Lesson 8: Intermolecular Forces 2.0

• Covalent Bonding

  The strongest type of bonding, formed by the sharing of electrons between 2 nonmetallic atoms
• Network Covalent Bonding

  • Polar covalent
  • Found in network solids including diamond (Cn), graphite (Cn), silicon carbide (SiC) and silicon dioxide (SiO2)
  • Substances are very hard and have very high melting points due to their structure in which all bonds are very strong and very directional, arranged in a three-dimensional network of covalently-bonded atoms
• Diamond

  • Each carbon atom is bound in three dimensions to four other atoms in a repeating tetrahedral pattern. Atoms are held in rigid positions resulting in high melting point and no electrical conductivity. A giant macromolecule is the result, where every atom is simultaneously attracted to neighbouring atoms by covalent bonds
• Graphite

  • Atoms are bonded together in layers. In each layer, each carbon atom forms three strong covalent bonds with three other atoms in a repeating trigonal planar pattern. The strong bonds account for graphite's high melting point. The fourth bond, between the layers, is weak and easily broken. The layers can therefore slide over each other such that graphite is soft and slippery
• Graphite

  • It's a good lubricant, especially at high temperatures
  • Electrons can travel through the layers so graphite is a good conductor of electricity
  • The structure also makes graphite lightweight yet strong
• Ionic Bonding

  The second strongest type of bonding, formed by the attraction of oppositely-charged ions to each other
• Crystal Lattice Structure

  • Ionic bonding produces an orderly three-dimensional ionic crystal lattice in which each ion is surrounded by ions of the opposite charge
  • Since the bonding is not concentrated between individual atoms, but involves all of the ions, ionic compounds are generally found as solids and have relatively high melting and boiling points
• Metallic Bonding

  Formed by the attraction of positive ions for valence electrons that are free to move from one unoccupied valence orbital to another

  
• Metallic Bonding

  • The structure of metals is believed to be an array of positive atomic nuclei in a "sea" of electrons = "electron-sea model"
  • Atoms in the metal become positive ions because their valence electrons are dispersed throughout the metal. The positive ions remain fixed but electrons are free to move, such that they can be simultaneously attracted to more than one nucleus, leading to a very stable structure

  
• Relative Strength of Bonding Forces(strongest to weakest)

  • Network Covalent
  • Ionic
  • Metallic
  • Hydrogen
  • Dipole-dipole
  • London dispersion
• How to determine which substance has the highest boiling or melting point

  1. Determine if substance is one of the network covalent solids

  2. Any ionic substance will have the second highest boiling/melting point

  3. Any metallic substance will have the third highest boiling/melting point

  4. Any molecular substances that are not network covalent solids will have the lowest boiling/melting point
• Polarizability: the ability of a substance to form a temporary dipole
• Temporary Dipoles: A molecule with no permanent dipole moment may still experience attractive forces due to its polarizability (the ability to distort electron cloud distribution)
• If a network covalent bond is between a metalloid and a nonmetal, the substance would present some metallic properties (e.g. SiO2 exhibits electrical conductivity as a solid, while other network covalent bonds usually don't)
• Properties of Ionic Bonds

  • ionic compounds are brittle, so if a force is applied, the like charges may align and repel each other, breaking the crystal lattice structure
  • can conduct electricity only in their liquid or aqueous state since the ions are mobile and can carry their charges around from one place to another
  • however, they cannot conduct electricity as a solid since the ions are rigidly held together and cannot carry their charges
• Physical properties of Metallic bonds

  • good conductors of electricity: if electrons from an external source enter one end of a wire, the moving electrons carry the charge from one place to another due to repulsive forces
  • good conductors of heat: energy is easily transferred through the metal because moving electrons can pass on kinetic energy by colliding with other particles within the metal
• More physical properties of metallic bonds

  • easily deformed: if a force is applied, one layer of metal ions moves across another, and no bonds are broken. The electrons flow between the ions forming a barrier between each other so they do not repel. Thus metals are malleable (sheets) and ductile (wires)
  • they can form alloys since the electrons in the valence shell can move around other atoms
  • can have variable melting/boiling points based on the amount of valence electrons and electronegativity (higher means stronger)