Chapter 5

• Elements

  Building blocks of matter
• Elements


  Pure substances that cannot be decomposed into simpler substances by chemical methods
• Elements

  • Sugar
  • Carbon
  • Water vapour
• Elements

  • Have fixed melting and boiling points
  • Can be classified as metals or non-metals
• Chemical symbols


  Chemists use them to represent elements<|>Each symbol is unique, consisting of one or two letters
• Elements and their symbols

  • calcium (Ca)
  • carbon (C)
  • hydrogen (H)
  • iron (Fe)
  • mercury (Hg)
  • neon (Ne)
  • silicon (Si)
  • sodium (Na)
• Classification of elements

  • metals
  • non-metals
  • metalloids
• Metals

  • Shiny (lustrous)
  • Mostly solids (except mercury)
  • High melting and boiling points (except sodium, potassium and mercury)
  • Good heat and electrical conductivity
  • Ductile and malleable
  • Sonorous
• Metalloids

  • Shiny (lustrous)
  • Solids
  • High melting and boiling points
  • Moderate heat and electrical conductivity
  • Brittle
• Non-metals

  • Dull (non-lustrous)
  • Gases, volatile liquids or solids
  • Low melting and boiling points (except carbon and silicon)
  • Poor heat and electrical conductivity (except graphite and diamond)
  • Brittle if solid
• Atoms


  Tiny particles that make up elements<|>Smallest particles of an element that have the chemical properties of that element<|>Atoms of different elements are different
• Molecules


  Groups of two or more atoms that are chemically combined (joined together)
• Diatomic molecules

  • hydrogen
  • oxygen
  • nitrogen
• Compounds

  Pure substances that contain two or more elements that are chemically combined in a fixed ratio
• The formation of a compound requires a chemical reaction which involves energy change
• Compounds


  • Have their own physical and chemical properties different from their constituent elements
  • Have fixed melting and boiling points
  • Can be decomposed by chemical processes
• Compounds cannot be separated by physical methods
• Chemical formula


  Shows the types of elements present and the ratio of the different atoms present
• Chemical formulas

  • lead(II) nitrate (Pb(NO3)2)
  • hydrogen chloride (HCl)
  • carbon dioxide (CO2)
  • carbon monoxide (CO)
  • sulfuric acid (H2SO4)
  • sulfurous acid (H2SO3)
  • ethanol (C2H5OH)
• Compounds have a fixed composition by mass
• Mixtures

  Made up of two or more substances that are not chemically combined
• Mixtures


  • petrol
  • air
  • muddy water
  • alloys such as brass, bronze and steel
• Mixtures


  • Not pure substances
  • Melt and boil over a range of temperature
  • Do not have a fixed composition by mass
  • Can be separated by physical methods
• Gaseous mixture


  • air
• Solid mixture


  • brass
  • steel
  • bronze
• Mixture of two elements

  • neon and hydrogen
• Mixture of two compounds

  • water and carbon dioxide
• Mixture of an element and a compound


  • hydrogen and ammonia
• Differences between mixture and compound

  • Separation: Mixtures can be separated by physical processes, compounds can only be broken down by chemical processes
  • Properties: Mixture properties are the same as its components, compound properties are different from its constituents
  • Energy changes: Mixture formation involves little or no energy change, compound formation involves an energy change
  • Composition: Mixture components can be mixed in any proportion, compound elements are combined in a fixed proportion
• In a mixture of iron filings and sulfur, iron filings can be separated with a magnet, but iron in iron(II) sulfide cannot be separated this way
• In a mixture of iron filings and sulfur, each component retains its properties, but iron(II) sulfide has different properties from iron filings or sulfur
• No chemical reaction takes place in a mixture, but a chemical reaction takes place to form a compound, usually with heat and light given off
• A mixture has no fixed composition, but a compound has a fixed composition