm)
A charged particle with a mass of 40 g is suspended in a Millikan oil drop apparatus that has a
separation of 4.0 cm between the charged plates. If the potential difference required for the
suspension is 1.633 x 1016 V, what is the number of excess electrons on the mass? (6)
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(review) In a Millikan oil drop experiment, five electrons are attached to a drop. The oil drop is
placed into an electric field created by parallel plates which are 3.0 cm apart. If the mass of the
oil drop is 3.0 g, what potential difference is required to cause the oil drop to accelerate upwards
at 3.0 m/s2? (1.4 x 1015 V)
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Bohr’s Hydrogen Atom (review)
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Describe how emission and absorption spectra of
Hydrogen are explained by the Bohr model of the
atom.
How much energy must a hydrogen electron in
energy level n = 1 absorb in order to jump to energy
level n = 2 and n = 3? (10.2 eV, 12.1 eV)
Why is a blue photon emitted when a hydrogen
electron jumps from energy level n = 5 to n = 2,
but a red photon is emitted when an electron jumps
from level n = 3 to n = 2?
What is the ionization energy for hydrogen in its
ground state?
What is the wavelength of a photon emitted when an electron in a hydrogen atom falls:
A.
From n = 4 to n = 2. (485 nm)
B.
From n = 5 to n = 1. (94.7 nm)
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A hydrogen atom absorbs a photon in moving from one radius of 5.29 x 10
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m to another of 1.32
x 10 m. What is the energy and wavelength of the photon?
(2.10 x 10-18 J, 94. 7 nm)
What energy is needed to ionize hydrogen from the n=2 state? How likely is this to occur?
Explain. (3.4 eV)
Determine the wavelength and frequency of the fourth Balmer line (n = 6 to n = 2) for hydrogen.
(412 nm, 7.3 x 1014 Hz)