chap 5 - structure and properties of material

• elements

  pure substance that cannot be broken down by physical or chemical means into simpler substance
• mixtures

  physical combination of two or more substance that is not chemically combined in any fixed ratio
• compound

  pure substance that is made up of two or more elements chemically combined in a fixed ratio
• elements

  • homogenous < same composition throughout the substance>
  • only one type of particles
  • represented by a ´o´ symbol
  • fixed composition
  • fixed melting/boiling point
  • distinct properties
  • cannot be separated by physical/ chemical means
• compound

  • homogenous < same composition throughout the substance>
  • represented by a formula
  • fixed melting/boiling point
  • properties different from the elements that made up the compound
  • separated by chemical means
• mixtures

  • non-homogenous < not the same composition throughout the substance>
  • no fixed chemical symbols/formula
  • not fixed composition
  • melts and boils over a range of temperature
  • properties same as the substance that made up the mixtures
  • separated by physical means
• formation of ionic compounds

  • metals transfer valence electrons to the non-metals
  • the resulting cation and anion attract each other to form giant ionic lattice structure <electrically neutral>
• structure determines properties

  • ionic compound has a giant lattice structure with strong ionic bonds between oppositely charged ions throughout the entire structure
• properties of ionic compounds

  • exists as solid at room temp. and pressure cuz it has high melting and boiling point
  • low volatility (difficult to evaporate) cuz it has high melting and boiling point that is far from the room temp.
  • reason

  > a large amount of energy has to be absorbed to overcome the strong electrostatic forces of attraction between cation and anion

  • generally soluble in water ( dissolve in water )
  • insoluble in organic solvent ( cannot dissolve in oil )
• properties of ionic compound

  • poor electrical conductor when in solid state

  > reason : the cation and anion are held in fixed position and are unable to move to carry electric current

  • good electric conductor when molten( melted)/aqueous( dissolved in water)

  > reason : giant ionic lattice structure breaks down, cation and anion are free to move. These mobile ions carry electric current
• properties of ionic compound

  • good electrical conductor of electricity when molten or aqueous state

  > reason: giant ionic lattice structure breaks down ,these mobile ions carry electric current

  • why solid cannot conduct electricity but molten/aqueous can?

  > ions that are in solid state are held in fixed positions in a giant lattice structure and there are no free ions to move to conduct electricity while in molten/aqueous state, the giant lattice structure breaks down and the ions are free to move to conduct electricity
• properties of simple covalent substance

  • exists as gas or liquid at room temp and pressure because of it has low melting and boiling point

  > only need a small amount if energy to overcome the weak intermolecular forces of attraction

  • high volatility cuz it has low melting and boiling point that is close to the room temperature
  • insoluble in water, soluble in organic solvent
  • poor electrical conduct when in all states of matter cuz no free electrons nor ions to carry electric current