Topic E Key Terms Flashcards
what were the observations and hypothesis did the Geiger-Marsden Experiment create?
Observation: Large majority of the α particles go straight through the gold leaf
Hypothesis: Most of an atom is empty space and therefore the mass of the atom must be held in a small region, called the nucleus
Observation: A small of the α particles are scattered.
Hypothesis: No useful hypothesis can be considered.
Observation: A tiny (1:8,000) number of α particles undergo LARGE ANGLE SCATTERING. (reflected back through more than 90 degrees)
Hypothesis: There must be a small, positively charged nucleus, that contains almost all of the mass of the atom.
What does Niels Bohr think about electrons in atoms?
Niels Bohr postulated that electrons can only exist in discrete quantised energy levels of the atom.
What did The Rutherford / Standard Model of the Atom conclude?
Since the large majority (>99%) of Alpha particles are not deflected the large majority of the atom is mainly empty space
A small number of Alpha particles undergo a small angle scattering (not useful)
A very small number (1/8000) undergo LARGE ANGLE SCATTERING (deflected more than 90o from their original course).
What does Z represent?
proton number
What is proton number
number of protons in the nucleus. The proton number dictates the element.
What does A represent?
nucleon number
what is nucleon number
number of protons plus neutrons in the nucleus.
What is a Nuclide?
A nuclide is a specific NUCLEUS with a specific number of protons and neutrons.
What are Isotopes
Different nuclides of the same element are called ISOTOPES. Isotopes are therefore NUCLIDES with the same number of protons but different numbers of neutrons.
What are Nucleons?
A nucleon is a particle in a nucleus, therefore a nucleon is a proton or a neutron.
What are Bohr Postulates?
The Bohr Postulate is that there is QUANTISATION OF THE ANGULAR MOMENTUM of the electrons in atoms. (Basically the electrons act as waves (HL material) and form standing waves around the outside of the atom.) Therefore the electrons can only exist in DISCRETE QUANTISED energy levels.
How does the emission line spectra work?
The electrical or thermal energy can …EXCITE…. the electrons into a higher energy level.
When the electron “relaxes” it emits a photon of the same energy as the energy change.
What are photons?
Photons were described in Einstein’s model of light.
Einstein said a photon is a packet of energy.
What happens to the energy of an alpha particle at the closest approach
At this distance r0, the entire initial kinetic energy of the α-particle gets converted into electrostatic potential energy of the system.
What is the photoelectric effect?
The photoelectric effect is the release of an electron from a metal surface after light/photons are incident.
What is the work function?
The work Function is the minimum energy required to free an electron from a metal surface. This is different for different metal.
What is the Photo-electron Effect?
The photo-electron effect is the release of an electron from a metal surface caused by the absorption of a photon.
How was the photoelectric effect discovered?
through Millikan’s Experiment (not Millikan’s oil drop expt)
What is stopping potential?
The PD required to stop the most energetic photoelectrons.
What is the threshold frequency?
the frequency below which no photoelectrons are emitted, regardless of the intensity or the time of illumination. At the threshold frequency; hf = 𝝓
Photo-electron effect Light observation
In classical physics - light is a wave: Light is a wave continuously transferring energy
In Quantum Physics The Einstein Model of light: Light is photons, which are discrete packets of energy
Photo-electron effect There is a threshold frequency below which no photoelectrons are emitted. (eg red light emitted no photoelectrons) observation
In classical physics - light is a wave: Lower frequency waves may not deliver enough energy to create a photo electron instantly, but given more time the light should deliver enough energy to release photo electrons.
In Quantum Physics The Einstein Model of light:
Any photon below the threshold frequency cannot be absorbed.
Only photons at or above the Threshold Frequency are absorbed and produce photoelectrons.
Photoelectrons are emitted instantaneously.
Photo-electron effect Regardless of the intensity the light below the threshold frequency emitted no photoelectrons. Observation
In classical physics - light is a wave:
Brighter light, ie. MORE INTENSE LIGHT (greater amplitude) will provide more energy and should emit photoelectrons.
In Quantum Physics The Einstein Model of light:
No photon below the threshold can be absorbed. Having more photons below the threshold frequency makes no difference, they are still not absorbed.
Photo-electron effect The frequency of the light is above the threshold frequency, eg. Blue light.
You increase the intensity of light. The current increases but the stopping potential stays the same.
Observation
In classical physics - light is a wave:
Light with a greater intensity provides more energy and should emit both more and more energetic photoelectrons
In Quantum Physics The Einstein Model of light:
More intense light means more photons.
However, all the photons have the same energy. E = hf
Therefore, more photoelectrons are released creating an increased current.
But the photoelectrons are equally energetic, so the stopping potential is unchanged.
Photo-electron effect You are above the threshold frequency, eg blue. You increase the Frequency of the light to for example violet. You get the same current but the stopping potential increases. Observation
In classical physics - light is a wave:
Light with a greater frequency provides more energy and should emit both more and more energetic photoelectrons
In Quantum Physics The Einstein Model of light:
The energy of each photon is greater E = hf
But there are the same number of photons. One photon can only release one photoelectron.
Therefore, the current is the same but the stopping potential increases.
What is the conclusion of the Compton scattering
Compton is additional evidence of the particle behaviour of light.
Compton Scattering can be considered as better evidence for the particle behaviour of light than the photoelectric effect:
KE is transferred (evident of particle behaviour)
Momentum is conserved (evident of particle behaviour)
Compton Shift (change of wavelength) is a result of energy exchange and cannot be explained through wave behaviour (eg. this is not refraction)
Why are some nucleus stable?
Coulomb Repulsion is BALANCED by Strong Nuclear Force Binding in a stable nucleus
What is Binding Energy?
Binding Energy is the work done in separating a bound nucleus into its separate isolated component nucleons.
What is mass defect?
Mass Defect is the mass difference (mass “lost”) between the separate isolated nucleons and the bound nucleus.
What is binding energy equivalent to?
Binding energy is the energy equivalence of the Mass Defect. ( E = mc2 )
What is the unified atomic mass unit (U)
The UNIFIED ATOMIC MASS UNIT (U) is one twelfth of the mass of a Carbon-12 ATOM.
What is the average binding energy per nucleon a measure of?
Stability
What is radioactive decay?
Radioactive decay is the spontaneous emission of Alpha, Beta or gamma radiation to allow the nucleus to move to a lower more stable energy level.
What are alpha particles?
What are Beta particles?
What is gamma?
