6) Shapes of Molecules and Intermolecular Forces

• What is the Valence Shell Electron Pair Repulsion Theory:

  • Valence(outer) electron exists in pairs
  • Electron pairs move far away as possible to minimise electrostatic repulsion.
  • Electron pairs are either bonding pairs or lone pairs
  • Strongest to weakest repulsion: Lone pair - Lone pair repulsion > Lone pair - Bonding pair repulsion > Bonding pair - Bonding pair repulsion.
  • Lone pairs reduces the bond angle by 2.5 degrees per lone pair (*when it substitutes a bonding pair*)
• Linear

  • Bonding Pairs: 2
  • Bond Angle: 180 degrees
  • Examples: CO2
• Trigonal Planar

  • Bonding Pairs: 3
  • Bond Angle: 120 degrees
  • Example: BF3
• Tetrahedral Arrangement:

  • Bonding PairS: 4
  • Bond Angle: 109.5 degrees
  • Example: CH4
  • 4 regions of electron density ---> 4 bonding pairs ---> equal electron pair repulsion ----> 109.5 degrees ----> tetrahedral arrangement
• Pyramidal

  • Bonding Pairs: 3
  • Lone Pairs: 1
  • Bond Angle: 107 degrees
  • Example: NH3
  • 4 regions of electron density ---> 3 bonding pair; 1 lone pair ---) unequal electron pair repulsion ---> pyramidal shape
• Non-linear

  • Bonding Pairs: 2
  • Lone Pairs: 2
  • Bond Angle: 104.5 degrees
  • Example: H2O
• Octahedral

  • Bonding Pairs: 6
  • Bond Angle: 90 degrees
  • Example: SF6
• What is electronegativity?


  The ability of an atom to attract the shared pair of electron in a covalent bond.
• What factors affect electronegativity values?

  • Size of nuclear charge (going across a period)
  • Atomic Radius (going up a group)
  • ≈ Shielding ≈
• How does electronegativity change going across a period?

  Electronegativity increases
• How does electronegativity change going up a group?

  Electronegativity increases
• What is the most electronegative atom?

  Fluorine
• How are polar bonds represented?

  Hydrogen has a lower electronegativity than chlorine, so hydrogen has a delta plus and chlorine has a delta negative.

  
• How are non-polar bonds represented?

  Atoms of the same element have the same electronegativity so the bonds are represented with a line.

  
• How do you decide whether the molecule is polar? (could be part if large)

  • Has polar bonds
  • Molecules is not symmetrical
  • As a result, dipoles do not cancel.

  
• How do you decide whether the molecules is non-polar?

  • Has no polar bonds

  or

  • Has polar bonds but the molecules is symmetrical
  • As a result, dipoles cancel.

  
• What is a polar covalent bond?

  A polar covalent bond is covalent bond in which the electron density is pulled more to one of the bonded atom, than the other, due to their difference in electronegativity.
• What are London Forces and how are they formed?

  • Type intermolecular force.
  • Variation in external electrical field strength creates temporary induced dipoles (shift in electron density of the molecule)
  • This induces temporary dipoles in neighbouring molecules.
  • The induced dipoles induces further dipoles on neighbouring molecules, which then attract each other.
  • Require 10KJ per mol to break - generally low melting and boiling point as it is a weak force.
  • e.g. H2, O2, Cl2

  
• What is needed for permanent dipole-dipole interactions?

  • Polar molecules contenting permanent dipoles (+don't cancel out).
  • Has London Forces as well due to temporary shifts in electron density (due to variation in external electrical field strength), which makes temporary shifts in electron density in neighbouring molecules.
  • Electric force of attraction occurs between the partial positive end of one polar molecule, and the partial negative end of another.
  • Require 25 KJ per mole to break - stronger
  • e.g. HCl, HBr

  
• What is the melting and boiling point of simple molecular substances?

  Low melting and boiling point due weak intermolecular forces (London forces and/or permanent dipole-dipole interactions.
• Is there a variation in boiling point in Noble Gases.

  • As you go down the group, the molecules have more electrons,
  • Stronger London Forces between molecules,
  • More energy is required to break/overcome the forces to separate the molecules.
• Can a non-polar (+simple molecular+) substance dissolve in polar solvent?

  No because the polar solvent has permanent dipole-dipole interaction between molecules and the non-polar simple molecular substance can only set up London Forces.
• Can a non-polar (+simple molecular+) substance dissolve in non-polar solvent?

  Yes because more London Forces can form.
• Can a polar (+simple molecular+) substance dissolve in a polar solvent?

  Yes because more permanent dipole-dipole interactions can form.
• Can a polar (+simple molecular+) substance dissolve in a non-polar substance?

  No because polar molecules have permanent dipole-dipole interactions but the non-polar only has London Forces.
• Why is ethanol a universal solvent?

  It has both London forces and permanent dipole-dipole interaction
• Do simple molecular substances conduct electricity?

  Molecules are not charged, so they do not conduct electricity (*dipole do not count in conductivity wise*)
• What is a hydrogen bond?

  The attraction between an electron deficient hydrogen atom on one molecule, and a lone pair of election on a electronegative atom of a neighbouring molecule.

  
• Why is the boiling point of water so high (+anomaly+) even though is a simple molecular substance (*it is polar*)

  Water has a relatively high boiling point because the hydrogen bonds between water molecules increase the energy required to separate water molecules.
• Why is ice less dense than water?

  • In this state, Hydrogen bonds fix the water molecules further apart.
  • D=M/V <---- if v increases, density decreases.
• Why is the surface tension of water high?

  Hydrogen Bonding keeps water molecules closely together.
• What are the 3 main types of intermolecular forces>

  • London Forces
  • Permanent dipole-dipole interaction
  • Hydrogen Bonding
• What type of properties are intermolecular forces responsible for?

  Physical properties - such as melting and boiling point
• What bond/ force break when a simple molecular lattice is melted?

  Only the weak intermolecular forces (not the strong covalent bonds)
• What type of atoms are involved in hydrogen bonding?

  The highly electronegative atoms examples: oxygen, nitrogen or fluorine.
• What is the strongest type of intermolecular force?

  Hydrogen Bonds
• How many hydrogen bonds can each water molecules form?

  • 2 hydrogen bond involving their hydrogen atom,
  • plus 2 more hydrogen bonds due to the 2 lone pairs on the oxygen atom.
  • 4 in total
• Why does water have a higher melting and boiling point than other group 6 hydrides?

  • Water can form hydrogen bonds between molecules,
  • Require more energy to overcome than the weaker permeant dipole-dipole interaction in other group 6 hydride,
  • Thus, higher melting and boiling point.