Shapes of Molecules and ions

• What is electron pair repulsion theory?

  A model used in chemistry to explain/predict the shapes of molecules and polyatomic ions

  Electrons are negatively charges so electron pairs repel one another
• The electron pairs surrounding a central atom determine the shape of the molecule or ion

  The electron pairs repel one another so that they are arranged as far apart as possible

  The arrangement of electron pairs minimises repulsion and thus holds the bonded atoms in a definite shape

  Different numbers of electron pairs result in different shapes
• What does a solid line represent?

  A bond in the plane of the paper
• What does a solid wedge represent?

  Comes out of the plane of the paper
• What does a dotted wedge represent?

  Goes into the plane of the paper
• A lone pair of electrons is slightly closer to the central atom, and occupies more space than a bonded pair. This results in a lone pair repelling more strongly than a bonding pair.
• What is the order of relative repulsions from lowest to highest?

  Bonded-pair/bonded-pair

  Bonded-pair/lone-pair

  Lone-pair/lone-pair
• Molecules with 4 electron pairs?

  The 4 electron pairs around the central atom repel one another as far apart as possible into a tetrahedral arrangement.

  Lone pairs repel more strongly than bonded pairs

  Therefore, lone pairs repel bonded pairs slightly closer together decreasing the bond angle
• What is a bond angle?

  The angle between the bonded pairs
• How much is the bond angle reduced by for each lone pair?

  2.5 degrees
• Tetrahedral shape?

  4 bonded pairs

  0 lone pairs

  109.5 degrees
• Pyramidal shape?

  3 bonded pairs

  1 lone pair

  107 degrees
• Non-linear shape?

  2 bonded pairs

  2 lone pairs

  104.5 degrees
• In molecules containing multiple bonds (double or triple), each multiple bond is treated as a bonding region. For example, the bonding and shape of carbon dioxide
• Linear shape?

  2 bonding regions

  180 degrees
• In carbon dioxide, the 4 bonded pairs around the central carbon atom are arranged as two double bonds, which count as two bonded regions. The two bonded regions repel one another as far apart as possible, giving the CO2 molecule a linear shape with all three atoms aligned in a straight line
• Electron pairs around the central atom repel each other as far apart as possible. The greater the number of electron pairs, the smaller the bond angle. Lone pairs of electrons repel more strongly than bonded pairs of electrons
• Trigonal planar shape?
• The bond angle in water is about 105 degrees because there are only two bonded pairs of electrons on oxygen, but also two lone pairs of electrons that repel the bonded pairs to give an overall bond angle of 105 degrees.
• Bond angles in trigonal planar molecules are approximately 120 degrees.
• A tetrahedron has four sides, each at right angles to the others. In this case, the four sides represent the four C-H single covalent bonds in methane. Each side represents a bond between the carbon atom and hydrogen atom. Because the four sides meet at right angles, they form a regular tetrahedron.
• Trigonal Planar shape?

  3 bonding regions/ electron pairs

  equal bond angles - 120 degrees
• Octahedral shape?

  5 electron pairs/regions

  90 degrees
• What causes an octahedral shape from six bonded pairs?

  SF6 - has 6 bonded pairs but forms a molecule that is an octahedral shape. The reason lies with what is meant by the shape of a molecule. The 6 Fluorine atoms are positioned at the corners of an octahedron - the octahedral shape with 8 sides is obtained by joining together the six corners occupied by fluorine atoms.
• The ammonium ion?

  NH4+, has 4 bonded pairs of electrons surrounding the central nitrogen atom.

  NH4+ ion has the same number of bonded pairs of electrons around the central atom as a methane molecule

  NH4+ has the same tetrahedral shape and bond angle - 109.5
• Carbonate, nitrate and sulfate ions?

  CO3 2-, NO3-, SO4 2-

  carbonate and nitrate have 3 regions of electron density surrounding the centre atom - same shape of a BF3 molecule - trigonal planar

  Sulfate ions have 4 centres of electron density around the central sulfur atoms - same shape as a methane molecule - tetrahedral