structure and bonding

• Ionic bonding


  Electrostatic attraction between positive and negative ions. It is a relatively strong attraction.
• How ionic compounds are held together

  1. They are held together in a giant lattice

  2. It's a regular structure that extends in all directions in a substance

  3. Electrostatic attraction between positive and negative ions holds the structure together
• Properties of ionic substances

  • High melting and boiling point (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−
  • SO4

  2−

  • NO3

  −

  • OH−
• Ionic compounds are electrically neutral, i.e. positive and negative charges balance each other
• How ionic compounds are formed (MgO case)


  1. Reaction of a metal with a non-metal

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

  3. Mg becomes Mg2+ and O becomes O2− (oxide)
• Covalent bond


  Shared pair of electrons between two atoms
• Properties of simple molecular covalent substances

  • Do not conduct electricity (no ions)
  • Small molecules
  • Weak intermolecular forces, therefore low melting and boiling points
• How intermolecular forces change as the mass/size of the molecule increases

  They increase. That causes melting/boiling points to increase as well (more energy needed to overcome these forces).
• Polymers


  Very large molecules (>100s, 1000s of atoms) with atoms linked by covalent bonds
• Thermosoftening polymers


  Special type of polymers; they melt/soften when heated. There are no bonds between polymer chains. Strong intermolecular forces ensure that the structure is solid at room temperature. These forces are overcome with heating - polymer melts.
• Giant covalent substances


  • Solids, atoms covalently bonded together in a giant lattice
  • High melting/boiling points – strong covalent bonds
  • Mostly don't conduct electricity (no delocalised e−)
  • Examples: diamond, graphite, silicon dioxide
• Properties of allotropes of carbon

  • Diamond: four, strong covalent bonds for each carbon atom, very hard, very high melting point, does not conduct
  • Graphite: three covalent bonds for each carbon atom, layers of hexagonal rings, high melting point, layers free to slide as weak intermolecular forces between layers, soft, can be used as a lubricant, conduct thermal and electricity due to one delocalised electron per each carbon atom
  • Fullerenes: hollow shaped molecules, based on hexagonal rings but may have 5/7-carbon rings, C60 has spherical shape, simple molecular structure, high tensile strength, conductivity
  • Nanotubes: cylindrical fullerene with high length to diameter ratio, high tensile strength, conductivity
  • Graphene: a single layer of graphite
• Metallic bonding


  Forces of attraction between delocalised electrons and nuclei of metal ions
• Properties of metals


  • High melting/boiling points (strong forces of attraction)
  • Good conductors of heat and electricity (delocalised electrons)
  • Malleable, soft (layers of atoms can slide over each other whilst maintaining the attraction forces)
• Alloys


  Mixtures of metal with other elements, usually metals. Different sizes of atoms distorts the layers, so they can't slide over each other, therefore alloys are harder than pure metals.
• There are no forces between spheres and atoms, molecules and ions are solid spheres – this is not true
• 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. A mixture will melt over a range of temperatures.
• The three states of matter

  • Solid
  • Liquid
  • Gas
• Nanoscience


  Science that 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)
• Fine and coarse particles

  • Fine particles (soot), 100-2500 nm diameter
  • Coarse particles (dust), 2500-105 nm diameter
• Nanoparticles have different properties to those for the same materials in bulk due to their high surface area to volume ratio