Chapter 6 - Shapes of molecules

• Describe the electron pair repulsion theory


  Electron pairs repel each other as far as possible due to negative charge. The arrangement of electrons minimises repulsion, holding bonds in a definite shape.
• What determines the shape of a molecule?

  The electron pairs surrounding a central atom
• What does a solid wedge represent?

  a bond that comes out of the plan of the paper

  
• What does a dotted wedge represent?

  a bond that goes into the plan of the paper
• Why do lone pairs repel more strongly than bonded pairs?

  lone pair of electrons slightly closer to the central atom and occupies more space than a bonded pair
• State the order of increasing repulsions between lone pairs and bonding pairs

  bonded pair/bonded pair < bonded pair/lone pair < lone pair/lone pair
• How much is the bond angle reduced by for each lone pair?

  2.5
• Do double bonds have the same repelling effect as single bonds?

  yes
• What is meant by the term 'electron density'?

  the probability of an electron being present at a specific location
• What is the shape and bond angle of a molecule with 2 bonded pairs of electrons?

  -linear

  -180

  -e.g. CO2

  
• What is the shape and bond angle of a molecule with 2 bonded pairs and 2 lone pairs?

  -non-linear

  -104.5

  -e.g. H2O

  
• What is the shape and bond angle of a molecule with 3 bonded pairs of electrons?

  -trigonal planar

  -120

  -e.g. BCl3

  
• What is the shape and bond angle of a molecule with 3 bonded pairs and 1 lone pair?

  -pyramidal

  -107

  -e.g. NH3

  
• What is the shape and bond angle of a molecule with 4 bonded pairs of electrons?

  -tetrahedral

  -109.5

  -e.g. CH4

  
• What is the shape and bond angle of a molecule with 5 bonded pairs of electrons?

  -trigonal bipyramidal

  -90 and 120

  -e.g. PCl5

  
• What is the shape and bond angle of a molecule with 6 bonded pairs of electrons?

  -octahedral

  -90

  -e.g. SF6

  
• What is the shape and bond angle of a molecule with 2 bonded pairs and 1 lone pair?

  -non-linear

  -120

  -SO2
• What is the shape and bond angle of an ammonium ion (NH4+)?

  -tetrahedral

  -109.5

  -dative covalent bond between central N atom and one of the H atoms

  
• What is the shape and bond angle of a hydronium ion (H3O+)?

  -trigonal pyramidal

  -107

  -dative covalent bond between central O atom and one of the H atoms

  
• What is the shape and bond angle of a sulfate ion (SO4 2-)?

  -tetrahedral

  -109.5

  -2 S=O double bonds and 2 O atoms with single bonds each contain one extra electron represented by a triangle
• What is electronegativity?

  The ability of an atom to attract the bonding electrons in a covalent bond
• When is the pair of electrons in a covalent bond equally shared?

  when the bonded atoms are the same or have the same electronegativity
• When is the pair of electrons in a covalent bond unequally shared?


  when the bonded atoms are different and have different electronegativities
• What is the Pauling scale used for?

  to compare different electronegativities
• Why are the pair of electrons in a covalent bond sometimes unequally shared?

  -nuclear charges are different

  -atoms may be different sizes

  -the shared pair of electrons may be closer to one nucleus than the other
• Describe the trend in electronegativity on the periodic table

  electronegativity increases across and up the periodic table, with the most electronegative atom being fluorine

  
• Why do noble gases not have Pauling numbers?

  they don't form compounds
• How can the type of bond of a compound be identified?

  -the difference in electronegativites

  
• What is a non-polar bond?

  a covalent bond in which electrons are shared equally
• What is a pure covalent bond?

  When the bonded atoms come from the same element and the electron pair is shared equally
• What is a polar bond?

  a covalent bond in which electrons are shared unequally
• What is the charge of the more electronegative element in a polar covalent bond?

  partial negative charge

  
• What is the charge of the less electronegative element in a polar covalent bond?

  partial positive charge

  
• What is a dipole?

  A molecule that has two poles
• What is a permanent dipole?

  the separation in partial charges across a polar bond, arising from different electronegativities
• When can dipoles cancel out?

  if the molecule is symmetrical and the dipoles act in opposite directions
• Describe what happens when an ionic compound dissolves in a polar solvent

  e.g. Na+ ions are attracted to oxygen (delta negative) and Cl- ions are attracted to hydrogen (delta positive)

  -the ionic lattice breaks down as it dissolves

  -in the resulting solution, the water molecules surround the ions
• Why is carbon dioxide considered non-polar?

  -the 2 C=O bonds each have a permanent dipole

  -2 dipoles act in opposite directions and exactly oppose one another as the molecule is symmetrical

  -the dipoles cancel out and the overall dipole is zero
• Why is water a polar molecule?

  -2 O-H bonds each have a permanent dipole

  -2 dipoles act in different directions but do not exactly oppose one another as the molecule is non-symmetrical

  -overall, the oxygen end of the molecule has a partial negative charge and the hydrogen end has a partial positive charge

  
• What are intermolecular forces?


  Weak interactions between dipoles of different molecules