The Nucleus Nucleons- the particles inside the nucleus: protons & neutrons Total charge of the nucleus: the # of protons (z) times the elementary charge (e-) Mass: both the proton & the neutron have a mass of 1 u 1 u – atomic mass unit = 1.67 x kg Mass of the nucleus = A x u (where A = # of protons & neutrons

The Nucleus What holds the nucleus together? Strong Nuclear Force – attractive force exerted by one proton on any other proton or neutron that is near it, or between neutrons. It is stronger than the electromagnetic force of repulsion between protons, therefore, the nucleus stays together. Range of force is very short, only the radius of a proton.

If a nucleon were to be pulled from the nucleus, work would have to be done. Therefore, a combined nucleus has less energy than a separated one. The difference is called binding energy Binding energy of the nucleus – comes from the nucleus converting some of its mass to hold nucleons together. E = mc 2 The mass of the assembled nucleus is less than the sum of the mass of the nucleons that compose it The Nucleus

The difference between the sum of the mass of the individual nucleons and the actual mass is called the mass defect. Binding energy = the energy equivalent of the missing mass E = mc 2 E = energy, m = mass in kg, c = 3.0 e 8 m/s The Nucleus

The Standard Model Four fundamental interactions in nature: Strong – binding of neutrons & protons into nuclei. (short-range) Electromagnetic – is responsible for the attraction of unlike charges & repulsion of like charges…binding of atoms & molecules. (long- range) Weak – short-range nuclear interaction responsible for beta decay (when a neutron changes to a proton w/in the nucleus, an electron is given off) Gravitational – long-range, holds the planets, stars & galaxies together

All particles can be classified into two categories: Leptons & Hadrons The difference between the two is whether they interact through the strong interaction. Leptons participate in the weak, gravitational & EM interactions Hadrons interact through all four interactions The Standard Model

Leptons Do not seem to break down into smaller units ~ truly elementary Currently, scientists believe there are six leptons: The electron, the muon, the tau, and a neutrino associated with each Each of the six also have an antiparticle The Standard Model

Hadrons Can be divided into mesons & baryons Mesons are unstable ~ not constituents of normal, everyday matter Baryons~ protons & neutrons All hadrons are made up of quarks (up, down, strange, charm, top & bottom) The difference between mesons & baryons is due to the number of quarks that compose them Associated with each quark is an antiquark of opposite charge Quarks have fractional electric charge The Standard Model

All elementary particles can be grouped into three families: 1.Quarks 2.Leptons 3.Force carriers - carry or transmit forces between matter –Photons – carry EM force –Gluons – carry the strong nuclear force that binds quarks into p & n and p & n into nuclei –Weak bosons – involved in weak nuclear reactions (beta decay) –Gravitron – yet undetected carrier of the gravitational force The Standard Model

Particles & Antiparticles The difference in the before and after particle energies of the nuclei led to the idea of an unseen neutral particle to be emitted with the beta particle. ~neutrino (actually an antineutrino) was discovered in 1956 The Standard Model

A free neutron – one in an unstable nucleus can decay into a proton by emitting a beta particle and an antineutrino with zero mass and charge, but carries momentum and energy 1 n 1 p + 0 e + 0 v Neutronprotonbeta antineutrino The Standard Model

When an isotope decays by emission of a positron (antielectron) a proton within the nucleus changes into a neutron. 1 p 1 n + 0 e + 0 v Protonneutron positron antineutrino The Standard Model

Antiparticle – particle of antimatter Example: positron ~the positron and the electron are the same mass and charge, but opposite signs. When they collide, they can annihilate one another, resulting in energy in the form of gamma rays. The amount of energy: E = mc 2 The Standard Model

The inverse of annihilation can occur. Pair Production – energy can be converted directly into matter. If a gamma ray with at least 1.02 MeV passes near the nucleus, a positron and electron can be produced. ***matter and antimatter must always be produced in pairs*** The Standard Model

Particle Conservation Quarks and leptons also have antiparticles which, when they collide with one another, can cause annihilation ~ they are transformed into photons or lighter particle-antiparticle pairs. The total number of quarks and leptons in the universe is constant. Consequently, the number of charge carriers is not conserved, but charge is. Force carriers can be created and destroyed if there is enough energy. The Standard Model