Chapter 24 Particle physics Flashcards
What was Thompson’s original model of the atom , plum pudding , features (2)
The atom was a cloud of POSITIVE CHARGE, with negative electrons dotted around
- the atom was overall NEUTRAL everywhere
So what did Rutherford exoeieemnt to try prove / disprove Thompson model entail
To fire alpha particles which are positive being helium nuclei at thin sheet of gold foil.
Observations would be collected on thr positon of where they ended up, using an observer that could measure the angle
If Thompson model correct what was expected to happen when alpha particles were fired through
And why!
Because the charge was supposedly neutral overall, the alpha particles weren’t supposed to be deflected at all and PASS STRAIGHT THROUGH
However as some were deflected, we now know to reject this model
What instead happened in the alpha scattering experiment, what three key observations were made
Instead if all passing through
1) most past through
2) about 1 / 2000 were deflected by a small angle
3) 1/10000 were deflected more than 90° back
Explain what the observations meant and how it changed rhe model
1) most of the particles going through = the atom was mostly empty space
2) the fact that some got deflected a bit = there is some mass in the centre of the atom , most of the mass = nucleus . Fact that most went through means most of the mass must be concentrated in the middle
3) the fact some deflected 90 = the mass in the centre must be POSITIVE
Why does the third observation explain that nuckeus had to be positive and not the second?
This is because even if the nucleus was negative, then there would be some small deflection as it gets ATTRACTED to the nucleus and deflected from straight path
But the atoms would not be completely rebounded, if the nuckeus was negative. This can ONLY HE ACHIEVED DUE TO REPULSION, AND THUS THE NUCLEUS MUST BE POSITIVE
What are considerations of the alpha particle source
4 reasons!
1) each particle must have same speed as the slower ones would be deflected more (more time to be deflected)
2) chamber must be vacuum or they will be stopped by air molecules different amounts
3) foil must be ideally one sheet thick so multiple scatterings don’t happen
4) half life of thr source must be long enough so that later readings don’t disappear due to radial active decay
Summary of alpha scattering
- objective
- observations
- conclusions
Considerations for experiment
Objective
To disprove Thompson, if this moons correct alpha particles go straight through if the charge overall neutral
Observations
1) most went straight through
2) 1/2000 deflected by small angle
3) 1/10000 deflected by more than 90°
Conclusions
1) most of atom empty space
2) there is some mass in the centre, most of the mass cocnentrsted there = nucleus
3) musr be positive if some deflected 90° back, otherwise did negative it would’ve been attracted and none would come complete way back
Considerations about alpha source
1) same speed or slower ones will be deflected more
2) gold sheet ideally one atom thick so doesn’t get deflected more than once
3) vacuum so not bump into air molecules
4) half life long enough ti record without decaying
How ti use method of CLOSEST APPROACH in two ways to estimate the UPPER LIMIT of radius of an atom , usign gold experiment
1) usign energy ideas
- an alpha particle travelling at a certain KE, would be transferred fully to EPE when it MOMENTARILY STOPS and starts to get deflected.
- thus = 1/2mv2 to Qq/4 pi e0 r, and rearrange ti find the closest approach
2) using potential ideas
- at the closest distance, the potential of both particles will be the same, nuckeus and alpha
- so equate them and find the distance r
What’s the problem about using closest approach? (2 ) big problems
-Is that more energetic alpha particles with higher energy will get closer to the atom
- only takes into account protons , due to their repulsion, whereas an isotope of an atom will have a bigger radius , but these calculations ignoring the neutrons will ignore this tok
What was misconception to why distance if closest approach gives radius of nuckeus rather than atom?
Distance if closest approach means the nuclei can pretty much TOUCH EACH OTHER, thus it refers to the radius of the nuckeus rather than the radius of the atom
Why did Rutherford exepiekrn give an upper limit, why is the nuckeus of the radius actually smaller?
Because he used the natural max ke an alpha particle could have , and thus the distance was a certain amount.
More energetic alpha particles will get closer, thus the actual raidus / diameter will be shorter, but it will be maximum 10-14 which he concluded at least
So what is the radius tuehrrford concluded and what is the acc raidus of nuckeus and atom
Radius Rutherford nucleus = 10^-14
Radius actual with more energetic alpha = 10^-15
Radius nuckeus is 10^-10, thus atom is 100000 bigger
What is the equation for raidus of a NUCLEUS
R = ro x A ^1\3
Wheee Ro is a cinstsnt
As you can see the higher the nucleon number rhe higher the radius
What is the mass of a nuckeus gonna be
As neutron and proton roughly have same mass = 1u
Mass gonna be nucleon number x u = AU
