RDB (EXPERIMENTAL)Scattering
Alpha particle (Rutherford) scattering experiments involved firing charged alpha particles at gold foil. Most of the nuclei went straight through the foil, while a small number were deflected, some at high angles. This showed that the atom is mostly empty space, with most of its mass concentrated in a small, positively charged nucleus. As an alpha particle approaches a nucleus, its kinetic energy is entirely converted to electric potential energy at its point of closest approach.
Electron scattering experiments involve firing electrons at high relativistic speeds. Special relativity is covered in detail in Our Place in the Universe.
Quarks
Protons and neutrons are composed of smaller elementary particles, quarks. A proton is formed by three up, up, and down quarks bound together by the strong force, while a neutron is formed by three down, down, and up quarks. The up quark has a charge of , while the down quark has a charge of . Hadrons are composite particles formed by quarks.
Exchange particles
The fundamental forces are mediated by exchange particles. The gluon is the exchange particle for the strong force.
Nuclear equations
Conservation of mass and conservation of energy individually are not true conservation laws. More generally, mass-energy is conserved, as demonstrated in nuclear reactions. For example, in nuclear fusion, the total mass of the products is lower than the mass of the reactants, with the difference in mass (mass defect) corresponding to the energy released, given by .
In nuclear reactions, charge and lepton number are both conserved. Leptons have a lepton number of +1, while anti-leptons have a lepton number of -1. In beta decay, a neutrino or anti-neutrino is also emitted along with a positron or electron respectively, in order to conserve lepton number. Neutrinos are very light, electrically neutral particles that barely interact with matter.
Quantum model of the atom
A simplified model of the atom can be described by treating electrons as quantum particles in a confined space. Atoms can be thought of as boxes that confine electrons, with the positive nucleus creating a potential energy well that does not allow electrons to escape.
As the electrons are constrained, they form standing waves, which means the electrons must have specific wavelengths for the standing wave (e.g. with one loop, two loops, etc.). This means the electron must only take discrete wavelengths, and by the de Broglie relationship, the electron must only be in discrete energy levels.
Evidence for discrete energy levels
Electrons in atoms occupy discrete energy levels. Evidence for this includes line spectra, where atoms emit or absorb electromagnetic radiation of specific wavelengths, corresponding to specific energy levels. Emission spectra have discrete lines, where each line is carried by photons of a discrete frequency, with energy given by , which corresponds to the difference in energy between two possible energy levels in the atom.
In the Franck-Hertz experiment:
- A filament is heated, causing electrons to be emitted through thermionic emission.
- Electrons accelerate towards a wire grid with a positive potential, and pass through.
- Past the wire grid, there is an anode at a lower potential than the grid, causing electrons to decelerate between the grid and anode.
- Electrons must have sufficient kinetic energy to reach the anode, where they go through a circuit through an ammeter.
- The whole setup is performed in a gas at a low pressure.
As the grid potential increases, the current (thus number of electrons reaching the anode) increases initially. Then, at certain p.d.s, the current drops. This is because the electrons have sufficient energy to knock electrons in gas atoms to a higher energy level when colliding, losing energy. These drops happen when the energies of the electrons are a multiple of the gas atom’s energy levels, causing inelastic collisions with the gas atoms.
Electron standing waves
Atoms can be considered as a ‘box’ that traps electrons (due to the potential well near the nucleus). These bound electrons form standing waves, with a de Broglie wavelength that makes an integer number of wavelengths fit within the box.