atomic structure

• structure of an atom:

  • small with a radius of 1$\times$10$^{-10}$
  • positively charged nucleus containing protons and neutrons surrounded by negative electrons
  • the radius of the nucleus is 1/10,000 that of the atom
  • mass is concentrated in the nucleus
  • electrons arranged in different distances from nucleus in energy shells
  • electron arrangements may change with the absorption of electromagnetic radiation - further from nucleus, higher energy level - or by the emission of electromagnetic radiation - closer to nucleus, lower energy level
• • electrons have no overall charge due to having the same number of positive protons as negative electrons
  • mass number is the number of protons and neutrons
  • atomic number is the number of protons
  • an atom of the same element that has the same number of protons but different number of neutrons is an isotope
  • atoms can become positive ions by losing electrons and negative ions by gaining electrons
• development of the atom:

  • before discovery of electron, atoms were thought to be tiny spheres that could not be divided
  • the discovery of electron led to the plum pudding model by jj thompson stating that an atom was a ball of positive charge with negative electrons distributed throughout it
  • the alpha scattering experiment by rutherford showed that the mass was concentrated in the centre, and that the nucleus was charged, and that the tom is mostly empty space. The nuclear model replaced the plum pudding model
• development of the atom:

  • niels bohr introduced the idea that the negative electrons were orbiting the nucleus at specific distances in energy levels or shells
  • later experiments stated that the positive nucleus could be subdivided into different particles, each having the same amount of positive charge, these were called protons
  • chadwick then proved the existence of neutrons in the nucleus
• radioactive decay:

  • some atomic nuclei are unstable
  • the nucleus gives out radiation to become more stable
  • this random process id called radioactive decay
  • activity is the rate at which a source of unstable nuclei decay
  • measured in becquerel (Bq)
  • count rate is the number of decays recorded each second ny a detector - geiger-muller tube
• nuclear radiation:

  • alpha particle ($\alpha$) - consists of 2 neutrons and 2 protons, same as a helium nucleus
  • beta particle ($\beta$) - high speed electron ejected from the nucleus as a neutron turns into a proton
  • gamma ray ($\gamma$) - electromagnetic radiation from the nucleus as a wave
  • a neutron
• nuclear equations:

  • the emission of different nuclear radiation causes a change in mass and charge
  • beta decay does not cause the mass to change but causes the charge to increase by 1
  • alpha decay causes the mass to decrease by 2 and the charge to decrease by 2
  • the emission of a gamma ray does not cause the mass or the charge of a nucleus to change

  
• half-lives:

  • radioactive decay is random
  • the half life of a radioactive isotope is the time it takes for the number of nuclei of the isotope in a sample to halve
  • or the time it takes for the count rate from a sample containing the isotope to halve
  • radioactive decay formula describes the remaining quantity of a substance over time
  • net decline ratio represents how much of the original emission rate has been lost
• contamination & irradiation:

  • the unwanted presence of radioactive atoms on an object
  • the hazard from contamination is due to the decay of the contaminating atoms
  • irradiation is the process of exposing an object to nuclear radiation
  • the object itself does not become radioactive
• range of nuclear radiation:

  • alpha - 5cm in air
  • beta - 1m in air
  • gamma - unlimited in air
• penetration of nuclear radiation:

  what will absorb each type of radiation:

  • alpha - paper/skin
  • beta - 5mm of aluminium
  • gamma - several cm of lead
• ionising power of nuclear radiation:

  ionisation is the process when electrons are knocked off and lost from an atom

  how easily it ionises atoms it hits

  • alpha - very ionising
  • beta - less ionising than alpha
  • gamma - not very ionising
• uses of radiation:

  • gamma -> medical, high penetrating power but low ionising power and short half life to target and destroy cancer cells and sterilises medical equipment, explore internal organs
  • gamma -> food production, increases shelf life as it is irradiated by gamma to sterilise and preserve food as it kills microorganisms
• uses of radiation:

  • alpha -> smoke alarms, long half life to last longer and low penetrating power, constant level of radiation is needed nad alpha gives it
  • beta -> material thickness, penetrates thorugh paper so when it has a high count rate the material is thinner than needed but when the count rate drops the material is thicker than needed
• hazards of radiation:

  • can cause cancer as ionising the DNA leads to mutations which can cause uncontrollable cell division
• safety precautions of radiation:

  • wear protective equipment, goggles and lab coat
  • store in lead lined containers to block radiation
  • minimise exposure
  • dispose properly following regulations
  • use geiger-muller tubes to monitor exposure in environment
• background radiation:

  • radiation that is around us all the time
  • comes from:

  1. natural sources - rocks, cosmic rays from space
  2. man made sources - fallout from nuclear weapons testing, nuclear accidents

  • the level of background radiation or radiation dose is affected by occupation and location
  • unit for radiation dose is sieverts
• nuclear fission:

  • splitting a large and unstable nucleus
  • spontaneous fissions are rare
  • a chain reaction is when the products of the reaction go to cause more reactions
  • the neutrons emitted are absorbed by other large nuclei
  • the chain reaction is controlled in a nuclear reactor with a control rod which absorb neutrons to stop them colliding with other nuclei
  • the explosion from a nuclear weapon is by an uncontrolled chain reaction
• process of nuclear fission:

  1. the unstable nucleus absorbs a neutron becoming more unstable
  2. the unstable nucleus splits into two smaller nuclei roughly equal in size
  3. radiation is emitted in the form of two or three neutrons as well as a gamma ray
  4. energy is released by the fission reaction
  5. the fission products have kinetic energy
• fission:

  • 2 isotopes usually used are plutonium and uranium
  • generates electricity as the energy released heats the water turning it to steam which turns a turbine
  • disadvantages are that a lot of radioactive waste that needs to be contolled therefore are buried but is still radioactive for thousands of years. destroys habitats
• fusion:

  • joining two light nuclei to form a heavier nucleus.
  • example of natural fusion is the stars
  • in the process, some mass may be converted into the energy of radiation
  • causes intense temperature and pressure
  • contained in a magnetic field to stop the vaporisation of the container
  • not a viable energy source as it is expensive to harness as more energy is needed to create it than it gives off to use it as a power source