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7
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J
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In 1911 British physicist Ernest Rutherford conducted experiments by sending alpha particles through gold leaf
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Rutherford proposed that alpha particles would travel straight through the atom without
deflection if Thomson’s “Plum pudding” model was correct
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Rutherford proposed the planetary model
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Topic 7 Page 2

Emission and Absorption Spectrum of Common Gases
When a gas in a tube is subjected to a voltage, the gas ionizes, and emits light
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A spectroscope acts
similar to a prism, in that it separates the incident light into its constituent wavelengths
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Each gas also has an absorption spectrum, caused by cool gases between a source of light and the scope
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Discrete means discontinuous, or separated
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Nuclear Structure
The word nucleon is used to denote a proton or a neutron
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The total number of nucleons (protons + neutrons) is called the mass (or nucleon) number
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stands for the nucleus of element , whose atomic number is

and mass number is
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Isotopes have the same number of protons, electrons,
but not neutrons
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The energy levels are labeled from the lowest to the highest as
to
is called the principal quantum number and goes all the way up to infinity
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In its ground state or unexcited state, hydrogen’s single electron is in the 1st energy level

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The atom de-excites if it jumps down to a lower state
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The equation for the energy of a photon is:
Where:
is the energy in Joules (convert from eV to J to put into equation)
is Planck's constant (
)
is the frequency of the photon
Energy level diagrams:

The closer the energy level is to the nucleus, the more negative it is
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" To ionize a groundstate electron (taking it from
to
), you would have to irradiate the gas with photons having energies of
or greater
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Any excess energy would remain in the form of the ionized
electron's kinetic energy
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The emission of particles and energy from a
nucleus is called radioactivity
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The emissions are called alpha particles, beta particles and gamma rays
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Note that the left hand side balances the right hand side
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In beta plus decay, instead of emitting an electron the nucleus emits its anti -particle, the positron
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Gamma Decay
In gamma decay a nucleus emits a gamma ray, in other words a photon of high -frequency electromagnetic radiation
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It just moves from a higher to a lower nuclear energy level (excited to unexcited)
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The daughter particles have a mass less than the original
particle
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The decay series is the set of decays that takes place until a given nucleus ends up as a stable
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The so called line of stability runs up the central white band which represents the stable nuclei
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Any given nucleus has a 50% chance of decaying within a time interval equal to the half-life
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Fundamental Forces and Their Properties
There are four fundamental interactions or forces in nature
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the electromagnetic interaction: this acts on any particle that has electric charge
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It has infinite range
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the weak nuclear interaction: it acts on protons, neutrons, electrons and neutrinos in order to bring about beta decay
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This makes up protons and neutrons and governs how they can change flavours
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the strong nuclear interaction: this (mainly attractive) force acts on protons and neutrons to keep then bound to each other inside nuclei
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the gravitational interaction: this is the force of attraction between masses
These 4 forces act over matter
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Can obstruct DNA and become malignant
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Alpha particles
- Really damaging to cells but can be deflected by a piece of paper
- Has high ionization property
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Beta particles
- Quite high penetration and reasonable high ionization strength
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2 - Nuclear reactions

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The unified atomic mass unit
The symbol for the unified atomic mass unit is
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From Einstein's theory of relativity, Energy

is equivalent to mass

according to the equation:

Where is the speed of light
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The mass of n ucleons when far apart is
greater than the mass of the nucleus
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The difference is known as the mass defect
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It can also be written as:

All values are in
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The binding energy of a nucleus is the work (energy) required to completely separate the
nucleons of that nucleus
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The higher the binding energy per nucleon, the more stable the nucleus
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If

is negative then no reaction will occur
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Remember that momentum conservation applies to nuclear physics as well
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When a
neutron is absorbed by a nucleus of uranium-235, uranium momentarily turns into
uranium-236
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Nuclear fusion is the joining of two light nuclei into a heavier one with the associated
production of energy
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3 - The structure of matter

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Probing Matter
In the Rutherford experiment, alpha particles were directed at a thin gold foil in an evacuated chamber
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The great majority of the alpha particles went straight through the foil
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The foil was very thin because to avoid
absorption of alpha particles and to avoid multiple scatterings
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Rutherford, Geiger and Marsden found that very occasionally, alpha particles were detected at very large scattering angles
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However, they could not understand the large angle scattering
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Since the electric force is
given by
to get a large force implies that the separation between the alpha particle and gold atom must be very small
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The nucleus would be very concentrated with a large positive charge
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The three elementary particles are the quarks, the leptons, and the exchange particles
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Quarks
There are six different 'flavours' of quarks
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Up, charm and top have electric charge
Down, strange and bottom have electric charge
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Antiparticles are denoted with a 'line' on top
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- When three quarks combine they form a baryon
- When a quark combines with an anti-quark they form a meson
A proton is a baryon made out of two up quarks and one down quark, so we say it is uud
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A pion is a meson
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Quarks have another property called baryon number
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All anti-baryons have baryon number

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In all reactions, the electric charge and baryon number are conserved, meaning that they have the same value on the left and right hand side
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There are six types of leptons
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We also have the anti-particles for these
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All leptons have a property called family lepton number, which is conserved in all interactions
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to all leptons and

to all anti-leptons
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Both particles experience a
force due to this
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A straight line pointing to the right represents an electron
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(e+)
A swiggy line represents a photon
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Think of it on a time axis
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A boson is all mediating particles
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A straight line pointing to the left represents an anti-particle
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This enables a quark flavour change
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They have mass and are called W+, W- and Z0
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Beta- decay:

Beta+ decay:

Beta- decay with quarks:

Topic 7 Page 20

Beta- decay with quarks:

Topic 7 Page 21

Colour
Quarks have a property called 'colour'
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Colour is
conserved
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g
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Example of colour equation:
quark

--->

○
red

gluon

+

○
---->

red & antiblue

quark
○

+

blue

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Confinement
Quarks only exist within hadrons
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It is not possible to observe isolated quarks
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The force between the quark and the anti-quark is constant no matter what their separation is
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It is impossible to free the quark, as it would require an infinite amount of energy
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The Higgs boson
The Higgs particle is responsible through its interactions, for the mass of the particles of the standard model, in particular the masses of the

Topic 7 Page 24

and the

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