Chemistry topic 3

Ionic Bonding

  • Occurs between a metal and a non-metal.
  • Electrons are transferred to achieve full outer shells (octet rule).
  • Ions formed attract, creating a giant ionic lattice.
  • Positive ions (cations) formed by losing electrons.
  • Negative ions (anions) formed by gaining electrons.
  • Ionic substances are brittle and don't conduct electricity in solid form, but do when molten or aqueous.
  • Electron density is low between ions.
  • Ion migration occurs in electrolysis due to electrostatic attraction.
  • Ionic bond strength increases with ion charge and decreases with ionic radius.
  • Polarisation: distortion of electron cloud. Cations with high charge and small radius have high polarising power. Anions with larger radius and charge are easily polarized.

Covalent Bonding

  • Occurs between two non-metals via electron sharing.
  • Strong electrostatic attraction between nuclei and shared electrons.
  • High electron density between bonding atoms.
  • Single, double, and triple bonds exist.
  • Shorter bonds are stronger.
  • Dative/coordinate bonds: both electrons from one atom, indicated by an arrow.
  • Macromolecular covalent structures have high melting points due to strong covalent bonds. Generally do not conduct electricity (except graphite and graphene).
  • Simple covalent molecules have low melting and boiling points due to weak intermolecular forces.
  • Examples:
    • Diamond: tetrahedral structure, very hard.
    • Graphite: hexagonal sheets, delocalized electrons, conducts electricity, layers slide.
    • Graphene: single 2D layer of graphite, strong, lightweight, conducts electricity.

Bond Polarity

  • Electronegativity: ability of an atom to attract electrons in a covalent bond.
  • Increases across a period, decreases down a group.
  • Polar bonds form if electronegativity difference is sufficient, creating partial charges (∂+\partial+ and ∂−\partial-).
  • Permanent dipole results from unequal sharing of electrons.
  • Polarisability: distortion of electron cloud of an anion by a cation. Smaller, highly charged cations are more polarising.
  • Polar molecules: overall polarity due to arrangement of polar bonds and molecular geometry.
  • Electronegativity difference between 0.4 and 1.7 indicates a polar covalent bond. Greater than 1.7 suggests an ionic bond.

Shapes of Molecules

  • Determined by electron pairs around central atom and their repulsion.

  • Lone pairs cause more repulsion, reducing bond angles by ~2.5° per lone pair.

  • Common Shapes:

    • Linear: 2 bonding pairs, 0 lone pairs, 180° angle.
    • V-Shaped: 2 bonding pairs, 2 lone pairs, ~104.5° angle.
    • Trigonal Planar: 3 bonding pairs, 0 lone pairs, 120° angle.
    • Triangular Pyramid: 3 bonding pairs, 1 lone pair, ~107° angle.
    • Tetrahedral: 4 bonding pairs, 0 lone pairs, 109.5° angle.
    • Trigonal Bipyramidal: 5 bonding pairs, 0 lone pairs, 180° and 120° angles.
    • Octahedral: 6 bonding pairs, 0 lone pairs, 90° angle.

    Bond length: average distance between nuclei.
    Bond angle: angle between covalent bonds.

Metallic Bonding

  • Giant lattice of positive ions surrounded by delocalized electrons.
  • Strong electrostatic attraction between ions and electrons.
  • Greater positive ion charge = stronger attraction.
  • Larger ion size = weaker attraction.
  • Good conductors due to mobile delocalized electrons.
  • Malleable because layers of ions can slide.
  • High melting points due to strong electrostatic forces.