4.4 - Atomic structure

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Structure of atom:

  • Atoms are around 1 × 10-10 meters in radius
  • Radius of nucleus is less than 1/10 000 of atom's radius. Most of matter is concerntrated in nucleus. 
  • Electron arrangements may change with absorbsion of emmission of EM radiation (further from nucleus = higher energy level)
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Mass number, atomic number and isotopes:

  • In an atom, electron number = proton number, no overall charge.
  • Atoms in same element have same number of protons. 
  • Atomic number = Number of protons, Mass number = Number of protons + neutrons.

(Smaller number on element's symbol = mass number)

  • Atoms of an elements with different neutron number = iscotopes
  • Atoms turn into positive ions if they lose 1 or more outer electron (s)
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Development of atom model:

  • New exerimental evidence may lead to a model being changed or replaced 
  • Before the electron discovery, atoms were thought to be tiny spheres that you could't divide.
  • Electron discovery lead to plum pudding model of atom - atom is ball of positive charge with negative electrons embedded in it.
  • Alpha patricle scattering experiment lead to conclusion that mass was concerntrated at the centre (nucleus), and the nuclues was charged. This model replaced the plum pussing model.
  • Niels Bohr adapted nuclear model suggesting electrons orbit nucleus at specific distances. - Bohr's theoretical calculations agreed with experimental observations.
  • Later experiements led to idea that positive charge of any nucleus could be subdivided into a whole number of smaller particles, each with the same amount of positive charge - protons was the name given to these partciles. 
  • James Chadwick's experimental work provided evidence for neutron's existance within the nucleus - this was approx. 20 yrs after nucleus became an accepted scientific idea. 
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Radioactive decay and nuclear radiation:

  • Some atomic nuclei are unstable, the nucleus gives out radiation as it changes to become more stable, it is a random process called radioactive decay. 
  • Activity = rate at which a source of unstable nuclei decays (measured in Becquerel - Bq)
  • Count rate = number of decays recorded each second by a detector (eg. Geiger-Muller tube)
  • nuclear radiation may be:

1) An alpha particle = 2 neutrons + 2 protons (same as a helium nucleus) emmitted from the nucleus. They are strongly ionising because of their size, can only travel a few cm in air and don't penetrate far into materials (stopped quickly) - are absorbed by a sheet of paper.

2) A beta particle = high speed electron ejected from the nucleus as a neuton turns into a proton. They are moderately ionising (relatively far into materials before colliding), travel a few meters in air and are absorbed by a sheet of aluminium approx. 5mm thick. For every beta particle emmitted, a neutron in the nucleus is turned into a proton.

3) A gamma ray = electromagnetic radiation from the nucleus (short wavelength). They are weakly ionising as they pass far through materials rather than colliding, travel a long distance in air and absorbed by thick sheets of lead, or meters of concrete.

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Uses of the nuclear radiation:

  • Alpha: Smoke detectors - ionises air particles causing a current to flow. If smoke is in the air it binds to the ions meaning the current stops and the alarm sounds. (beta and gamma do not create enough ions to make the air in the gap conduct electricity)
  • Beta: Test thickness of metal sheets - detector measures amount of radiation passing through and adjusts thickness rollers accordingly (particles are not immediately absorbed like alpha radiation, but don't penetrate as far as gamma.)  + Test faults in pipes - radioactive isotope injected and traced from outside (alpha particles would be absorbed by soil, gamma rays would pass through metal pipe). 
  • Gamma: Inject/ swallow certain radioactive iscotopes to track their progress around the body using an extrenal detector - this allows a display where the strongest reading is coming from. (eg. iodine-123, which is absorbed by thyroid gland like normal iodine 127, but gives off radiation which can show whether the thyroid is taking in iodine as it should. Usually the isotopes are gamma (never alpha) so radiation passes out without much ionisation. They should have a short half life so radiation quickly disappears.
  • Any type: Dating - any radioactive substance decays showing you the age if traced back 
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Nuclear equations:

  • Nuclear equations are used to represent radioactive decay.
  • Alpha particle:
  • Beta particle:
  • The emission of different types of nuclear radiation may cause a change in mass and/or charge of the nucleus
  • Alpha causes both charge and mass to decrease. Eg .
  • Beta causes the the charge of the nucleaus to increase but not the mass. Eg.
  • The emission of gamma rays doesn't cause the mass or charge of the nucleus to change.
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Half lives:

