Structure & Properties

• Ionic substances


  • Have giant ionic crystal lattice structures
  • Consist of large numbers of alternating positive and negative ions held together by strong electrostatic forces of attraction
• Hardness and brittleness of ionic substances


  • Hard as the strong electrostatic forces of attraction cause the ions to resist motion
  • Brittle as the ionic lattice is deformed when a strong force is applied
• Element

  A pure substance that cannot be broken down into two or more simpler substances by chemical methods
• Compound


  A pure substance containing two or more elements that are chemically combined in a fixed ratio
• Separating compounds


  1. Thermal decomposition - exposing the compound to strong heat

  2. Electrolysis - passing electric current through the compound
• Mixture


  Substances not in fixed ratios, can consist of elements and/or compounds
• Separating mixtures


  1. Filtration

  2. Chromatography

  3. Distillation
• Ionic substances


  • Soluble in water, insoluble in organic solvents
  • Conduct electricity in liquid/molten and aqueous states

  
• Solid ionic compounds do not conduct electricity 


  • The oppositely-charged ions are held in fixed positions by strong electrostatic forces of attraction between them. Thus, the ions are not free to move to conduct electricity.
• Covalent bonds


  Strong bonds holding atoms together in simple covalent molecules
• Intermolecular forces


  Weak forces holding simple covalent molecules together
• Simple covalent molecules


  • Low melting and boiling points
  • Insoluble in water, soluble in organic solvents
  • Unable to conduct electricity
• Giant covalent structures


  Atoms connected by strong covalent bonds
• Giant covalent structures


  • Diamond
  • Graphite
  • Silica/Silicon dioxide
• Giant covalent structures


  • High melting and boiling points, insoluble in water and organic solvents
• Polymer


  Macromolecule consisting of many covalent molecules joined into chains
• Carbon allotrope

  • Graphene
• Polymers


  • Most are solids at room temperature due to their large size
  • Do not have a fixed melting or boiling point
  • Typically soften over a range of temperatures when the weaker intermolecular forces of attraction are overcome by the molecular vibrations with higher kinetic energy
• Macromolecules


  • Most are insoluble in water and soluble in organic solvents
  • Cannot conduct electricity in any states due to the absence of mobile ions and electrons
• Graphene


  A carbon allotrope obtained by extracting one layer of carbon atoms from graphite
• Carbon nanotubes


  A carbon allotrope with a structure like a sheet of paper rolled into a tube and glued together where the edges meet
• Fullerenes


  Carbon allotropes that may be filled with sensitive biochemical molecules, before being delivered to target sites in patients' bodies
• Covalent molecules are held together by weak intermolecular forces of attraction
• Atoms in covalent structures are connected to one another via strong covalent bonds
• Simple covalent molecules have low melting and boiling points
• Giant covalent structures have high melting and boiling points
• Most covalent structures are unable to conduct electricity, except for graphite
• Alloys


  Alloys are a mixture of a metal with one or more other elements physically combined, not chemically combined.
• Ionic substances can conduct electricity in both the liquid/molten and aqueous states


  The strong electrostatic forces of attraction between the ions are overcome and the giant ionic lattice structure breaks down. Thus, the ions are now free to move (mobile) to conduct electricity.
• Ionic substances have high melting and

  boiling points.


  The electrostatic forces of attraction between oppositely-charged ions is very strong. Hence, a lot of heat

  energy is required to overcome the strong ionic bonds.
• Ionic substances are hard but brittle.

  1. Ionic substances are hard as the strong electrostatic forces of attraction cause the ions to resist motion.
  2. Ionic substances are brittle as the ionic lattice is deformed when a strong force is applied.
• Giant Covalent Structures

  The atoms in giant covalent structures are connected to one another via strong covalent bonds.

  
• Diamond and Graphite

  • are allotropes of carbon.
• Diamond

  • Each carbon atom in diamond forms covalent bonds with

  four other carbon atoms.

  • Diamond has a tetrahedral arrangement.
• Graphite


  • Each carbon atom in graphite forms covalent bonds with three other carbon atoms.

  • has a layered structure of hexagonal rings of carbon atoms.

  • Between each layer, there are weak

  forces of attraction

  are present.
• Silicon Dioxide (sand)

  • made up of silicon and oxygen atoms linked to each other via strong covalent bonds.

  • The overall ratio of silicon atoms to oxygen atoms

  is always 1 : 2.