18: Looking Inside the Atom Flashcards
Rutherford experiment disproved which model for the atom?
describe the model
The Thompson model or plum pudding model
atoms were a plum pudding with electrons embedded throughout a positive sphere
how do you investigate the structure of an atom
with scattering experiments
particles are accelerated to a high energy and directed at a target
there are detectors surrounding the target track to identify particles created and those scattered in coliision
Describe Rutherford’s experiment
A stream of alpha particles from a radioactive source was fired at a very thin gold foil.
The angles at which the particles were scattered were recorded
some passed straight through (so most atom is empty space)
but a few of the alpha particles bounced right back from the foil being scattered at angles greater than 90 degrees
What could Rutherford conclude after his experiment?
The atom is mainly empty space.
the core must be massive on an atomic scale to deflect alpha particles through large angles, but its much smaller than an atom as very few particles deflected through more than 90 degrees
alpha particles deflected due to electric repulsion between positively charged alpha particles and positive core in gold atoms.
The centre of the atom must have a large, positive charge. Rutherford named this the nucleus.
The nucleus must be tiny, but massive (mass).
what happens to the deflection angle if the alpha particles are slowed down
how is scattering affected if the nuclei has less electric charge
relationship between scatter angle and inverse square law
what happens to the deflection angle if the alpha particles have more energy
if alpha particles are slowed down more would be deflected at greater angles since the nucleus would be able to turn them back more easily
nuclei of smaller electric charge scatter alpha particles less strongly
the pattern of number of alpha particles scattered at different angles fits the pattern expected from the inverse square law for electric repulsion
angle decreases and fraction deflecting through an angle decreases because the nucleus cannot deflect as easily because the deflecting force acts for less time
What did Rutherford discover about charge at the centre of the atom? How did he know?
Some of the alpha particles were deflected through large angles, so the centre of the atom must have a large positive charge to repel them.
What value do you need when you are estimating the distance of closest approach of an alpha particle that has been fired at a gold nucleus?
The alpha particles initial kinetic energy
kinetic energy of alpha particle at its closest approach
0 kinetic energy
what is electrical potential energy equal to at a large distance from the nucleus
at a large distance from the nucleus, alpha particle electrical potential energy is equal to its initial kinetic energy
equation for initial kinetic energy
charge on alpha particle
charge on gold particle
alpha: 2e = 2 *(1.6*10-19)
gold: 79e = 79 * (1.6*10-19)
What value do you need to find the charge of the nucleus?
The atom‘s proton number, Z
What is the charge on an alpha particle?
+2e
What holds the nucleus together? What stops the nucleus flying apart due to the overall positive charge?
The strong interaction force
What are hadrons?
Particles feel the strong interaction e.g. protons and neutrons
What are hadrons made of?
Fundamental particles called quarks. Must be more than one quark
Name four different hadrons
Protons, neutrons, sigmas, mesons
Which particle is the only stable hadron?
The proton
What will all hadrons, except for protons, do?
Decay into other particles
What does the neutron decay into?
It is an unstable particle that decays into a proton
A neutron decaying into a proton is an example of what kind of decay? what is this caused by?
It is an example of beta minus decay which is caused by the weak interaction
What is the half life of a free neutron?
Half life of 15 minutes
What is a free neutron?
A neutron that isn’t held in the nucleus
Describe leptons
They are fundamental particles that don’t feel that strong interaction. They interact with other particles via the week interaction and gravity (and the electromagnetic force if they’re charged)
What are the two types of lepton we need to know about and what are the symbols?
Electrons and neutrinos
e and ν (nu)
Describe neutrinos
They have zero, or almost 0, mass and zero electric charge – so they don’t do much. Neutrinos only take part in week interactions. They can pass right through Earth without anything happening to it
What is lepton number?
The number of leptons
What is conserved in a particle reaction?
Lepton number and baryon number
Energy, charge and momentum
Classifying particles: if there is only one fundamental particle what can it be?
A fermion or a boson
Classifying particles: if the are more than one fundamental particle what is the particle?
Composite
Classifying particles: what two types of fermion can you have?
Leptons and quarks
Classifying particles: What is a composite particle composed of quarks called?
Hadron
Classifying particles: what two types of hadrons are there? What is the difference between the two?
