OCR A-Level Physics/Fields, Particles and Frontiers of Physics/Particle Physics

The Nuclear Atom.
In 1908, Ernest Rutherford had identified alpha radiation emitted by radioactive materials consisted of fast-moving, positively charged particles. In Rutherford's scattering experiment, a beam of alpha particles were sent through gold foil to observe how they deflected. A fluorescent screen detector emitted flashes of visible light when hit by alpha particles. The results showed that the vast majority of the alpha particles travelled straight through the gold foil without being deflected. A small number were deflected through angles less than 90 degrees. One in every eight thousand alpha particles was deflected at an angle greater than 90 degrees, meaning it would effectively 'bounce back' in the direction which it came.
Deductions.
The Ernest Rutherford experiment led to the following conclusions;
The diameter of the nucleus is around formula_1mm and the diameter of the atom is around formula_2mm.
Model of the Atom.
Niels Bohr proposed that electrons can only occupy certain energy levels, or shells. Later, Rutherford proposed the name proton for the positively charged particle in the nucleus of an atom. In 1932, James Chadwick discovered the neutron.
Atomic Notation.
The "atomic number" is the number of protons within the nucleus, this is often denoted by formula_3
The "nucleon number", formula_4, is the number of protons and neutrons found in the nucleus of an atom. 
"Isotopes" are the same element that contains different number of neutrons. 
An atom can be represented by formula_5, where formula_6 is the element symbol, we can say that the number of protons = formula_3, the number of electrons = formula_3, and the number of neutrons = formula_9
Strong Nuclear Force.
Force between nucleons.
The attractive force is responsible for keeping the nucleus together cannot be the gravitational force of attraction because the force is too small and around formula_10 times smaller than the size of the electrostatic force of repulsion. Since the electrical force between two protons is a force of repulsion and the force of gravity is far too small to keep them together, we know there must be a different kind of force acting between protons, called the strong nuclear force.
Properties of strong nuclear force.
The strong nuclear force decreases rapidly with distance, and doesn't extend beyond adjacent protons and neutrons within the nucleus. The force must act between nucleons, and is independent of charge. Some of the properties are as follows:
Equilibrium Separation.
For two neutrons to be in equilibrium, the resultant force must equal zero. Neutrons have no charge, no electrical charge, and negligible gravitational charge, we focus on the strain nuclear charge for answers. 
When th strong nuclear force equals zero, they have separation formula_15. If formula_15 increased then a large force of attraction would pull them back. If formula_15 decreased then a large force of repulsion would push them back into equilibrium 
Nuclear Density.
As we know the atom diameter is around 10 000 times greater than the diameter of the nucleus. Following that the density of the atom as a whole should be considerably smaller than the density of the nucleus's the vast majority of the mass is contained within the nucleus.
Nuclear radius.
The relationship between the radius of the nucleus and the nucleon number isn't linear. Experiments have shown that the relationship between the nuclear radius, formula_18, and the nucleon number formula_4, is given by formula_20, where formula_21 is a constant. So the radius is directly proportional to the cube root of the mass number. formula_21 is has the value of formula_23
Nuclear density.
Density is defined as mass per unit volume and nuclear density is stated as formula_24, where formula_25 is the mass of the nucleon. This leads to: formula_26
Simplifying this gives the equation for the mean density of the nucleus given by formula_27. All the values in the equation are constants at nucleon level and independent of the mass number. This means that all atomic nuclei have the same density.
Fundamental Particles.
Classification of particles.
Subatomic particles are divided into two main groups - Hadrons and Leptons
Hadrons are composed of quarks. Quarks are fundamental particles, meaning they can't be broken down into smaller particles. Protons and neutrons are hadrons and are both composed of three quarks. All hadrons experience a strong nuclear force. 
Leptons are fundamental particles, for example an electron is a lepton. Each lepton has it's own neutrino. All leptons experience a weak nuclear force but do not experience the strong nuclear force. 
Antiparticles.
Every particle has its own 'opposite particle' called an "antiparticle". An electrons antiparticle is the positron. All antiparticles have the same mass as the particle, but the opposite charge. 
When particles interact with their antiparticles they annihilate each other, with their combined mass is converted into energy. 
Quark Properties.
Beside having mass, and charge, quarks also have other properties such as strangeness, charm, baryon and lepton numbers, and spin. Each one has a number that measures this. 
The three quarks that were initially proposed were the up, down and strange quarks. We can see from the table their properties:
Using this model of quarks and antiquarks we can see how certain particles and antiparticles are constructed
Decay of particles.
The quark nature of nucleons can be used to explain beta-plus and beta-minus radioactive decay. Many unstable hadrons decays due to weak interaction by transformation of their component quarks.