atomic structure

• development of the atom

  democritus: imagined that if you continuously cut matter in half, you would reach a part where you can no longer cut it in half

  atom came from "atomus" meaning "indivisible"
• plum pudding model

  1877: discovery of the electron

  JJ thompson proposed the plum pudding model, where the atom was made of a positive substance with electrons in it, which agreed with beliefs at the time:

  • solids cannot be squashed, therefore the things that make them up must be olid throughout
  • rubbing two solids together pften results in static charge so there must be something outside the atom which can be transferred as atoms collide
• development of the atom

  1909: Ernst Rutherford's gold foil experiment

  using gold foil: can be flattened to only a few atoms wide.

  positively charged alpha particles were fired at the gold leaf

  most passed straight through, suggesting most of atom is empty space

  some were deflected suggesting that the nucleus was positively charged (because they're both positively charged: same charges repel)

  some bounced straight back suggesting very massive/ concentrated center

  > leading to a small, dense positively charged nucleus (no neutrons yet) and negatively charged electrons orbiting around it
• development of the atom

  Niels Bohr

  he discovered, through his calculations, that electrons orbit the nucelus at different distances (shells) according to their energy levels

  calculations agreed with observations from experiments
• development of the atom

  1932: James Chadwick discovered the neutron

  we now use this altered model to this day

  By doing this, he was able to explain that since particular chemicals burn with certain-coloured flames, the pattern of energy released by electrons in the chemical reaction must be the same for every single atom of that element.

  Therefore, electrons cannot be arranged at random, but they must have fixed levels of energy within each type of atom.
• development of the atom

  further discoveries led to the discovery of protons
• developments of the atom

  Chadwick used a version of Rutherford's experiment, using a sheet of beryllium and a paraffin block instead of gold foil. He was able to prove that a proton-sized neutral particle - now known as the neutron - existed.
• size of the atom

  • the radius of an atom is about 0.1 nm (1 × 10^-10 m)
  • the radius of a nucleus (1 × 10^-14 m) is less than 1/10,000 of the radius of an atom
• Atoms of an element with the same number of protons and electrons but different numbers of neutrons.

  measured against 1/12 of carbon 12 isotope
• Atoms can, however, lose or gain electrons due to collisions or other interactions. When they do, they form charged particles called ions:

  • if the atom loses one or more electrons, it becomes a positively-charged ion
  • if the atom gains one or more electrons, it becomes a negatively-charged ion
• An atom’s nucleus can only be stable if it has a certain amount of neutrons for the amount of protons it has.

  Elements with fewer protons (ones near the top of the periodic table) are stable if they have the same number of neutrons and protons.

  However as the number of protons increases, more neutrons are needed to keep the nucleus stable.

  >For example lead, lead-206 has 82 protons and has 124 neutrons.

  Nuclei with too many, or too few, neutrons do exist naturally but are unstable and will decay by emitting radiation.
• alpha radiation

  If the nucleus has too few neutrons, it will emit a ‘package’ of two protons and two neutrons called an alpha particle.

  An alpha particle is also a Helium-4 nucleus, so it is written as 2(top) 4(below)He 

  alpha particle is written the same just with "a" instead of He

  Alpha decay causes the mass number of the nucleus to decrease by four and the atomic number of the nucleus to decrease by two.

  
• beta particle

  If the nucleus has too many neutrons, a neutron will turn into a proton and emit a fast-moving electron.

  This electron is called a beta (β) particle

  A beta particle has: relative mass of zero and as the beta particle is an electron, written as −1 0e.

  also written as −1 0𝛽.

  beta particle comes from nucleus not shells

  neutrons can split into a positive proton (same mass, positive charge). An electron (balance the positive charge) is ejected at high speed and carries away a lot of energy.

  atomic number increases by one and the mass number remains the same.
• gamma radiation

  After emitting an alpha or beta particle, the nucleus will often still be too ‘hot’ and will lose energy (similar way to how a hot gas cools down by emitting infrared radiation)

  High energy particles will emit energy as they drop to lower energy levels.

  (energy levels in the nucleus are much higher than in the gas) the nucleus will cool down by emitting a more energetic electromagnetic wave, a gamma ray.

  Gamma ray emission causes no change in the number of particles in the nucleus (atomic number and mass number remain the same)
• neutron emission

  Occasionally a neutron can be emitted by radioactive decay.

  • This can occur naturally - absorption of cosmic rays high up in atmosphere can result in neutron emission (rare at the Earth’s surface).
  • it can occur artificially - the work done by James Chadwick firing alpha particles at Beryllium resulted in neutrons being emitted from that.
  • A further example of neutron emission is in nuclear fission reactions, where neutrons are released from the parent nucleus as it splits.

  Neutron emission: mass number of the nucleus to decrease by one, the atomic number remains the same
• powers of ionising radiation

  alpha (α)

  blocked by paper and skin, alpha radiation has high ionising power and has a range of < 5cm in the air

  beta (β)

  blocked by 3mm aluminium foil, beta radiation has low ionising power and has a range of approximately 1m in the air

  gamma (y)

  blocked by lead or concrete, gamma radiation has very low ionising power and have a range of >1km in the air
• measuring ionising radiation

  All types of radioactive decay can be detected by a Geiger-Muller tube, or G-M tube.