If there are two quarks it is a meson
If there are three quarks it is a baryon
What are the six different quarks?
Up and down
Charm and strange
Top and bottom
What are the six types of leptons?
Electron neutrino, muon neutrino, tau neutrino
Electron, muon, tau
What are the four types of bosons?
Week force, strong force, photon, gluon
What is a positron?
Positrons have identical mass to electrons but they carry a positive charge
An anti-electron
Does every particle have an anti-particle? Describe the mass and charge of the antiparticle?
Yes (but not bosons). Same mass, but with opposite charge
What has a baryon number of +1?
Matter baryons
What has a baryon number of -1?
Anti-matter baryons
What has a baryon number of zero?
Anything that isn’t a baryon
What has a lepton number of +1?
Matter leptons
What has a lepton number of -1?
Antimatter leptons
What has a lepton number of zero?
Anything that isn’t a Lepton
What is the relative charge of a proton, neutron, electron, neutrino?
Proton +1
Neutron 0
Electron -1
Neutrino 0
What is the relative charge of the antiproton, antineutron, positron, antineutrino?
Antiproton -1
Anti-neutron 0
Positron +1
Antineutrino 0
What can you assume about neutrinos in particle collisions?
That they have zero mass and zero charge
From energy you can create matter and what?
Antimatter
What happens when energy is converted into mass?
You get equal amounts of matter and antimatter
What is pair production?
When you fire two protons at each other at high-speed and you’ll end up with a lot of energy at the point of impact. This energy might be converted into more particles.
If an extra proton is formed then there will always be in antiproton to go with it. It’s called pair production
What is each particle antiparticle pair produced from?
A single photon
When/where does pair production happen?
If one gamma ray photon has enough energy to produce that much mass. It also tends to happen in a nucleus, which helps conserve momentum
What is the most common pair to be produced in a proton collision? Why?
Electron positron pairs because they have a relatively low rest mass
How can you calculate the minimum energy, and therefore the minimum frequency and maximum wavelength, a photon must have a pair production to occur?
Use E equals MC squared
What is the opposite of pair production?
Annihilation
When does annihilation occur?
When a particle meets it antiparticle
Describe annihilation
All the mass of the particles and antiparticles gets converted to energy, in the form of a pair of identical photons
Why do we not see many antiparticles?
Generally they can only exist for a fraction of a second before they annihilate
How can you work out the minimum energy of each photon produced in annihilation? What assumption do you have to make?
The combined energy of the photons will be equal to the combined energy of the particles, so 2 E = 2MC²
Assume that the particles have negligible kinetic energy
What can you use to observe the pair production and annihilation?
A cloud chamber
What is a cloud chamber?
A large box filled with alcohol, or water, vapour
Explain how you can use a cloud chamber to observe pair production and annihilation
When a high energy particles passed through the cloud chamber, they ionised alcohol particles along their path
The vapour in the cloud chamber condenses around these ions, forming a trail of alcohol droplets (a ‘cloud’) along the path of the charged particle
Most cloud chambers include a magnetic field at right angles to the direction of particle motion. Why?
A moving charge in a magnetic field experiences a force, so the path of charged particles, like electrons and positrons, will bend as they pass through the chamber. Positively and negatively charged particles are deflected in opposite directions.
What happens to a particle speed as it travels through the cloud chamber? What does this mean about the shape of their trail?
They slow down, as ionising alcohol particles uses energy.
This means the paths of charged particles when the magnetic field is present are spirals, rather than circles
Which photons don’t produce trails? Why?
Gamma ray photons are only weekly ionising – they pass through the alcohol of the cloud chamber without interacting with it very much
Describe and explain what happens when antiproton enters a cloud chamber
The antiproton and proton collide. Hadrons are created and shoot off from the point of collision
There are an equal number of clockwise and anticlockwise hadron tracks because charge is conserved
What are the antiparticles of hadrons made up from?
Anti quarks
Which quarks are protons and neutrons made up from?
Up (u) and down (d)
What is the relative charge of an up and a down quark?
Up +2/3
Down -1/3
What is the relative charge of an anti-up and an anti-down quark?
Anti-up -2/3
Anti-down +1/3
Where did evidence for quarks come from?
Came from hitting protons with high energy electrons.
