3.2.1 Particles

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Constituents of the atom

The model of the atom:

proton

Mass: 1.67*10^-27kg

charge 1

electron

Mass: 9.11*10^-31 kg

charge: 1

specific charge = charge / mass

An isotope is an atom with the same number of protons but a different number of neutrons

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Stable and unstable nuclei

The strong nuclear force is what keeps nuclei stable.

It is repulsive at ranges less than 0.5 fm

attractive at ranges between 0.5 and 3 fm

and has a negligible effect at ranges greater than 3 fm

Which means that it holds the particles in the nucleus around 0.5 fm apart

Because protons are positively charged, they repel.

When combining the strong nuclear and electromagnetic forces we can determine if a nucleus is stable.

If there are too many neutrons in the nucleus, then they will force the protons to be further apart.

If the protons are pushed far enough apart then they will no longer be held together by the strong force, and will instead break apart

This is either:

Alpha Decay

left with 2 nuclei and an alpha particle​​​​​

Beta decay

left with 2 nuclei and a beta particle

it is either:

The neutrino was originally hypothesized to account for the imbalance in the energy of these equations.

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Particles, antiparticles and photons

For every type of particle, there is a corresponding antiparticle.

A particle and its antiparticle have:

The same mass

opposite charge

the same rest energy

Light can act like a particle. The particle of light is called a photon and has the equation:

When a particle and a corresponding antiparticle meet, they are annihilated, and two photons with combined energy equal to the combined rest energy of the particles.

A single photon with enough energy can also become a particle and antiparticle.

The equation for rest energy is:

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Particle interactions

there are four fundamental forces.

gravity

electromagnetic

weak nuclear

strong nuclear

Particles interact through exchange particles.

The exchange particle for the electromagnetic force is the virtual photon

The weak interaction occurs in beta + and - decay, as well as electron-proton collision. The exchange particle is the W +/- boson

We use Feynman diagrams to represent these interactions

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Classification of particles

Hadrons

Hadrons are affected by the strong interaction

There are two types of hadrons:

Baryons

Proton

neutron

Mesons

pion

kaon

Each baryon has a baryon number of one, and antibaryons have a baryon number of -1

Baryon number is conserved in all interactions.

The proton is the only stable baryon, and all other baryons eventually decay into them.

The pion is the exchange particle for the strong interaction.

The kaon decays into the pion

Leptons

electron

muon

neutrino

Each lepton has a lepton number. +1 for leptons, and -1 for antileptons

Lepton number is conserved only in weak interactions.

Muons decay into electrons.

Strange Particles

Strange particles are produced through the strong interaction but decay through the weak interaction.

Strange number is conserved only in strong interactions and can change during weak interactions.

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Quarks and antiquarks

baryons are made from 3 quarks OR 3 antiquarks

leptons are made from a quark-antiquark pair

knowledge of charm, top, and bottom quarks is not needed

The decay of the neutron is:

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Applications of conservation laws

In beta+ and beta- decay, quark character can change

The conservation laws can be used to determine whether a reaction is possible or to predict the missing particles in a decay.

Energy and momentum are always conserved