Radioactive

• Isotopes


  Atoms of the same element that have the same number of protons but a different number of neutrons
• Isotopes


  • They have the same atomic number but a different mass number
• Isotopes are important for learning about radioactivity
• Atomic number


  The number of protons in the nucleus of an atom
• Mass number


  The total number of protons and neutrons in the nucleus of an atom
• The number of protons is what defines the element, the number of neutrons can vary
• Since protons are positively charged and neutrons are neutral, the nucleus of an atom has a particular overall charge
• Atomic mass


  The number of protons plus the number of neutrons in an atom
• Isotopes


  • Carbon-12 and carbon-14
• Components of an atom

  • Electrons surrounding the nucleus
  • Nucleus containing protons and neutrons
• Most elements have different isotopes, but there are usually only one or two stable isotopes
• The other isotopes tend to be unstable and radioactive, which means they give out nuclear radiation and may decay into other elements
• Types of nuclear radiation

  • Alpha
  • Beta
  • Gamma
  • Neutron
• Alpha particle


  Two neutrons and two protons - the same as a helium nucleus
• Alpha particles


  • They have a relative mass of 4 and a charge of +2
  • They are relatively large and heavy and slow moving
• Beta particle


  An electron, with virtually no mass and a charge of -1
• Beta particles


  • They move quite fast and are quite small
  • For every beta particle emitted, a neutron turns to a proton in the nucleus
• Gamma ray


  A type of electromagnetic radiation emitted by a nucleus to get rid of excess energy after emitting an alpha or beta particle
• Gamma rays


  • They have no mass and no charge, they are just energy
• If a nucleus contains a lot of neutrons, it may just throw out a neutron
• The number of protons stays the same, but it now has a different nucleus, so it becomes an isotope of the same element
• Radioactivity is the process of unstable nuclei emitting radiation
• Radioactive decay rate


  The higher the radioactivity, the faster the radiation has to travel, the more ionising it is
• Alpha and beta particles would be blocked by thin aluminium, gamma rays would be blocked by thick lead
• Balancing nuclear equations


  1. For alpha decay: mass number decreases by 4, atomic number decreases by 2

  2. For beta decay: mass number doesn't change, atomic number increases by 1

  3. For gamma decay: mass number and atomic number don't change
• A thorium (Th) atom


  Decays into a uranium (U) atom with 92 protons and 146 neutrons by emitting an alpha particle
• Electrons can absorb EM radiation and move up to higher energy levels, then fall back down emitting the same amount of energy as radiation
• An atom is ionised if it loses an electron
• Ionising radiation


  • Alpha, beta and gamma radiation can ionise atoms
  • The ionisation power is different for different ionising radiations
• Fluorescent tubes


  1. Electrons are accelerated through mercury vapour, ionising some mercury atoms and producing free electrons

  2. These collide with other mercury atoms, exciting their electrons to higher energy levels

  3. When the excited electrons fall back down, they emit UV radiation

  4. This UV radiation is absorbed by a phosphor coating, which then emits visible light
• Half-life


  The time taken for half the radioactive atoms in a sample to decay
• The radioactivity of a sample always decreases over time
• When excited electrons return to their original energy levels

  They emit radiation in the ultraviolet range of the electromagnetic spectrum
• Phosphor


  A compound that coats the inside of the tube and absorbs the ultraviolet radiation, causing the electrons to move to higher energy levels
• Electrons then cascade down the energy levels

  Emitting different frequencies of radiation, all in the visible part of the electromagnetic spectrum
• When an electron absorbs a positive amount of energy, it moves up one or more energy levels, then falls back to lower levels, emitting EM radiation
• Positive ionization


  When an electron is knocked out of an atom, leaving it with a positive charge
• Half-life


  The average time taken for a radioactive source's activity (or count rate) to halve
• The activity of a radioactive sample always decreases over time
• To calculate half-life from numbers


  1. Count how many half-lives it took to fall to a certain level

  2. Calculate the half-life by dividing the total time by the number of half-lives