The way the electrons scattered showed that there were three concentrations of charge (quarks) inside the proton
What 3 quarks are a proton made up of? An anti-proton?
Proton: up, up, down
Anti-proton: anti-up, anti-up and an anti-down
What 3 quarks are neutrons made from? Antineutrons?
Neutrons: up, down, down
Anti: anti-up, anti-down and an anti-down
If you blasted a proton with enough energy, could you separate out the quarks? Why?
No. The energy just gets changed into more quarks and anti-quarks. It’s pair production and it makes mesons
What is quark confinement?
The fact that you can never get free quarks. Adding energy to try and split the quarks in a proton up, just makes a new quark, anti-quark pair. Eg. A new meson
Gluons provide force between…
Quarks
What are gauge bosons?
Exchange particles. Virtual particles which are used to let one particle ‘know’ that another one is there
What are the four fundamental forces?
Strong, electromagnetic, weak, gravity
What are the gauge bosons for each force?
Strong: gluon
Electromagnetic: photon
Weak: W-, W+, Z0
Gravity: Graviton?? No evidence yet for it
Which particles are effected by each force?
Strong: hadrons only
Electromagnetic: charged particles only
Weak: all types
Gravity: all types
Because gluons cause a force, you can think of them as [] as well as particles
Fields
What happens as you try and separate quarks? In terms of gluons. What happens if you keep pulling?
You increase the energy of the gluon field, increasing the attraction between them
If you keep pulling, eventually the energy in the gluon field will be enough that it produces a quark-antiquark pair
Particle accelerators cause…
High energy collisions
What is a linear accelerator?
A particle accelerator that is a long straight tube containing a series of electrodes
Describe how a linear accelerator works
Alternating current is applied to the electrodes so that their charge continuously changes between positive and negative
The ac-current is timed so that the charged particles are always attracted to the next electrode in the accelerator and repelled from the previous one
A particle’s speed will increase each time it passes and electrode – so if the accelerator is long enough particles can be made to approach the speed of light
The high energy particles leaving a linear accelerator collide with a fixed target at the end of the tube
What is a cyclotron?
It is a circular particle accelerator
How do cyclotrons work?
A cyclotron uses 2 semicircular electrodes to accelerate protons or other charged particles across the gap
An alternating potential difference is applied between the electrodes – as the particles are accelerated from one side to the other their energy increases, i.e. they are accelerated.
A magnetic field is used to keep the particles moving in a circular motion
The combination of the electric and magnetic fields makes the particles spiral outwards as the energy increases
What is the advantage of a synchrotron compared with a cyclotron or a linear accelerator ?
A synchrotron can produce particle collisions with much higher energies than either a linear accelerator or cyclotron
What keeps the particles moving in a circular path in focused beams in a synchrotron?
Electromagnets
How can synchrotrons produce particles with energy is reaching from 500 GeV to several TeV?
Are using electromagnets
Explain how a synchrotron works
The beam travels around a very large circle.
The beam is accelerated by electrodes.
detectors are placed around the circle
Magnets are used to focus and deflect the beam
Two beams can be accelerated in opposite directions for collision beam experiments
Which equation you can use to find a force experience by a particle, due to the magnetic field, in a synchrotron or a cyclotron?
F=qvB
What is the disadvantage of a synchrotron?
They are really expensive to build and run, so they need to be funded internationally
What are inertial frames?
Reference frames that aren’t accelerating. Einstein’s theory of special relativity only works in inertial frames
No particle that has mass can…
Move at the speed greater than or equal to the speed of light, c
Which two assumptions does Einstein’s theory of special relativity rely on?
Physical laws have the same form in all inertial frames
The speed of light in free space is invariant
What happens when you increase the kinetic energy of a mass, like a particle in an accelerator?
The object becomes more massive
What do particle accelerators have to do to compensate for the relativistic mass of the accelerating particles?
They have to alter their magnetic and electric fields
Electrons in atoms have [] energy levels
Discrete
Describe an electron on an energy level
Electrons in an atom can only exist in certain well-defined energy levels. Each level is given a number, called the principal quantum number of the electron in that state, with n=1 representing the electron’s lowest possible energy – its ground state
Why are all the electron energies negative when talking about energy levels?
This is because of the way the zero energy is defined – it is the energy of an electron when it is not bound by an atom.
