bonding and structure

• Ionic bonding


  The electrostatic force of attraction between oppositely charged ions formed by electron transfer
• Formation of ionic compounds

  1. Metal atoms lose electrons to form +ve ions

  2. Non-metal atoms gain electrons to form -ve ions
• Ionic bonding


  • Stronger and higher melting points when ions are smaller and/or have higher charges
  • E.g. MgO has higher melting point than NaCl as Mg2+ and O2- are smaller and have higher charges than Na+ and Cl-
• Giant ionic lattice


  Regular 3D pattern of ions in an ionic solid
• Ionic bonding is between ions and all their surrounding oppositely charged ions
• Typical physical properties of ionic compounds

  • High melting points due to strong electrostatic attractive forces
  • Non-conductor of electricity when solid as ions are held tightly in lattice
  • Good conductor of electricity when in solution or molten as ions are free to move
  • Usually soluble in aqueous solvents
• Covalent bond


  Strong electrostatic attraction between a shared pair of electrons and the nuclei of the bonded atoms
• Dative covalent bond


  Shared pair of electrons in the covalent bond come from only one of the bonding atoms
• Compounds with dative covalent bonds

  • NH4+
  • H3O+
  • NH3BF3
• Average bond enthalpy


  Measurement of covalent bond strength, larger value means stronger bond
• Types of bonding


  • Ionic
  • Covalent
• Structures of covalent compounds

  • Simple molecular
  • Giant ionic lattice
• Only use the words molecules and intermolecular forces when talking about simple molecular substances
• Properties of ionic and molecular compounds

  • Boiling and melting points
  • Solubility in water
  • Conductivity when solid
  • Conductivity when molten
  • General description
• Molecular shapes


  • Linear
  • Trigonal planar
  • Tetrahedral
  • Trigonal pyramidal
  • Bent
  • Octahedral
• Electronegativity


  Relative tendency of an atom in a covalent bond to attract electrons to itself
• F, O, N and Cl are the most electronegative atoms
• Polar covalent bond


  Bond with unequal distribution of electrons, producing a charge separation (dipole)
• The element with the larger electronegativity in a polar compound will be the δ- end
• Symmetric molecules with all bonds identical and no lone pairs will not be polar even if individual bonds are polar
• Induced dipole-dipole interactions


  Temporary dipoles that form in molecules and can induce dipoles in neighbouring molecules
• Factors affecting induced dipole-dipole interactions

  • More electrons in molecule increases chance of temporary dipoles forming, making interactions stronger
• Permanent dipole-dipole forces


  Stronger than induced dipole-dipole, occur between polar molecules
• Permanent dipole forces occur in addition to induced dipole-dipole interactions
• Induced dipole-dipole interactions occur between all molecular substances and noble gases, not ionic substances
• Hydrogen bonding


  Stronger than other intermolecular forces, occurs between H atom attached to N, O or F which has a lone pair
• Water can form two hydrogen bonds per molecule due to its high electronegativity and two lone pairs
• Compounds that can form hydrogen bonds

  • Alcohols
  • Carboxylic acids
  • Proteins
  • Amides
• Hydrogen bonding occurs in addition to induced dipole-dipole interactions
• Molecular structures


  • Iodine (I2 molecules held by induced dipole-dipole)
  • Ice (water molecules held by hydrogen bonding)