Rutherford Scattering

Question 1

In which direction will the number of a particles per second being scattered be a maximum

Answer 1

Straight on

Question 2

State what most a particles being straight on suggests about the structure of the atoms in the metal

Answer 2

The atom consists mainly open space

Question 3

A small number of a particles are scattered through 180 degrees.
Explain what this suggests about the structure of the atoms in the metal

Answer 3

Most of the mass of an atom is contained in the nucleus
The nucleus contains a positive charge
Charge is concentrated at the nucleus

Question 4

Name the force that is responsible for the deflection of an alpha particle at a nucleus

Answer 4

Electrostatic force

Question 5

Why is it necessary to remove the air from the chamber in a Rutherford scattering experiment

Answer 5

to prevent deflection of the a particle by the air

Question 6

Give two conclusions that can be deduced about nucleus from the results of such an experiment

Answer 6

1) nucleus has positive charge
2) nucleus contains most of the mass
3) nucleus is small compared to the separation between nuclei

Question 7

What force or interaction is responsible for Rutherford scattering

Answer 7

Electrostatic

Question 8

Why is it essential for there to be a vacuum in the chamber for the experiment

Answer 8

To prevent the alpha particles being absorbed or scattered by air molecules

Question 9

What observations made from the experiment supported that the nuclear radius of gold &laquo_space;atomic radius

Answer 9

little or no deflection by a majority of alpha particles

Question 10

What observations made from the experiment supported most of the mass of an atom of gold is contained in its nucleus

Answer 10

Some alpha particles suffer large deflection

Question 11

Why is it necessary to remove air from the experiment’s apparatus

Answer 11

a particles have a short range in air (0.4m)

Question 12

Explain why the gold foil should be very thin

Answer 12

the alpha particles must not be absorbed by the foil - must only be scattered once

Question 13

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

Answer 13

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

Question 14

Why is electron diffraction used to establish the size of a nucleus rather than the diffraction of visible light

Answer 14

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).

Question 15

What is the plum pudding model

Answer 15

Atoms were spheres of positive charge with negatively charged electrons stuck inside them

Question 16

Rutherford was the first to suggest that…

Answer 16

atoms did not have uniformely distributed charge or density

Question 17

Rutherford’s investigation of radioactivity (not nuclear radius) showed that: (2)

Answer 17

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

Question 18

Rutherford showed the existence of _____ by…

Answer 18

a nucleus by probing the structure of the atom by firing a narrow beam of alpha particles at thin gold foil

Question 19

Why must the gold foil be thin

Answer 19

to prevent the alpha particles from possible scattering more than once

Question 20

Why was the experiment done in an evacuated chamber

Answer 20

so that the alpha particles were not stopped by the air molecules (roughly 0.4m range in air)

Question 21

Why did all the alpha particles have the same kinetic energy

Answer 21

so they have the same speed otherwise slow alpha particles would deflect more than faster moving alpha particles on the same initial path

Question 22

Why was a circular detector placed around the gold foil and alpha source

Answer 22

to detect the deflected alpha particles

Question 23

The alpha source must have a long enough half-life otherwise…

Answer 23

readings would be lower than earlier readings due to decay

Question 24

What did the plum pudding model predict would happen in Rutherford’s experiment

Answer 24

alpha particles would deflect slightly as an even distribution of positive charge.

Question 25

How was the nuclear radius determined via this experiment

Answer 25

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

Question 26

What happened in the experiment

Answer 26

most alpha particles passed straight through
some were deflected slightly
some were deflected at angles > 90 degrees

Question 27

What did the results of the experiment show? (3)

Answer 27

• 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

Question 28

The further from the nucleus the particles approach, the _____ the angle of diffraction therefore…

Answer 28

smaller

when they are too far away they do not deflect at all and pass straight on

Question 29

How is deflection affected by a change in proton number, kinetic energy and neutron number

Answer 29

• 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

Question 30

Nuclear diameter is of the order

Answer 30

1fm = 10^-15m