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Chapter 18 - Looking Inside the Atom
Terms in this set (28)
What is pair creation? What energy does the gamma photon need?
This is when a gamma photon of sufficient energy (at least 2mc²) changes into an electron and a positron (anti-electron) as it passes by a massive nucleus.
Needs to be at least 2mc² to conserve energy of the electron and positron (as mc² is their minimum possible energy).
When can the total energy of a photon be estimated with E = Pc?
When its speed is near the speed of light (as Eₜ = γmc², and P = γmc).
How can alpha particles be made to go closer to the centre of the atom's nucleus?
Alpha particles have a fixed energy, so they need to be accelerated quicker to give them more total energy and get them closer.
How are alpha particles scattered by the nucleus?
Alpha particles are +vely charged, so are deflected accordingly by the +vely charged nucleus, with some coming straight back or deflected away from the nucleus.
What did the Rutherford alpha particle scattering experiment show?
. The atomic core must be massive to deflect alpha particles by large angles,
. Alpha particles are very small (otherwise they would just bounce back rather than being deflected),
. The nucleus must be +vely charged to deflect the particles (electrostatic repulsion).
What variations of the Rutherford alpha particle scattering experiment were carried out?
. Slower alpha particles means less energy, so they were deflected at greater angles,
. Nuclei with smaller charge means less scattering of alpha particles,
Number of particles alpha particles scattered at different angles fir inverse square law for electrical repulsion.
Why is a massive nucleus needed for pair creation?
The gamma photon must pass near the nucleus so that the nucleus can absorb some of the momentum of the photon (so that it can be conserved).
How are electrons scattered by the nucleus?
Electrons have opposite charge to nucleus (-ve to +ve) so curve in towards the nucleus, due to electrostatic attraction.
How is the speed of a particle (usually electron) calculated?
Equate E = qV and E=1/2mv²
qV = 1/2mv²
(2qV)/m = v²
v = √((2qV)/m)
Where q = charge, v = velocity, V = Voltage (accelerating p.d.) and m = mass of particle.
Why does γ = 1 at rest for E=γmc²?
γ = 1 because it takes in the relativistic effects of travelling close to the seed of light. At rest, there are no relativistic effects, so γ = 1.
What is the diffraction minimum on the Rutherford scattering curve?
Electrons have a de Broglie wavelength, so they can diffract. The minimum on the curve is when the diffraction of the electrons superimpose with one another, cancelling each other out and creating a minimum.
The first minimum occurs when sinθ = (1.22λ)/d.
How are electrons given a small enough wavelength to diffract?
They need an incredibly small wavelength to diffract, so to achieve this they are accelerated to very high speeds (several hundred MeV).
This is because λ ≈ (hc)/E at high speeds.
What is the proton?
A Hydrogen Nucleus
What are the atomic charge and mass numbers on an element?
The top number (Mass) shows the number of protons and neutrons an element has.
The bottom number shows the neutrons, and therefore the charge that the element have (as neutrons have no charge).
This means that the mass of an element can change, without changing its charge (by removing/adding neutrons).
Why are particular elements unstable?
The number of protons in a neutral-charged element also determines the number of electrons. Since electrons also determine the chemical properties of a material, this explains why some elements are more/less stable than others.
Changing mass (by adding/removing neutrons) can also make an element unstable.
What charges do up and down quarks have? What combinations of quarks do neutrons and protons have?
UP QUARK = +2/3e,
DOWN QUARK = -1/3e.
Protons have quark combination uud (for +e charge),
Neutrons have quark combination udd (for 0 charge).
What must antiprotons and antineutrons be made up of?
They must be made up of combinations of antiquarks (rather than quarks), which have the opposite charge to quarks.
How are new particles made?
Electrons strike quarks, where the electron is scattered at a large angle. Along with this, a jet of new particles, mainly mesons, is made.
This jet of particles follows the direction that the electron collided with the quarks.
What are gluons?
Gluons are the particles that hold quarks together inside the nucleus. This gluon interaction keeps the nucleus together, and is also known as the strong force.
What are leptons?
These are "little particles" that interact through the weak interaction (so do not take part in the strong interaction). This weak interaction is responsible for beta decay.
Examples of leptons are electrons and neutrinos.
What are hadrons?
These are particles, held together by quarks, which take part in the strong interaction.
Examples of hadrons are protons and neutrons.
What are neutrinos (and antineutrinos)?
These are leptons that are present to conserve lepton number in reactions.
What happens in Beta decay?
A neutron turns to a proton, and an electron is emitted (in B+ decay, a proton turns to a neutron and a positron is emitted).
A neutrino/antineutrino is also emitted to conserve lepton number accordingly.
The neutrinos/antineutrinos carry away the excess energy of the reaction, also conserving this part of the reaction.
Why were there dips in the Franck and Hertz graph? What did the graph show?
At the first dip in the graph, the electrons had enough energy to collide with gas molecules in the chamber and knock them to a higher energy level, meaning they didn't have enough energy to reach the anode (so the current dropped).
The drops in current showed that the inelastic collisions occurred at specific electron energies, showing that electrons have fixed energy levels.
How do electrons form their own "standing waves"?
An atom is trapped in a potential energy well of the nucleus, due to the attractive forces (nucleus is +ve and electron is -ve).
These wells can be thought of as boxes, where electron standing waves can form.
How do energy levels increase for an electron?
The increase in energy levels follows a 1/n² relationship, where n is the energy level number.
If the energy level has an energy value > 0, the electron is no longer bound to the nucleus and can escape (the atom ionises).
What is the lowest energy level for Hydrogen?
-13.6eV is the lowest energy level for Hydrogen. This means that the ionisation energy for hydrogen is 13.6eV.
The energy levels for hydrogen are given by:
13.6eV/n² = Eₙ.
How is potential energy and kinetic energy related to electrons escaping?
Electrons have -ve potential energy because they are attracted (bound) to the nucleus, but +ve kinetic energy due to their de Broglie wavelength.
The minimum possible atomic radius is when Ep +Ek = 0.
If the magnitude of the kinetic energy is greater than the magnitude of the potential energy, the electron escapes and the atom ionises.
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