bonding, structure and the properties of matter

• Ionic bonding is the electrostatic attraction between positive and negative ions
• Ionic compounds are held together in a giant lattice structure
• The structure extends in all directions in a substance
• The electrostatic attraction between positive and negative ions holds the structure together
• Properties of ionic substances:

  • High melting and boiling point (due to strong electrostatic forces between oppositely charged ions)
  • Do not conduct electricity when solid (ions in fixed positions)
  • Conduct when molten or dissolved in water (ions are free to move)
• Examples of positive ions: Na+, Mg2+, Al3+, Ca2+, Rb+

  Examples of negative ions (anions): Cl−, Br−, SO42−, NO3−, OH− (chloride, bromide, sulfate, nitrate, hydroxide)

  When working out a formula of an ionic compound, it is important to ensure that positive and negative charges balance each other
• Ionic compounds are formed through the reaction of a metal with a non-metal

  Electron transfer occurs - metal gives away its outer shell electrons to non-metal

  In the case of MgO, Mg becomes Mg2+ and O becomes O2− (oxide)
• Covalent bond is a shared pair of electrons between two atoms
• Simple molecular covalent substances:

  • Do not conduct electricity (no ions)
  • Consist of small molecules
  • Have weak intermolecular forces, resulting in low melting and boiling points
• Intermolecular forces increase as the mass/size of the molecule increases

  This causes melting/boiling points to increase as well (more energy needed to overcome these forces)
• Polymers are very large molecules with atoms linked by covalent bonds

  Thermosoftening polymers melt/soften when heated

  There are no bonds between polymer chains, and strong intermolecular forces ensure the structure is solid at room temperature
• Giant covalent substances:

  • Solids with atoms covalently bonded together in a giant lattice
  • Have high melting/boiling points due to strong covalent bonds
  • Mostly do not conduct electricity (no delocalised electrons)

  Examples include diamond, graphite, and silicon dioxide
• Allotropes of carbon:

  Diamond:

  • Four strong covalent bonds for each carbon atom
  • Very hard with a high melting point
  • Does not conduct electricity

  Graphite:

  • Three covalent bonds for each carbon atom
  • Layers of hexagonal rings
  • High melting point, soft, and can conduct thermal and electricity

  Fullerenes, nanotubes, and graphene also exhibit unique properties
• Metallic bonding is the forces of attraction between delocalised electrons and nuclei of metal ions
• Properties of metals:

  • High melting/boiling points
  • Good conductors of heat and electricity
  • Malleable and soft due to layers of atoms that can slide over each other
• Alloys are mixtures of metal with other elements, usually metals

  They are harder than pure metals because different sizes of atoms distort the layers, preventing them from sliding over each other
• The amount of energy needed to change state from solid to liquid or liquid to gas depends on the strength of the forces between the particles of the substance

  The nature of the particles involved depends on the type of bonding and the structure of the substance

  The stronger the forces between the particles, the higher the melting point and boiling point of the substance
• A pure substance will melt or boil at a fixed temperature, while a mixture will melt over a range of temperatures
• The three states of matter are solid, liquid, and gas
• Nanoscience studies particles that are 1 - 100nm in size
• Uses of nanoparticles:

  • Medicine (drug delivery systems)
  • Electronics
  • Deodorants
  • Sun creams (better skin coverage and more effective protection against cell damage)
• Nanoparticles have different properties from those of the same materials in bulk due to their high surface area to volume ratio