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What is the point of learning about models that turn out to be wrong?
Is the way that we tell the history leaving out any experiments that don't lead to the result we want. Is this history or chronology?
A summary of all notes that you need to know: IB Physics NOTES
7.1 Sub Pages
A Big Broad Overview:
The Thomson model
In the early 20th century the accepted model of the atom was referred to as the "Plum Pudding" model. This model was devised by JJ Thomson after his discovery of the electron. The model has electrons in a spherical cloud of positive charge as shown below.
The electrons have negative charge and the atom overall is neutral.
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The Rutherford model
In 1911 Ernest Rutherford modified the accepted model of the atom based on results of experiments carried out by Hans Geiger and Ernest Marsden where alpha particles were fired at thin sheets of metal foil. This model consists of a small but massive nucleus with positive charge surrounded by electrons orbiting the nucleus like planets around the sun, as shown to the right.
Measurement of Mass of Electron
Measurement of Mass of Electron
Electric current is the movement of charged particles called electrons. These can be seen in the Thomson's experiment. See if the direction of the force agrees with Flemings LHR
Using this apparatus it is possible to find the charge/mass ratio for an electron. This is done by balancing electric and magnetic forces so the electrons travel in a straight line. This isn't on the syllabus but it's good application of electric and magnetic fields.
For the case shown the following can be determined:
The electric field strength (E = V/d = F/q)
The electric force on the electron and its direction (F = k qq/ r^2 or F=Eq)
The magnetic force on the electron and its direction (F = qvB)
The balanced forces show that V = Bdv.
Scattering experiments - Geiger-Marsden Experiment
Scattering experiments - Geiger-Marsden Experiment
To find out what is inside a box you could shoot a bullet at it.
For each of the examples given here, describe what would happen to the bullet.
Shooting a bullet at matter would not reveal anything about the structure of the atom for that you need a much smaller projectile. Some materials emit fast moving, very small (1/50 the mass of a gold atom), positive, particles called alpha particles. These can be fired at thin sheets of matter to see what happens. Before discussing the results let's make a prediction
(NOS) scientists often use evidence from experiments to define models that are used to make predictions.
Early models described the atom as a + ball with - charges embedded in it, often called the plum pudding model.
In a neutral atom, what must be true about the + and - charge.
Explain how a balloon becomes charged when rubbed using the plum pudding model.
If atoms are like plum puddings and alpha particles like bullets explain what would happen if an alpha particle would be fired at a thin gold foil only a few atoms thick.
Use the PhET simulation below to investigate alpha particle scattering by nuclei.
Directions:
Click on the blue button to fire the alpha particles.
Check the Traces box in order to see the tracks taken by the alpha particles.
The deflections of the alpha particles by whole atoms, including nucleus and electrons can be observed by clicking on the nucleus surrounded by circles in the upper right hand corner.
The deflections of the alpha particles by a single nucleus can be observed by clicking on the model of the nucleus.
Investigate, then within your notes explain the effect of changing the number of protons or number of neutrons in the nucleus on the deflection of the alpha particles.
Both the videos below will give further explanations of the Gold Foil Experiment (Geiger-Marsden Experiment).
Emission Spectra: The Bohr Model
Emission Spectra: The Bohr Model
The Bohr Model Simulation
In 1913 Niels Bohr proposed a model, particularly for hydrogen, based on observations of the emission spectra of hydrogen gas.
The Bohr model has electrons orbiting the nucleus in discrete energy levels and the electrons can make instantaneous jumps from one energy level to another.
The Bohr Model in the Classroom.
Use the Spectrum Tube Carousel shown below in order to view the emission spectra of a variety of gases.
Set up the apparatus shown below with a spectrometer to view the light from the gases. The spectrometer has an adjustable slit at one end which allows the light through into the collimator. This light is then refracted by the prism and can be observed with the telescope. Make sure that the slit is lined up with the light emitted from the gas tube. Rotate the gas tubes around the carousel in order to view the different gases.
The diagram below shows how the main lines in the emission spectrum of hydrogen are formed.
Violet light, with a shorter wavelength and larger refractive index, is refracted more than the red light.
The Hydrogen (H_2) spectrum as seen through the spectral tubes. Showing the lines at 434 nm, 486 nm and 656 nm respectively. The 410 nm line is often difficult to see with the spectral tubes.
The Helium (He) spectrum as seen through the spectral tubes.
The diagram below shows, in a simple format, the emission spectra for most elements of the periodic table.
Additional Notes: HERE
Quantification of Bohr's Model
Quantification of Bohr's Model
The energy E of a photon of light is proportional to its frequency f according to the equation. The change in energy within the Bohr model, describes the change in energy level within the orbital shells.
where h is Planck's constant and has a value of 6.63 x 10-34 J s.
Since:
then combining:
SHOW THAT:
In your notes, create a graph of both Energy vs Frequency AND Energy vs. Wavelength.
Include important information on your graphs (meaning of gradient/slope, value of the inverse constant (k).
Show that the units within the equation are balanced.
The wavelength of an emitted photon is inversely proportional to the magnitude of the energy jump.
An electron that makes a bigger energy jump between two energy levels will emit a photon with a shorter wavelength but HIGHER energy.
Why are energy levels negative?
Why are energy levels negative?
The Origin of -13.6 eV
The Origin of -13.6 eV
Absorption and Emission Spectra
Absorption and Emission Spectra
Sample Bohr's Model Calcs
Sample Bohr's Model Calcs
In both Joules (J) and electron volts (eV), determine the energy of a photon of blue light (435nm) emitted in the hydrogen spectrum. E = 4.57x10^-19 J = 2.85 eV
Photons of energy 2.3eV are incident on a low-pressure vapour. The energy levels of the atoms in the vapour are shown
What energy transition will occur when a photon is absorbed by the vapor?
A. –3.9eV to –1.6eV ✓
B. –1.6eV to 0eV
C. –1.6eV to –3.9eV
D. 0eV to –1.6eV
A simple model of an atom has four energy levels. What is the maximum number of different frequencies in the emission spectrum of that atom?
A. 4
B. 6 ✓
C. 10
D. 25
Bohr Exam Style Problems:
Bohr Exam Style Problems:
The diagram to the right shows the energy level diagram of a hydrogen atom.
The associated spectrum to the diagram to the right is also shown. The transition labelled A in the top diagram gives the spectral line labelled B in the spectrum diagram.
(a) (3 marks)
Show that the frequency of spectral line B is about 4.6 × 1014 Hz.
Calculate the wavelength represented by line B
The hydrogen atom is excited and its electron moves to level 4. (2 marks)
How many different wavelengths of electromagnetic radiation may be emitted as the atom returns to its ground state?
Calculate the energy, in eV, of the longest wavelength of electromagnetic radiation emitted during this process.
(c) In a fluorescent tube, explain how the mercury vapour and the coating of its inner surface contribute to the production of visible light. (3 marks)
Total 8 marks)
Each title will take you to a different problem w/ solution.
Q1 - Hydrogen spectrum and fluorsecent light - 8 mark question
Q2 - Spectrum and related energy jumps - 8 mark question
Q3 - Explaining how spectra occur - 11 mark question
Q4. - Ionisation and excitation - 12 mark question
Q5. - Hydrogen atom ionised by electron bombardment - 12 mark question
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