shapes of molecules

• VSEPR theory

  Valence-shell electron-pair repulsion theory; because electron pairs repel, molecules adjust their shapes so that valence electron pairs are as far apart as possible
• Linear

  • central atom has two bonding pairs of electrons
  • no lone pairs
  • 180 eg. BeCl2
• Trigonal Planar

  • central atoms have three bonding pairs
  • no lone pairs
  • repulsion of charge clouds is the same
  • 120 eg. Bf3
• Tetrahedral

  • central atom has 4 bonding pairs
  • no lone pairs
  • charge clouds repel each other equally
  • 109.5 eg.NH4+
• Trigonal pyramidal

  • central atom has 3 bonding pairs
  • one lone pair
  • smaller bond angles between bonding pairs and larger bond angles between the lone pair and bonding pairs
  • 107 eg. NH3
• Bent

  • central atom has two bonding pairs
  • two lone pairs
  • 104.5 eg. H20
• trigonal bipyramidal

  • central atom has 5 bonding pairs
  • no lone pairs
  • 90 120 eg. PCl5
• Octahedral

  • central atom has 6 bonding pairs
  • no lone pairs
  • 90 eg. SF6
• square pyramidal

  • central atom has 5 bonding pairs
  • one lone pair
  • 90 eg. ClF5
• square planar

  • central atom has 4 bonding pairs
  • two lone pairs
  • 90 eg. XeF4
• electronegativity

  -the power of an atom to attract the pair of electrons in a covalent bond towards itself
• nuclear charge

  -attraction exists between +ve protons and -ve e-

  -increase in proton no. increases nuclear attraction

  -increased nuclear charge results in increased electronegativity
• atomic radius

  -distance between nucleus and electrons in outermost shell

  -e- closer to nucleus are more strongly attracted

  -increased atomic radium results in decreased electronegativity
• shielding

  -filled energy levels can mask the effect of nuclear charge causing outer electrons to be less attracted to the nucleus

  -addition of extra shells and subshells will cause outer e- to experience less attractive force from the nucleus

  -increased number of inner shells and subshells will result in a decreased electronegativity
• trends in electronegativity - down a group

  -decrease

  -nuclear charge increases

  -each element has an extra filled electron shell which increases shielding

  -addition of extra shells results in larger atomic radii

  -decrease in attraction between nucleus and outer bonding electrons
• trends in electronegativity - across a period

  -increases

  -nuclear charge increases

  -shielding remains relatively constant

  -nucleus has increasingly strong attraction which results in smaller atomic radii
• intramolecular forces

  forces within molecules. Forces caused by the attraction and repulsion of charged particles
• intermolecular forces

  -forces of attraction between molecules

  -VDW

  -permanent dipole - attractive forces between two neighbouring forces

  -hydrogen bonding - special type of permanent dipole

  -intra stronger than inter
• intermolecular forces - strongest to weakest

  
  

  covalent bonding

  hydrogen bonding

  permanent dipole dipole

  induced dipole
• polarity

  -Molecules having uneven distribution of charges

  -when 2 molecules in covalent bond have same electronegativity the bond is non polar

  -different electronegativities means e- will be drawn to one side

  -e- distribution is asymmetric

  -greater electronegativity difference = more polar
• permanent dipole-dipole

  -forces between two molecules that have permanent dipole

  -delta+ end in one molecule and delta- end of neighbouring molecule are attracted towards each other

  -polar molecules have permanent dipole

  -always have +ve and -ve end
• induced dipole-dipole

  -exists between all atoms or molecules

  -VDW forces

  -electron charge cloud in non-polar mol. are constantly moving

  -cloud could be more one side - causes temporary dipole

  -temporary dipole can induce a dipole on neighbouring molecules

  -delta+ on one molecule and delta- of another are attracted

  -because cloud is constantly moving the dipoles are temporary
• hydrogen bonding

  -strongest form of intermolecular bonding

  -a species has an O, N or F which are very electronegative atom bonded to H

  -when hydrogen is bonded ONF the bond becomes highly polarised

  -hydrogen becomes delta+
• properties of water - high melting and boiling

  -strong intermolecular forces of hydrogen bonding between molecules
• properties of water - high surface tension

  -surface tension is the ability of a liquid surface to resist any external forces

  -water molecules at the surface are bonded to other water molecules through H bonds

  -molecules pull downwards the surface molecules causing surface to become compressed

  -increases waters surface tension
• properties of water - density

  -solids are denser than liquids as particles are more closely packed

  -water molecules are packed into an open lattice

  -this way of packing and long bond lengths means water molecules are slightly further apart in liquid form

  -ice has lower density than liquid water by 9%