radioactivity

• describe the differences between the plum pudding model and the nuclear model?

  plum pudding doesn't show nucleus or shells and doesn't show neutrons and protons unlike the nuclear model
• what is an isotope?

  Isotopes are forms of an element that have the same number of protons but different numbers of neutrons
• what is half-life ?

  the interval of time required for one-half of the atomic nuclei of a radioactive sample to decay
• what is contamination?

  Contamination: presence of unwanted substances on a surface or in a material.
• what is irradiation?

  Irradiation: exposure to radiation, which can kill or inactivate microorganisms.
• how do you protect yourself from irradiation?

  keeping the sample in a lead box
• what is ionisation?


  the radiation from a radioactive substance can knock electrons out of atoms. the atoms become charged when they lose electrons, this is ionisation.
• why did Rutherford's experiment lead to the plum pudding model being replaced?


  the results from the experiment meant that the plum pudding model was disproved as it was inaccurate.
• what is the significance of Rutherford's nuclear model?

  it proved that atoms had a tiny positively charged nucleus but is mostly empty
• why is alpha radiation's contamination more important than irradiation?

  contamination is more concerning as alpha is unable to pass through even a sheet of paper due to their large mass, let alone human skin. also, if swallowed due to contamination, it would be damaging as it could not get out. a way to prevent this is by wearing gloves when handling samples.
• the plum pudding model - jj Thompson


  contains gas like spheres

  mass is evenly distributed throughout

  the body is positive

  there are negative things floating within
• Rutherfords experiment 


  alpha particles are passed through gold foil

  the apparatus is in a vacuum

  there is a zinc sulphide screen

  flashes happen when alpha particles hit the screen

  the screen can be moved around the apparatus

  result = most particles passed through, some were deflected and some were reflected from the foil.
• atomic mass and number


  atomic mass is above the element on the periodic table and atomic number is below the element on the periodic table.
• atoms


  atoms are not indivisible

  there is a heavy nucleus in the centre

  positively charged protons are in the nucleus

  protons + neutrons = nucleons

  negatively charged electrons orbit the nucleus
• what were the conclusions of Rutherfords experiment?


  there must be lots of empty space within the atom

  there is a positive nucleus causing the deflection

  nucleus must be small and heavy
• what is radioactive decay?


  the spontaneous emission of penetrating and ionising radiation from the nucleus of an atom.
• why does radioactive decay occur?


  it occurs in order to become more stable, by changing the amount of protons in the nucleus.
• what can stop the different types of radiation from being harmful?

  alpha = can be stopped by paper and only has a range of a few cm in air

  beta = can be stopped by a thin sheet of metal (aluminium) and has a range of 1m in air

  gamma = can be stopped by thick lead and has an unlimited range in air
• what is alpha radiation?


  it consists of particles, composed of two protons and two neutrons, it has the greatest ionising power.
• what is gamma radiation?


  it consists of electromagnetic radiation from the nucleus of an atom , it is uncharged so has no mass so its emission doesn't change the amount of protons or neutrons in a nucleus. so the mass and charge of the nucleus are both unchanged.
• what is beta radiation


  it consists of fast moving electrons emitted from the nucleus, it is less ionising than alpha but more ionising than gamma. beta has too many neutrons compared to its protons. a neutron in the nucleus changes to a proton and a beta particle (an electron) which is instantly emitted to the relative mass is zero and its relative charge is -1.
• what happens when an unstable nucleus particles emits a beta particle?

  the atomic number goes up by one and its mass number is unchanged and a neutron turns to a proton.

  the charge of the nucleus is increased but the mass is unchanged

  numbers on the top = total no. of protons and neutrons after change is equal to the number before the change

  numbers on bottom = shows that the total charge is equal to the charge before the change
• what happens when an alpha particle is emitted?