  The radiation ionises the gas inside

  >charged particles move across the chamber and get counted as charges rather like an ammeter.
• half life

  It is not possible to say which particular nucleus will decay next, but given that there are so many of them, it is possible to say that a certain number will decay in a certain time. 
• half life

  the time it takes for half of the unstable nuclei in a sample to decay or for the activity of the sample to halve or for the count rate to halve. Count-rate is the number of decays recorded each second by a detector, such as the Geiger-Muller tube.

  - The half-life of radioactive carbon-14 is 5,730 years. If a sample of a tree (for example) contains 64 grams (g) of radioactive carbon after 5,730 years it will contain 32 g, after another 5,730 years that will have halved again to 16 g.
• calculating isotopes remaining

  • a fraction - a ½ of a ½ of a ½ of a ½ remains, which is ½ × ½ × ½ × ½ = 1/16 of the original sample
• irradiation

  Process of exposing an object to a source of radiation. Eg fruit exposed to gamma rays in order to destroy bacteria is said to have been irradiated.

  it also applies radiation from atomic nuclei

  Irradiation from radioactive decay can damage living cells. This can be put to good use as well as being a hazard.
• uses of irradiation - sterilisation

  Irradiation can be used to preserve fruit sold in supermarkets by exposing the fruit to a radioactive source - typically cobalt-60.

  The gamma rays emitted by the cobalt will destroy any bacteria on the fruit but will not change the fruit in any significant way.

  The process of irradiation does not cause the irradiated object to become radioactive.
• advantages and disadvantages of irradiation

  Advantages

  • sterilisation can be done without high temperatures
  • it can be used to kill bacteria on things that would melt

  Disadvantages

  • it may not kill all bacteria on an object
  • it can be very harmful - standing in the environment where objects are being treated by irradiation could expose people’s cells to damage and mutation
• contamination

  Contamination occurs if an object has a radioactive material introduced into it.

  >An apple exposed to the radiation from cobalt-60 is irradiated but an apple with cobalt-60 injected into it is contaminated.

  As with irradiation, contamination can be very useful as well as being potentially harmful.
• contamination - water leaks

  Water supplies can be contaminated with a gamma-emitting radioactive isotope to find leaks in pipes .

  if theres a leak: contaminated water seeps into the ground, causing a build-up of gamma emissions in that area.

  >>The build-up of gamma emissions can be found using a Geiger-Muller tube.

  >This makes it easier to decide where to dig to find the leak.

  The isotope used for this must:

  • be a gamma emitter
  • have a half-life of at least several days (allow the emissions to build up in the soil)
  • not be poisonous to humans as it will form part of the water supply
• advantages and disadvantages of contamination

  ADVANTAGES

  • Radioactive isotopes can be used as medical and industrial tracers
  • Use of isotopes with a short half-life means exposure can be limited
  • Imaging processes can replace some invasive surgical procedures

  DISADVANTAGES

  • It can be difficult to ensure that the contamination is fully removed so small amounts of radioisotope may still be left behind
  • radioactive isotopes may not go where wanted
  • Exposure to radioactive materials can potentially damage healthy cells
• irradiation vs contamination

  • irradiation Occurs when an object is exposed to a source of radiation outside the object
  • contamination Occurs if the radioactive source is on or in the object
  • irradiation Doesn’t cause the object to become radioactive
  • A contaminated object will be radioactive for as long as the source is on or in it
  • irradiation Can be blocked with suitable shielding
  • Once an object is contaminated, the radiation cannot be blocked from it
  • irradiation Stops as soon as the source is removed
  • It can be very difficult to remove all of the contamination
• effects of radiation on the body

  Radioactive materials are hazardous. 

  Nuclear radiation can ionise chemicals within a body, which changes the way the cells behave.

  It can also deposit large amounts of energy into the body, which can damage or destroy cells completely.
• effects of radiation on the body

  • Eyes: High doses can cause cataracts.
  • Thyroid: Radioactive iodine can build up and cause cancer, particularly during growth.
  • Lungs: Breathing in radioisotopes can damage DNA.
  • Stomach: Radioactive isotopes can sit in the stomach and irradiate for a long time.
  • Reproductive organs: High doses can cause sterility or mutations.
  • Skin: Radiation can burn skin or cause cancer.
  • Bone marrow: Radiation can cause leukaemia and other diseases of the blood.
• reducing risks

  • keep radioactive sources like technetium-99 shielded (preferably in a lead-lined box) when not in use
  • wear protective clothing to prevent the body becoming contaminated should radioactive isotopes leak out
  • avoid contact with bare skin and do not attempt to taste the sources
  • wear face masks to avoid breathing in materials
  • limit exposure time - so less time is spent around radioactive materials
  • handle radioactive materials with tongs in order to keep a safer distance from sources
  • monitor exposure using detector badges, etc