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Flashcards in Mod 8 U2A Deck (60)
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1

What was the method of JJ Thomson's experiment

  1. Acceleration of electrons from cathode (negative) to the anode (positive)
  2. Deflection of electrons by magnetic fields. Cathode ray was deflected with a magnetic field allowed radius of the curvature of the path to be determined. qvB = mv2/r -> q/B = v/Br
  3. Deflection of electrons by both magnetic and electric fields. Electric field was applied to bring cathode ray into a straight path. Effectively balancing electric and magnetic forces. v = E/B
  4. Using size of deflection and measured values of E and B q/m could be determined

2

What were the results of Thomson's experiment

  • Thomson measured q/m of around 1.5 x 1011 C/kg
  • Much larger than charge to mass ratio of all other known charged particles (ions) hence indicating discovery of a new particle
  • cloest value was that of hydrogen atoms which was 2000 times smaller 
  • Thomson found q/m for cathode rays was the same for different gases and different electrode materials providing evidence that electrons are present in all matter. 

3

Conclusion of Thomson's experiment

  • The electron is a particle which is: fundamental, subatomic and negatively charged
  • Cathode rays are streams of electrons flowing from the cathode to the anode 
  • Thomson was only able to measure q/m, could not seperate q or m. This was accomplished later by Millikan. 

4

Outline the production of cathode rays in cold discharge tubes

  1. A small trace of ions and free electrons are present in the residual gas
  2. A high voltage is applied between the cathode (negative) and the anode (positive), creating a strong electric field in the tube
  3. The strong electric field accelerates the ions towards the cathode and electrons towards the anode (cathode rays)
  4. As the ions and electrons accelerate through the tube they collide with other molecules and ionise them, releasing more free electrons
  5. As the electrons accelerate through the discharge tube they collide with the air molecules as a result light is produced

5

What are the three consequences of an electron (cathode rays) colliding with an an atom or molecule

  1. Low energy collision. Electron is deflected (electrical resistance)
  2. High energy collision. Some of the kinetic energy the electrons acquired as a result of the high potential difference dk = qV is converted to light energy (making cathode rays visible)
  3. Very high energy collision. The electron ionises the atom or molecule, resulting in an ion and extra free electron. 

6

Original cathode ray experiments and their conclusions 

  1. Maltese cross, travel in straight lines 
  2. Paddle wheel, have momentum and hence mass (inaccurate and was actually due to the radiometric effect)
  3. Deflection by magnetic fields, must be charged
  4. Cathode rays carry negative charge, negatively charged

7

Diagram of deflection of magnetic fields in Thomson's experiment

8

Method of Millikan's oil drop experiment

  1. Drops of oil are sprayed into a region above a pair of parallel plates 
  2. The drops become ionised either through friction with the nozzle of the spray or by illuminating them with X-rays
  3. Drops can fall into the region between the plates. This region contains an electric field set-up by a voltage across the plates
  4. Once between the plates the drops can be viewed through a microscope. The drops can be accelerated up or down by adjusting teh voltage on the plates, and hence the electric field
  5. The voltage is adjusted such that the drops are suspended (qE = mg, q = mg/E)

9

Results of Millikan oil drop experiment

  1. q = mg/E (after balancin qE and mg)
  2. E = V/d (known) and g = 9.8ms-2 (just need mass)
  3. Mass of oil drop was calculated using the oil drops terminal velocity, when Fnet = 0 since air resistance balances out gravitational force, and measured their diamter to find mass
  4. Then use mass of oil drop to calculate the charges of many oil drops and worked out the charges of the oil drops were always an integer multiple of some constant
  5. Constant was charge of an electron, this value was then used with Thomson's q/m to work out the mass of an electron.

10

Outline Thomson's Plum Pudding model of the atom (Disproved by Gieger Marsden) 

  • Thomson suggested that an atom is a sphere of positive charge in which electrons are embedded like raisins in a plum pudding
  • No protons in the PP model as they had not yet been discovered

11

Outline the method of the Geiger-Marsden experiment

  1. A narror beam of alpha particles from a radioactive source was aimed at a thin foil of metal such as gold
  2. A zinc sulfide screen was used to detect the position of the deflected alpha particles (Zinc sulfide fluoresces when struck by alpa particles)

12

Theoretical prediction of the Geiger-Marsden experiment

  • On the basis of the Thomson model of the atom Rutherford expected the alpha particles to pass through the foil virtually undeflected
  • Based on Thomson's model, Rutherford expected that the gold atoms would not affect the positive alpa particles because in gold atoms there is no concentation of of charge and mass large enough to deflect the alpha particles 

13

Why doesn't Rutherford's model work

  • Cannot explain why atoms are stable
  • Cannot explain the aborption and emission spectra of atoms

14

Experimental results of the Geiger Marsden experiment

  • Most of the alpha particles passed through undeflected as expected 
  • Some were very significantly deflected
  • A few bounced back straight towards the source

15

Conclusion of the Geiger Marsden experiment

  • Rutherford concluded that there must be a small, massive, positively charged core in the atoms that causes the alpha particles to deflect
  • Analysis was:
  • No deflection, most particles, alpha particles do not interact with the nucleus
  • Significant deflection, some particles, alpha particles interact weakly with the nucleus
  • Reflected to source, very few particles, alpha particles pass close to nucleus and interact strongly

16

Outline details of Rutherfords model (does not work)

  • The nucleus is a central region that is very small relative to the total size of an atom
  • The nucleus contains virtually all of the mass and all the positive charge of the atom
  • The positive charge of the nucleus is to be balanced by the negative charge on the electrons outside the nucleus to maintain electrical neutrality
  • The electrons orbit around the nucleus in uniform circular motion. 