  its atomic number goes down by two and its mass number goes down by four

  the mass and charge of the nucleus are both reduced

  numbers along top = total number of protons and neutrons after change

  numbers along bottom = show number of protons after change is equal to number before change
• neutron emission 


  neutrons are emitted by some radioactive substances as a result of alpha particles colliding with unstable nuclei in the substance, such collision causes these nuclei to become even more unstable and emit a neutron which can pass through substances much easier than alpha or beta as they are uncharged.
• what is contamination?


  the presence of radioactive material on surfaces, objects or in the environment that can cause harm to living beings and the environment
• what is half-life?

  the time taken for half the radionuclides to decay, this is the same as the time taken for the activity to fall to half the initial value.
• what affects half life?


  the isotope

  independent of temperature

  independent of pressure

  independent of chemical composition/reaction
• calculations using half-life 


  no. of half lives = original activity / current activity

  number of half lives = original mass / current mass

  time taken = number of half lives x time for half life

  net decline = fraction that has decayed

  1/2n (n=number of half lives)
• how does c-14 work in living things?

  c-14 beta decays in living things, its half life is 5730 years

  when dead, c-14 production ceases

  with time, activity due to c-14 reduces

  therefore time since death can be calculated

  c-14 to c-12 ratio is a long term constant

  c-14 is produces in the atmosphere.
• isotopes and their half-lives 


  pa-235, beta, 70seconds

  pu-239, alpha, 24000 years

  ra-226, gamma, 1600 years

  sr-90, beta, 29 years
• how does background radiation occur?


  natural = natural minerals, radon gas, cosmic radiation from space, c-14/ wood beta decay

  man made = nuclear weapons, nuclear accidents, nuclear energy fallouts, mining bringing radioactive minerals to the surface.
• how us nuclear radiation used in medicine?


  radioactive tracers = patient drinks water containing a tiny bit of radioactive iodine, if kidney is blocked substance goes up into kidney but does not go out again and can be seen on detector. iodine is used as it has a half-life of 8 days so it is long enough to be used for the test, but also is able to have decayed within a few weeks, its emit gamma radiation so can be detected outside the body and it decays stably.
• how are gamma cameras used?


  patient is infected with gamma emitting radioactive isotope solution, it is absorbed by organ and a gamma camera detects the radiation emitted by the solution. gamma rays pass through holes in the thick lead grid infant of the detector but only detects rays from nuclei directly infront of it. detector signals build up an image of where the radioactive isotope is located in the organ.
• how can radiation treat cancer?


  radioactive implants are used to destroy cancerous tumours, it is emitted from an isotope of cobalt which has a half-life of five years. gamma radiation used for better penetration.
• what is nuclear fission?

  an atom is struck by a neutron causing the nucleus to split into two smaller fragments of nuclei of roughly equal size and to release several neutrons. fission can also happen very rarely without a neutron being absorbed, spontaneous fission.
• what does an atom release after undergoing fission?

  two or three neutrons at high speeds

  energy in the form of gamma radiation plus kinetic energy stored in the fission neutrons and fragment nuclei
• what are chain reactions?

  as two or three neutrons are released in nuclear fission, these neutrons can collide with other nuclei causing a chain reaction of fission.
• what happens inside a nuclear reactor?


  a nuclear reactor has uranium fuel rods spaced out evenly in the reactor core .

  the reactor core contains fuel rods, control rods, and water at a high pressure.

  the fission neutrons are slowed down by collisions with the atoms in water molecules which is vital as fast neutrons don't cause further fission of U-235. the water is a moderator as it slows down fissions.

  control rods in the core absorb surplus neutrons to keep chain reaction under control , depth of rods adjusted to maintain a steady chain reaction.
• what is nuclear fusion?

  spontaneous nuclear fusion happens in stars

  small nuclear fusion into larger ones but the nuclei repel so the temperature and pressure must be very high so that the kinetic energy in the nuclei is then enough too overcome repulsion. the products are mostly stable (not radioactive) and lots of energy is released resulting in stars shining.

  nuclear fusion has a very high energy density and is only spontaneous at high temps and pressures.