Rutherford Scattering Flashcards
In which direction will the number of a particles per second being scattered be a maximum
Straight on
State what most a particles being straight on suggests about the structure of the atoms in the metal
The atom consists mainly open space
A small number of a particles are scattered through 180 degrees.
Explain what this suggests about the structure of the atoms in the metal
Most of the mass of an atom is contained in the nucleus
The nucleus contains a positive charge
Charge is concentrated at the nucleus
Name the force that is responsible for the deflection of an alpha particle at a nucleus
Electrostatic force
Why is it necessary to remove the air from the chamber in a Rutherford scattering experiment
to prevent deflection of the a particle by the air
Give two conclusions that can be deduced about nucleus from the results of such an experiment
1) nucleus has positive charge
2) nucleus contains most of the mass
3) nucleus is small compared to the separation between nuclei
What force or interaction is responsible for Rutherford scattering
Electrostatic
Why is it essential for there to be a vacuum in the chamber for the experiment
To prevent the alpha particles being absorbed or scattered by air molecules
What observations made from the experiment supported that the nuclear radius of gold «_space;atomic radius
little or no deflection by a majority of alpha particles
What observations made from the experiment supported most of the mass of an atom of gold is contained in its nucleus
Some alpha particles suffer large deflection
Why is it necessary to remove air from the experiment’s apparatus
a particles have a short range in air (0.4m)
Explain why the gold foil should be very thin
the alpha particles must not be absorbed by the foil - must only be scattered once
Explain why the count rate from the detector straight on from the source is much greater than the detector situated in a 45 degree deflection position.
What can be deduced from this observation about the structure of the atom and the properties of the nucleus of gold
majority of a particles pass straight through
most do not pass close enough to be deflected
shows that atoms consist mainly of open space
nucleus much smaller than the atom
most of the mass of the atom is contained in the nucleus
nucleus is positively charged
Why is electron diffraction used to establish the size of a nucleus rather than the diffraction of visible light
it is more accurate because the de Broglie wavelength of the electrons can be made to be of the order 1fm (because the de Broglie wavelength is inversely related to the speed of the particle, by accelerating it quickly enough you reduce the wavelength significantly however relativistic effects must be accounted for) which is equal to the order of the nuclear diameter whereas the wavelength of visible light is around 700nm which is much longer. The more appropriate wavelength of the electrons provides more accurate results upon its diffraction to determine nuclear radius = 0.61*lambda/sinX. The small electron wavelength means it diffracts significantly as its wavelength can be made to be similar to that of a nuclear diameter giving significant diffraction compared to a wavelength much greater than the size of the nucleus (i.e. that of visible light which would give much less diffraction).
What is the plum pudding model
Atoms were spheres of positive charge with negatively charged electrons stuck inside them
Rutherford was the first to suggest that…
atoms did not have uniformely distributed charge or density
Rutherford’s investigation of radioactivity (not nuclear radius) showed that: (2)
1) radiation ionised air making it conduct electricity
2) was two types: alpha (absorbs easily) and beta (penetrating)
3) Gamma was later discovered (most penetrating)
named after order they were discovered
Rutherford showed the existence of _____ by…
a nucleus by probing the structure of the atom by firing a narrow beam of alpha particles at thin gold foil
Why must the gold foil be thin
to prevent the alpha particles from possible scattering more than once
Why was the experiment done in an evacuated chamber
so that the alpha particles were not stopped by the air molecules (roughly 0.4m range in air)
Why did all the alpha particles have the same kinetic energy
so they have the same speed otherwise slow alpha particles would deflect more than faster moving alpha particles on the same initial path
Why was a circular detector placed around the gold foil and alpha source
to detect the deflected alpha particles
The alpha source must have a long enough half-life otherwise…
readings would be lower than earlier readings due to decay
What did the plum pudding model predict would happen in Rutherford’s experiment
alpha particles would deflect slightly as an even distribution of positive charge.
How was the nuclear radius determined via this experiment
The distance away from the nucleus that the particles could reach before they were deflected due to the electrostatic force of repulsion (between the positively charged alpha particles and the positively charged nucleus) could be measured because the initial kinetic energy = potential energy at the point of deflection. At this point, kinetic energy and speed = 0
The distance away from the nucleus > the nuclear radius but gives a rough approximation/a maximum value for the nuclear radius
What happened in the experiment
most alpha particles passed straight through
some were deflected slightly
some were deflected at angles > 90 degrees
What did the results of the experiment show? (3)
- nucleus at centre of atom and +vely charged as repelled close alpha particles
- most of the atom was empty space as most particles passed straight through
- nucleus of atom was small as few particles deflected back
- most of mass must be in the nucleus
The further from the nucleus the particles approach, the _____ the angle of diffraction therefore…
smaller
when they are too far away they do not deflect at all and pass straight on
How is deflection affected by a change in proton number, kinetic energy and neutron number
- increase in proton number increases deflection because there is an increase in positive charge thus increasing the electrostatic force of repulsion between the positively charged alpha particles and the positively charged nucleus. So less protons = less deflection
- increase in kinetic energy of alpha particles allows more particles to pass closer to the atom because they will have more momentum and so there is a larger force for the electrostatic force to overcome
- changing the neutron number has no effect on deflection as have no charge so the electrostatic force of repulsion is unaffected
Nuclear diameter is of the order
1fm = 10^-15m