17

Explain why Rutherfords model cannot explain why atoms are stable

 

  • Electrons orbit nucleus in uniform circular motion which requires centripetal force
  • Hence the electrons are accelerating
  • According to Maxwell's theory accelerating charges emit EM waves
  • Meaning electrons would lose energy by EMR
  • Losing energy would cause them to orbit increasingly close to the nucleus, with increasing frequency, ultimately leading to collapse of the atom
  • This does not happen as atoms are stable

18

Explain why Rutherford's model cannot explain the absorption and emission spectra of atom

  • At the time of Rutherfords work scientists had characterised the emission and absorption spectra of different element
  • Hydrogen and other elements absorb only specifc wavelengths and emit light only at the wavelengths they absorb. 
  • Rutherford's model had no structure for the electrons, they were free to orbit at any radius, hence they should be able to absorb and emit radiation at any wavelength

19

What is the main cause for Rutherford's model being unable to explain the stability of atoms and absorption and emission spectra

  • Problems arise due to the classical nature of Rutherford's model.
  • It treats the motion of the electrons using Newton's laws and makes predictions based on Maxwell's theories
  • These are both classical physics theories which do not work at a subatomic level, this requires quantum physics.  

20

What are Bohr's postulates

  1. Only certain electron orbits are stable
  2. Radiation is absorbed or emitted when electrons transition from one stable orbit to another (higher energy they absorb energy, if they move to a lower energy they emit energy), difference in energy is Ei - Ef = hf
  3. The stable electrons have quantised angular momentum Ln = mvr = nh/2pi

21

Outline Rutherford's identification of the proton

  1. Rutherford observed that hydrogen nuclei were produced when alpha particles were fired at nitrogen
  2. From this he concluded that nitrogen nuclei (and hence all nuclei) contained hydrogen within them
  3. The nuclear reaction that was occuring in his experiment was: 147N + alpha -> 178O + 11H
  4. Rutherford proposed that the nucleus of hydrogen consisted of a fundamental building block which made up all other nuclei
  5. He named this particle the proton in 1920

22

Outline method of James Chadwick's disovery of Neutrons

  • Polonium is used as an alpha source and the emitted alphaa particles are directed at a piece of beryllium
  • When struck the berryllium emitted mysterious "neutral rays" which were highly penetrating
  • Paraffin wax (which contains a lot of hydrogen) was placed in the path of the mysterious radiation. When the rays hit the paraffin, they knocked protons out of it
  • Due to their charge, the protons were able to be detected by a Geiger counter and their energy and speed could be characterised

23

Explain the conclusion of Chadwick's experiment

  • The following reaction was taking place between the alpha particles and the berrylium which resulted in the release of neutrons: 94Be + alpha -> 126C + 10n
  • Neutrons travelled at high speed and collided with the nuclei in the target 
  • Collisions caused the nuclei to be knocked out
  • Nuclei was easily detected as they were charged their mass was known and the speed could be measured
  • This allowed energy and speed to be measured 
  • Using conservation of energy and momentum, Chadwick showed that whatever was colliding with the nuclei must have been an uncharged particle with a mass close to that of the proton i.e. a neutron

24

What is the emission spectrum of a chemical element

  • This is the spectrum of frequencies of electromagnetic radiation emitted due to an electron undergoing a transition from a high energy state to a lower energy state 
  • Ei - Ef = hc/lamda

25

What is the absorption spectrum of chemical elements

  • This is the spectrum of frequencies of electromagnetic radiation absorbed due to an electron undergoing a transition from a low energy state to a higher energy state
  • Ei - Ef = hc/lamda

26

Which problems of Rutherford's model did Bohr's model fix

  • Bohr's atomic model was stable by definition
  • Provided an explanation of the atomic spectrum of hydrogen

27

Outline how Bohr was able to explain the existence of line spectra

  1. Electrons exist in stable orbits only which have quantised angular momentum and energy, En = -13.6eV/n2, Ln = nh/2pi
  2. Electrons will absorb and emit photons when they transition from one stable orbit to another
  3. Since the energy of the stable orbit is fixed, the energy difference and hence photon energies will be fixed 
  4. Wavelengths absorbed will be fixed

28

General flowchart for extended response question

  1. Impact on scientific thinking (what is it about) Describe
  2. Change (before and after) Evaluate
  3. How did ..... improve our understanding of ..... (Evaluate)

29

Which quantum number does the Balmer series correspond to

2

30

What is significance of the Balmer series, any number n = 3-6 to 2 

This transition of any n value to n = 2 produces wavelengths in the visible light spectrum