Atoms, Elements and Compounds

• Atom

  The simplest part of an element that can exist
• Atoms

  • Arranged in neat rows and columns in a solid element
  • Can be broken down into subatomic particles
• Element


  A substance that cannot be broken down into simpler substances by chemical means
• There are approximately 100 naturally occurring elements
• Periodic table

  A tabular arrangement of the chemical elements, ordered by their atomic number, electron configuration, and recurring chemical properties
• Chemical reaction

  When two or more elements or compounds interact to form new substances
• Chemical reactions


  • Result in new substances being made
  • Often involve a detectable change in energy (e.g. release of heat, light, sound)
• Compound


  Two or more elements chemically combined in fixed proportions
• Chemical reactions


  • Sodium + Chlorine -> Sodium chloride
  • Hydrogen + Oxygen -> Water
• Fixed proportions

  The atoms in a compound combine in a specific ratio
• Fixed proportions in compounds

  • Water (H2O): 2 hydrogen to 1 oxygen
  • Magnesium chloride (MgCl2): 1 magnesium to 2 chlorine
  • Aluminium oxide (Al2O3): 2 aluminium to 3 oxygen