All electrons that are bound to the atom have negative energies.
The higher the energy level, i.e. the larger the value of n, the more energy in the electron has and the less negative the energy
Relating to energy, when is an electron ionised?
An electron is “free“ and no longer bound to the atom when it has an energy of zero or more – the atom becomes ionised
How can an electron moves down an energy level?
By emitting a photon
When an electron moves down and energy level, what is the consequence of there being definite energy levels?
The energy of each photon emitted can only take certain values
What is the energy carried by each emitted photon equal to?
The difference in energy is between the two levels that the electron has moved between
What happens when an atom absorbs a photon?
An electron moves up to another energy level
What two things are produced as a result of quantisation of electron energies?
Line emission and absorption spectra
Why do you electrons, protons and neutrons obey the Pauli exclusion principle?
They are fermions
What is the Pauli exclusion principle? What does this mean for electrons in energy levels?
This states that no to a fermions can be in exactly the same quantum state at the same time. In the context of energy levels, that means no more than two electrons can be in the same energy level at the same time.
The spectrum for white light is []
Continuous
When do you get a line absorption spectrum?
When light with a continuous spectrum passes through a cool gas
Which states will be electrons in a cool gas being?
At low temperatures, most of the electrons in the gas atoms will be in their ground states
Which wavelengths are missing from the continuous spectrum when the light comes out the other side of the gas?
Photons of the correct wavelength are absorbed by the atoms to excite the electrons to high energy levels.
These wavelength and missing
What shows that a wavelength has been absorbed in a spectrum?
Dark lines
What do emission spectra show?
The wavelength of the photons emitted when an electron falls into a lower energy level
What does an emission spectrum look like?
They are made up of a series of bright lines corresponding to the wavelength of the photons emitted
If you compare the absorption and emission spectra of a particular gas, what will you see?
The dark lines in the absorption spectrum match up to the bright lines in the emission spectrum
What are line spectra evidence for?
Energy levels
What are fluorescent tubes evidence for?
Energy levels
When thinking about electrons around a nucleus, what is the consequence of thinking of electrons as both a particle and a wave?
When they orbiting around a nucleus, they ought to behave like standing waves
What should the wavelength of the electron waves do? When talking about energy levels
It should fit the circumference of the orbit a whole number of times
What is equal to the number of complete waves that fit in the circumference at a certain energy level?
The principal quantum number
Why can you think of electrons being standing waves between two fixed walls?
Because you can think of the electrons as being trapped by a potential well made by the nucleus
When will an electron be able to escape the potential well of the nucleus?
When it has a total energy of zero or more
How did Rutherford know that the nucleus was massive compared to electrons?
Most of the mass must be in the nucleus, since the fast alpha particles (with high momentum) are deflected by the nucleus
What did Rutherford find out about the size of the nucleus? How did he know?
Very few particles are deflected by angles greater than 90°, So the nucleus must be tiny.
How do you fluorescent tubes work? Quick step-by-step
Free electrons produced
They change energy levels, which results in the emission of UV photons
Phosphorous coating absorbs these, more electrons change energy levels, causing the emission of visible light photons
What do fluorescent tubes do? (one sentence)
They use excited electrons to produce light.
How do you fluorescent tubes work? (1):
First, more free electrons must be produced, how is this done?
A high voltage is applied across mercury vapour, which accelerates free electrons that ionise some of the mercury atoms, producing even more free electrons
How do you fluorescent tubes work? (2):
What happens with the newly produced free electrons?
When the free electrons collide with electrons in other mercury atoms, the electrons in the mercury atoms are excited to high energy levels. When they return to their ground states, they emit UV photons
How do you fluorescent tubes work? (3):
What happens to the UV photons, how does this lead to the emission of visible light?
A phosphorus coating inside the tube absorbs these photons, in turn exciting its electrons to much higher orbits. These electrons then cascaded down the energy levels, emitting photons in the form of visible light
Where is the first minimum in electron scattering?
sinθ = 1.22λ/d
where λ is the de Broglie wavelength
and d = diameter of the nucleus
Electron scattering:
On the graph of number of electrons scattered, against angle, how do you find the angle of the first minimum?
The first dip in the curve is the angle of the first minimum. You can use that angle for
sinθ = 1.22λ/d
