Atomic Structure (Physics)

• • 1800 – John Dalton

  o before the discovery of electrons, atoms were thought to be tiny, undividable spheres

  • 1897 – J. J. Thomson

  o he discovered electrons and made the plum pudding model of the atom:

  o an atom is a ball of positive charge with negative electrons embedded inside it

  o therefore the atom is neutral (no overall charge)
• • 1909-1913 – Ernest Rutherford

  o he fired alpha particles at thin gold foil (the alpha scattering experiment)

  o from the plum pudding model, the particles should’ve passed straight or been slightly deflected

  o most did pass straight through, but some were deflected more than expected

  o this led to the Rutherford nuclear model where the mass of the atom is concentrated at the centre in a

  tiny, positively charged nucleus (which repelled the positive alpha particles) surrounded by a large cloud

  of electrons
• • 1913 – Neil Bohr

  o adapted the nuclear model suggesting that electrons orbited the

  nucleus at specific distances called energy levels (proven with

  theoretical calculations and experimental data which agreed)

  o this is called the Bohr model or Nuclear Model
• • Activity: the rate at which a source of unstable nuclei decays (this is measured in becquerel, Bq)

  • Count-rate: is the number of decays recorded each second by a detector

  • Ionising power: how easily radiation can knock electrons off an atom
• o an alpha particle (α)

  ▪ alpha radiation is when an alpha particle is emitted from the nucleus

  ▪ an alpha particle consists of 2 protons and 2 neutrons (a helium nucleus)

  ▪ highly ionising but weakly penetrating (absorbed by a sheet of paper and only travel 5cm in air)
• o a beta particle (β)

  ▪ a fast-moving electron ejected from the nucleus as a neutron turns into a proton

  ▪ beta particles have virtually no mass and a charge of -1

  ▪ moderately ionising and moderately penetrating (absorbed by 5mm of aluminium

  and travels up to 3m in air)
• o a gamma ray (γ)

  ▪ waves of electromagnetic radiation released by the nucleus due to changes in

  electron arrangement

  ▪ weakly ionising (just passes through objects)

  ▪ highly penetrating (absorbed by 5cm of lead and travels very far in air)

  o a neutron (n)

  ▪ neutrons are emitted to rebalance the atomic and mass number of an atom
• • alpha decay (a transmutation)

  o causes the atomic number to decrease by 2 and mass

  number to decrease by 4

  o the charge of the nucleus decreases too as protons

  are lost
• • beta decay (a transmutation)

  o does not change the mass of the nucleus (a neutron

  is lost but a proton gained, equal mass)

  o the number of protons increases by 1, increasing the

  positive charge the nucleus
• • gamma decay

  o the emission of a gamma ray doesn’t change the

  atomic or mass number of the atom, it solely

  gets rid of excess energy from the nucleus
• • contamination: unwanted presence of radioactive materials on or in an object

  o the hazard of contamination is the decay of the contaminated atoms releasing ionising radiation (the

  object becomes radioactive)

  o lasts a long time (until the source is removed or decays fully)

  o e.g. radioactive dust settling on your skin (your skin is contaminated)
• • irradiation: exposure to nuclear radiation

  o the irradiated object doesn’t become radioactive

  o lasts a short period of time (only while exposed to the source)
• • outside the body

  o Beta and gamma sources are the most dangerous because they can penetrate the skin and damage

  delicate organs whereas alpha particles cannot penetrate the skin
• • inside the body

  o Alpha sources are the most dangerous because they do all their damage in a very localised area

  (contamination is the main worry when working with alpha sources).

  o Beta sources are less damaging inside the body because the radiation is emitted across a wider area.

  o Gamma sources are the least dangerous inside the body because most of the rays pass straight out the

  body and they have the least ionising power.
• • Background radiation is caused by:

  o natural sources

  ▪ cosmic rays – radiation from space (usually from the sun and

  other stellar phenomena)

  ▪ radiation from rocks – radon gas is released from rocks and soil containing traces of uranium

  (the amount of radon released depends on rock type of the area)

  o man-made sources (smaller proportion)

  ▪ nuclear fallout – radioactive particles propelled into atmosphere by nuclear bombs and disasters

  ▪ medical rays – radioactive sources are used for imaging and treatment
• • Nuclear fission is the splitting of a large unstable nucleus e.g. uranium-235 or plutonium-239 into smaller nuclei

  o spontaneous fission is rare, for fission to occur an unstable nucleus has to absorb a thermal neutron

  Process

  • neutron absorbed by nucleus making it unstable nucleus

  • the unstable nucleus splits into 2 smaller, roughly equal sized daughter nuclei, 2-3 neutrons and gamma rays

  • all fission products have kinetic energy (energy is released by the fission reaction)

  • if uncontrolled, the neutrons released can go on to cause more fission reactions (a chain reaction)
• Nuclear reactors

  • in nuclear reactors, uranium or plutonium undergoes fission in the fuels cells which are surrounded by a

  moderator which slows down the neutrons released to control the chain reaction

  • control rods absorb excess neutrons to control the energy released

  o if lowered more neutrons are absorbed, reducing the energy released

  o if raised less neutrons are absorbed, increasing the energy released

  • the energy released, heats up the coolant which produced steam at the heat exchanger, the steam powers a

  turbine that generates electricity
• Nuclear fusion

  • Nuclear fusion is the joining of 2 light nuclei to form a heavier nucleus

  • takes place in stars mainly

  o deuterium + tritium → helium + neutron

  • any mass lost is converted to energy (releases more energy than fission)

  • difficult to reproduce on earth as the +ve charged nuclei repel each other but they have to be within 10-15m of

  each other to overcome the repulsion

  o this is possible on the sun due to the immense heat and pressure