2.4 - Shapes of Molecules & Ions

• valence shell electron pair repulsion theory (VSEPR)

  valence electrons repulse each other because they have like charges

  the larger the electron density the more repulsion there is
• what does electron repulsion determine

  the shape and bond angles in molecules & ions
• bond angle


  angle between two covalent bonds in a molecule or giant covalent structure
• order of electron repulsion - most to least

  lone pair+ lone pair (biggest bond angle)

  lone pair + bonding pair (second biggest bond angle)

  bonding pair + bonding pair (bond angles are the smallest)
• why repulsion between lone pairs is greater than lone pairs & bonding pairs

  in lone pairs orbitals are smaller as electrons aren't shared between atoms So have a greater electron density So more repulsion

  in bonding pours electrons are shared between atoms so orbitals are bigger so changes are more Spread out so lower charge density so less electron repulsion
• electron density


  measures the probability of an electron being at any given point in time
• determining shape of molecules with no lone pairs

  dran dot and cross diagram to determine number of electron regions by determining number of lone pairs & bonding pairs

  1 bonding/lone pair = 1 electron region

  determine arrangement of electron areas in terms of repulsion so determine Shape & bond angles of molecules
• determining shapes of molecules with lone pairs

  drown dot and cross diagram & determine number of electron regions

  determine shape of molecule based on number of electron regions

  lone pairs force bonding pairs closer together so affect bond angles

  when drawing molecule ignore lone pairs
• determining shapes of molecules with multiple bonds

  dram dot & cross diagram & determine number of electron regions

  double or triple bonds count as one electron region

  determine shape and take into account any lone pairs

  when drawing molecule ignore lone pairs & draw 2 lines for double bonds or 3 for triple bonds
• why does xenon react with fluorine

  xenon is a very large atom with lots of shielding on its outer electrons allowing fluorine which is very electronegative to attract valence electron of xenon so they form stable compounds

  XeF4
• explain the structure & bond angle in XeF4

  Xe has 6 areas of electron density so has an octahedral shape with 90° between pairs

  but since Xe has 4 bonding pairs & a lone pairs the shape is square planar with 90° between pairs because lone pairs have a greater electron density so repel more
• What is the total number of electron pairs for a linear shape?

  3
• What is the number of lone pairs in a linear shape?

  0
• How many bonding pairs are present in a linear shape?

  3
• What is the shape of a molecule with 3 electron pairs?

  Linear
• What is the total number of electron pairs for a trigonal planar shape?

  4
• How many bonding pairs are present in a trigonal planar shape?

  3
• What is the number of lone pairs in a trigonal planar shape?

  1
• What is the shape of a molecule with 4 electron pairs and 1 lone pair?

  Trigonal planar
• What is the total number of electron pairs for a bent (V-shaped) molecule?

  4
• How many bonding pairs are present in a bent (V-shaped) molecule?

  2
• What is the number of lone pairs in a bent (V-shaped) molecule?

  2
• What is the total number of electron pairs for a trigonal bipyramidal shape?

  5
• What is the shape of a molecule with 4 electron pairs and 2 lone pairs?

  Bent (V-shaped)
• How many bonding pairs are present in a trigonal bipyramidal shape?

  3
• What is the number of lone pairs in a trigonal bipyramidal shape?

  2
• What is the shape of a molecule with 5 electron pairs and 2 lone pairs?

  T-shaped
• What is the total number of electron pairs for an octahedral shape?

  6