Chapter 24 Again Flashcards
Thompson original model of the atom
Plum pudding model which modelled as a sea of positive charge spread out, with negative electrons embedded in between
The sea was pudding and the electrons plum
What was the main feature of plum pudding modelling
That as the positive charge was SPREAD out, the atom was NEUTRAL EVERYWHERE
What did Rutherford and Co do to prove / disprove Thompson model
Alpha scattering gold foil experiment
How was alpha scattering experiment conducted
A radiacotive source decayed to produce beams of alpha particles which were targeted at a thin sheet of metal foil, which was only a few atoms thick
They would then observe how the particles got scattered , if they did at all, using a microscope and moving this around at different angles
Where it would collide it would produce a spec of light, that’s how they detected
What was EXPECTED TO HAPPEN if plum pudding was correct
But what happened which meant…
If it was correct, as the atom was overall neutral, even though alpha particles straight positive, they should’ve all just passed straight through, and not be deflected at all
But the fact that some were deflected meant Thompson automatically wrong, and a new model suggested
What were the observations
1) that most of the alpha particles DID go through
2) that some were deflected by a small angle, 1/2000
3) that very few were deflected COMPLETELY at angles of more than 90° , 1/10000
What did the 3 observations mean?
2nd important !
1) the fact that most passed through, meant that the atom was MOSTLY EMPTY SPACE
2) the fact that some were deflected suggested there had to be some charged mass concentrated at the centre of the atom
- the fact that most went through suggested most of the mass was concentrated at the centre, which was the nucleus !
3) the fact that some were deflected if angles MORE THAN 90 meant this charged mass HAD TO HE POSITIVE , ( if negative it would never deflect , as alpha positve)
Just combine 1 and 2
- the fact that most of alpha particles went through but some were deflected suggested the atom was mostly empty space, and that most of the mass of the atom was concentrated at the centre, which had to be charged. This is the nucleus
2) more than 90, suggested positively charged
Why does third explain psotive and 2nd doesn’t
This is because fact some were deflected, could be deflected due to attraction if centre was negative, can’t tell from afar
But fact deflected more than 90, meant middle had to be positive, as this can only be achieved via repulsion
What were some specific considerations of the experiment ?
LEARN
1) each particle must have the same speed , else the slower ones would be deflected more (more time to be deflected)
2) chamber must be evacuated or else they would be stopped by air molecules by different amounts
3) foil must be ideally one atom thick so MULTIPLE scatterings didn’t occur
4) half life of the source must be long enough such that later readings didn’t disappear when time to meander!
How to use closets apporsch to find an UPPER LIMIT FOR THE RADIUS of an atom
1) energy ideas, the closest rhe alpha particle gets is when all Ke is transfered to EPE, find this distance
2) potential idea
- find the potential by using v = wd / Q of alpha
Now equate this to potential of the GOLD at distance R
- apparently they will both have the same potential at the closest approach
What are 2 problems with usign closest approach ideas?
1) more energetic particles will get closer to the nucleus, so if we can achieve this we can get better upper bound
2) but only takes into account the PROTONS, it could have infinite neutrons and we would get same answer, but obviously the radius is more…
How does the closest approach even work
Why is it AN UPPERBOUND
using energetic alpha particles will give us better values
1) the distance you find , is the radius of the alpha nuclei, + the distance in between and the radius of the GOLD NUCLEI
- the radius of alpha nuclei is small compared to gold, so Ignore
- the distance in between the alpha and the nuclei is why we have an UPPER BOUND, the radius will defo be smaller than this, but don’t know
So hat is the raidus of the gold Rutherford concluded
What is it generally
And what about atom
Rutherford got 10^-14m
In general nuckei have 10^-15m radius
And radius of atom is 10^-10 m
So atom much bigger then nuckeus !
Radius of a nucleus
REMEMBER NUCLEUS NOT ATOM
R = r0 A ^1/3
Here it shows the raidus is promotional to cube root of molar mass
But increasing A does increase raidus
R0 is constant, proton radius if they give
What is the mass of a nucleus gonna be?
Mass = protons and neutrons x AU
Now protons and neutrons only give mass as electron very small
Density of all nuclei therefor?
Is CONSTANT , use the equation, the A cancels out and you left with bunch of constant
How to find density of atom?
Mass / volume
This time mass is AU
And volume is 4/3 pi r cubed
But the RADIUS is just 10^-15, take this as crude approximation
You’re not gonna get a constant value, this time different
Remember small prefixes, pico femto
PETA big?
Pico is -12, femto is -15
PETA = 15
Think about helium nuckeus, what are the forces acting in each nucleon
Why must they’re be a third ?
Between each = gravitational attractive
Between protons = electrostatic force of repulsion
In short distances , one fm, calcuakted force of repulsion, it’d huge (230N)
- grvaitiaonl is TOO SMALL to keep this in place
- hence a third force is at play, strong nuclear
The extra force is the STRONG NUCLEAR force , fundamental, what is it
It’s a force that ACTS BETWEEN anythign based on distances, thus acts on all nucleons
- short range force
- between 0.5 fM and 3fm it’s ATTRACTIVE
- below 0.5 FM it’s reuplsiove
How does this show on a groan and why
From 0 to 0,5 fm infinity to 0, positive
- positve force means REPULSION
From 0.5fm to 3fm , negative = attraction
So how does string nuckear explain why helium nuclei is stable
2 POINTS COVER BOTH (delve into nature )
As the protoms are about 1fm away from each other, the string nuckear force acts ATTRACTIVE, which keeps them together, and counters the electrostatic force of attraction
AND the fact that from 0.5fm and below it’s repulsive, the nucleons can never get too close to each other such that they CRASH
Again explain how strong nuclear force allows for the helpline nuclei to be stable
Over 1fm, the two protons experience very high force of repulsion electrostatic
But the strign nuclear force is attractive for 0.5fm to 3fm, hence attraction provided counters the repulsion and holds them togther
AS REUPLSIVE for less than 0.5fm, they can never get too close to each other as to crash, and this shown by positve on the graph