  • Radioactive decay is random 
  • Half-life of a radioactive isotope = time taken for number of nuclei of the isotope in a sample to halve OR  time taken for count rate (activity) from a sample containing the isotope to fall to half it's initial level.
  • Half-life can be used to show you the rate at which a source decays even though radioactive decay is completely random.
  • When a radioactive nucleus decays it becomes stable.
  • Short half-life = activity falls quickly, can be dangerous due to high radiation emission at start 
  • Long half-life = activity falls slowly, can be dangerous as nearby areas are exposed o source constantly over th whole period.
  • Some isotpes take hours, some take millions of yrs - none ever reach 0 activity.
  • Radioactive iscotopes have a wide range of half-life values. 
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Radioactive contamination:

  • Radioactive contamination = (physical contact) unwanted presence of materials contaminating radioactive atoms on other materials. The contamination hazard is due to decay of contaminating atoms but the type of radiation emitted affects the level of hazard. Gloves and tongs should be used when handling sources to avoid particles gettig stuck on skin - protective suits + masks can prevent inhalation.
  • Irradiation = (going near) exposing an object to nuclear radiation, (the irradiated object doesn't become contaminated/radioactive). keeping sources in lead-lined boxes, standing behind barriers or in a different room using remote controlled arms can reduce the effects.
  • Outside the body, gamma and beta are the most dangerous as the can penetrate the body to delicate organs. - alpha is blocked by skin + easily blocked by air. High levels of any irradiation are dangerous but esecially gamma and beta. 
  • Inside the body, alpha are the most dangerous as they damage in a localised area - so contamination rather than irradiation is the main concern with alpha sources. Beta radiation is absorbed over a wider area inside the body so is less damaging, and some passes out the other side. Gamma mostly pass out so are the least damaging inside the body.
  • Data/ findings into studies of effects of radiation should be published and shared with ofther scientists for peer reviews.
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Background radiation:

  • It's around us contantly, coming from natural sources eg. rocks and cosmic rays from space, and man-made sources eg. fallout from nuclear weapons testing and nuclear accidents.
  • Level of background radiation and radiation dose may be affected by occupation and your loaction.
  • Radiation dose (tells you risk of harm) is measured in Sieverts (Sv), where 1 Seivert = 1000 millisieverts (mSv).
  • You should always measure and subtract backdround radiation from results to avoid systematic error.
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Risk and uses:

  • Radiation can enter living cells, ionise atoms and molecules within them leading to tissue damage. Lower doses cause minor damage without killing cells  which can give rise to mutant calls which divide uncontroallbly. Higher doses tend to kill cells completely causing radiation sickness. 
  • Radiotherapy can treat cancer with radiation - high doses of ionising radiation kill all living cells. Gamma rays are directed in dosages to kill cancer cells without damagin too many nromal ones. Radiation emitting implants (usually beta emitters) can be put next to/inside tumours. Damage is enevitably done to healthy cells causing the patient to feel ill, but the cancer can be sucessfully killed off.
  • Every situation carries benefits and risks when using radiation. (eg. tracers which can diagnose life-threatening diseases, but carry a small risk of cancer). Prolonged exposure poses future risks and many side-effects, but radiotherapy may eliminate someone's cancer completely. The percieved risk varies from person to person but is not the same as the actual risk of a procedure.
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Fusion and fission:

  • Nuclear fission is the splitting of a large, unstable nucleus (eg. uranium or plutonium).
  • Spontaneous fission is rare. Usually, for it to occur the unstable nucleus must first absorb a neutron.
  • The nucleus undergoing fission splits into two smaller nuclei, roughly equal in size, and emits two or three neutrons plus gamma rays.
  • Energy is released by the fission reaction.
  • All of the fission products have kinetic energy. The neutrons may go on to start a chain reaction.
  • The chain reaction is controlled in a nuclear reactor to control the energy released. The explosion caused by a nuclear weapon is caused by an uncontrolled chain reaction.
  • Nuclear fusion is the joining of 2 light nuclei to form a heavier nucleus. In this process some of the mass may be converted into the energy of radiation which is acrried away. 
  • Fusion releases more energy than fission for a given mass of fuel, though reactors are hard to build due to the high temperatures and pressures needed.
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