Structure 2.1-2.3 (bonding)

• boron = exception to the octet rule (incomplete octet)
• number of bonds increases = strength of bonds increases
• smaller cation = stronger electrostatic attraction between cation and delocalised electrons
• small ionic radius + high charge = strongest ionic bond
• polyatomic ion


  groups of joined atoms
• ionic lattice


  regular, repeating 3-dimensional arrangement of alternating cations and anions

  • oppositely charged ions are as close together as possible
  • like charged ions are as part apart as possible
• lattice enthalpy


  the energy needed to separate one mole of a solid ionic compound into a gaseous state
• higher charge= higher lattice enthalpy
• smaller ionic radii= higher lattice enthalpy
• determinants of lattice enthalpy

  • charges of the ions involved
  • size (ionic radii) of the ions involved
• endothermic


  processes which require/absorb energy
• solubility in water


  when an ionic compound dissolves in water:

  • ionic bonds within the solid lattice are broken
  • hydrogen bonds between adjacent water molecules
  • ion-dipole bonds are formed between ions and water molecules
• saturated


  when no more solute can be dissolved in that solution at that temperature
• solubility

  the maximum amount of that substance that can be dissolved in a given quantity of solvent at a given temperature
• covalent bond


  shared pair of electrons
• Bond length (covalent substances)


  distance between two atoms:

  • maximizes attraction of the two positive nuclei for the shared pair of electrons
  • minimizing the repulsion between the two positive nuclei
• increase in number of bonds = increase in bond strength
• increase in number of bonds = decrease in bond length
• metallic lattice


  regular, repeating 3 dimensional lattice of delocalised electrons and cations
• larger ionic radius, negative electrons are further away from the positive nucleus, hence weaker electrostatic attraction
• greater charge, greater density, greater charge density, stronger electrostatic attraction between cations and delocalised electrons, stronger metallic bonding
• smaller ionic radius, greater charge density, stronger electrostatic attraction between cations and delocalised electrons, stronger metallic bonding
• substitutional alloys


  made from elements with similar atomic size:

  • as the ionic radii is similar, one element can be substituted for another
  • the slightly different size restricts the movement of the cations within the lattice
  • creates a harder and more durable alloy, making it less malleable
• interstitial alloys


  made from elements with smaller atomic size:

  • small atoms fit into the gaps between the larger atoms
  • form strong-directional element-element bonds
  • making the alloy harder, stronger and less malleable
• Alloys


  mixtures of metals and other element, that retain the physical properties of a metal. Enhances the properties:

  • increased hardness
  • improved corrosion resistance
  • variation in color
  • lower melting point
• explain why alloys are harder than pure metals

  The difference in atoms sizes between metals restricts the movement of the metallic lattice, making them harder

   
• outline the compositional difference between substitutional and interstitial alloys

  Substitutional alloys are metal mixtures with atoms of similar size and chemical properties. Interstitial alloys include atoms much smaller than the base metal atoms.