Topic 7: Atomic Physics Flashcards
Nuclear model of atom
Mass and positive charge concentrated in tiny nucleus with negative electrons around it.
Limitation of nuclear model
A classical atom would decay quickly since the electrons would spiral into the nucleus.
Atomic energy levels
Bohr and Planck proposed distinct energy levels for electrons (quanta).
Nuclide
A combination of protons and neutrons that form a nucleus.
Isotope
Nuclei with the same number of protons but different numbers of neutrons.
Nucleon
The particles in the nucleus (proton or neutron).
Natural radioactive decay
A random and spontaneous process in which an unstable nucleus emits a particle (disintegration). The element of the nucleus changes. The emission of an alpha or beta particle or a gamma ray are the three possible processes.
Exponential decay
The rate of decay decreases exponentially with time. The number of nuclei remaining (and the activity rate) reduces by half in a constant time.
Alpha particle
Consists of two protons and two neutrons (= helium nucleus). Has approximately 5 MeV kinetic energy. Travels at approx 5% speed of light.
Beta minus particle
Consists of one electron, often travelling at close to the speed of light. They have a range of speeds and kinetic energies depending on the element and the anti-neutrino.
Beta minus decay
In the nucleus, a neutron changes into a proton and an electron (which is emitted as a beta minus particle).
Anti-neutrino
Particle emitted with beta minus particle.Carries away some of the kinetic energy.
Gamma ray
High energy (also frequency) electromagnetic radiation emitted by nucleus following alpha and beta decay (which left the nucleus in an excited state).
Ionisation
Ionisation can be thought of as ‘damage’ to the medium the radiation is passing through. The greater the ionisation, the less the penetration into the medium, since the radiation more rapidly loses energy. Thus the most ionisation is by alpha and the least (almost none) by gamma.
Nuclear strong force
The force that holds the particles of a nucleus together. It is strong enough to overcome electrostatic repulsion of protons. It is very short range (less than three nucleon radii), so larger nuclei are less strongly held together by this force and can be unstable..
Transmutation
The process by which a nucleus changes by adding or removing nucleons. The process is called ‘artificial’ if an instrument such as a particle accelerator or nuclear reactor is required to provide the initial energy.
Binding energy
The energy released when a nucleus assembles from its component nucleons.
Mass defect
Mass defect = mass of component nucleons - mass of nucleus
Einstein mass-energy equivalence
ΔE = Δmc2. The mass defect is equivalent to a change in energy which is calculated using this equation.
MeV
Mega-electron-volt is a convenient unit for measuring nuclear energies. The energy an electron would have if it were accelerated through 1 000 000 V
Nuclear fission
A process where a large nucleus splits to make smaller ones. This is permitted when the BE/ nucleon of the products is higher than the initial nucleus. The difference in mass/ energy is released.
Nuclear chain reaction
A uranium nucleus can be split by an incident neutron. The fission reaction produces more neutrons. If one of the neutrons produced also causes fission, then a chain reaction will follow. If the number of fissions is greater than one, the reaction will go out of control.
Nuclear fusion
A process where small nuclei can combine to make a larger nucleus. This is permitted when the BE/ nucleon of the product is higher. The difference in mass/ energy is released.
Radioactive half-life
The time taken for half of the nuclei in a sample to decay. OR The time taken for the activity to reduce to half of its original value
Unified atomic mass unit
The mass of one twelfth of the nucleus of a carbon-12 isotope.
Half life
Time taken for half the nuclei in the sample to decay
