Matter and radioactivity

• Solids, liquids and gases

  States of matter
• Density


  • Measured in g/cm³
  • Depends on the material
• Density of materials


  • Iron: 7.8 g/cm³
  • Water: 1.0 g/cm³
  • Air: 0.00129 g/cm³
• Density

  The amount of mass per unit volume
• Measuring density

  1. Measure mass

  2. Measure volume

  3. Divide mass by volume
• Methods to measure volume

  • Regular shape: Use ruler
  • Irregular shape: Eureka can, measuring cylinder
• There is little difference between the density of a liquid and its corresponding solid
• The particles in solids, liquids and gases are tightly packed
• Atom

  Smallest unit of an element that retains its properties
• Atom

  • Contains a nucleus with protons and neutrons
  • Has electrons orbiting the nucleus
• Atomic models

  • Democritus: Atoms are indivisible spheres
  • J.J. Thomson: Plum pudding model with electrons in a positive sphere
  • Rutherford: Gold foil experiment showing a dense nucleus
  • Bohr: Electrons orbit the nucleus in specific energy levels
• Atoms have a radius of 1x10^-10 meters
• Proton

  Positively charged particle in the nucleus
• Neutron

  Neutral particle in the nucleus
• Electron

  Negatively charged particle orbiting the nucleus
• Atomic number

  Number of protons in the nucleus
• Mass number

  Number of protons and neutrons in the nucleus
• Protons and neutrons make up most of an atom's mass
• The atomic number defines the element, but the mass number can differ due to different numbers of neutrons
• Isotope

  Atoms of the same element with different numbers of neutrons
• Ion

  Atom that has gained or lost one or more electrons, giving it a net positive or negative charge
• Atoms normally have a neutral overall charge, but can become ions by gaining or losing electrons
• Radioactive decay

  The process where an unstable nucleus emits radiation to become more stable
• Radioactive decay

  • Occurs when a nucleus has too many or too few neutrons for the number of protons
  • Can involve emission of alpha particles, beta particles, gamma rays, or neutrons
• Types of radioactive decay

  • Alpha decay
  • Beta decay
  • Gamma emission
  • Neutron emission
• Alpha particle

  A helium nucleus (2 protons, 2 neutrons) emitted during radioactive decay
• Beta particle


  A high-energy electron emitted from the nucleus during radioactive decay
• Gamma ray


  High-energy electromagnetic radiation emitted from the nucleus during radioactive decay
• Radioactive decay is a random process, but the rate can be described statistically using half-life
• Half-life


  The time it takes for half of the radioactive nuclei in a sample to decay
• The activity of a radioactive sample is measured in becquerels, which is the number of decays per second
• Calculating remaining radioactivity


  For each half-life, the remaining activity is half of the previous amount
• Concrete


  A type of material
• Types of radioactive decay

  • Alpha
  • Beta
  • Gamma
• Types of materials

  • Aluminium
  • Lead
• Range in air


  • Very low
  • 330 m
  • Greater than 1 km
• Radioactive decay


  A random process
• Count rate


  The number of decays recorded each second by a detector
• Half-life


  The time it takes for half of the unstable nuclei (in a sample) to decay
• Activity


  Measured in becquerels