bonding and structure

• ionic bonding

  ionic bonds are present in compounds usually consisting of a metal and a non-metal

  electrons are transferred from the metal atom to the non-metal atom

  oppositely charged ions are formed which are bonded together by electrostatic attraction

  metal ion is positive, non-metal ion is negative
• giant ionic lattices

  each ion is electrostatically attracted to ions of the opposite charge in all directions

  it takes significant energy to overcome these strong electrostatic forces between the ions

  e.g NaCl
• high melting and boiling points in giant ionic lattices 


  the strong electrostatic attractions between the positive and negative ions in the giant lattice must be overcome for the lattice to break apart; this requires a lot of energy

  the greater the charge, the stronger the electrostatic forces between the ions
• solubility in giant ionic lattices

  polar water molecules break down an ionic lattice by surrounding each ion to form a solution

  the slight charges within the polar substance are able to attract the charged ions in the giant ionic lattice

  this means the lattice is disrupted and ions are pulled out of it
• electrical conductivity in giant ionic lattices

  in a solid ionic lattice:

  the ions are firmly locked in place and unable to move to carry electric charge

  when melted or dissolved:

  the ions can move freely and carry electric charge through the liquid
• covalent bonding

  the strong electrostatic attraction formed between a shared pair of electrons and the nuclei of the bonded atoms

  this bonding creates a stable molecule - a group of 2 or more atoms held together by covalent bonds
• single covalent bonding 


  atoms bonded by one shared pair of electrons
• multiple covalent bonding 


  atoms that share more than one pair of electrons with another atom to form a multiple bond
• dative covalent bonding 


  both shared electrons come from just one of the bonding atoms rather than one electron coming from each atom
• for a dative covalent bond to form between two atoms

  one atom must have a lone pair of electrons to donate

  the other atom must be electron deficient

  this type of bonding is represented by an arrow showing the direction of electron donation from the atom with the lone pair to the electron-deficient atom.
• a measure of the average energy of a bond (and therefore the energy that would be needed to break it) is called the average bond enthalpy
• 2 bonding pairs, 0 lone pairs 


  shape: linear

  bond angle: 180 degrees

  
• 3 bonding pairs, 0 lone pairs 


  shape: trigonal planar

  bond angle: 120 degrees

  
• 4 bonding pairs, 0 lone pairs 


  shape: tetrahedral

  bond angle: 109.5 degrees

  
• 3 bonding pairs, 1 lone pair 


  shape: trigonal pyramidal

  bond angle: 107 degrees

  
• 2 bonding pairs, 2 lone pairs 


  shape: non-linear / bent

  bond angle: 104.5 degrees

  
• 5 bonding pairs, 0 lone pairs 


  shape: trigonal bypyramidal

  bond angles: 120 degrees, 90 degrees

  
• 6 bonding pairs, 0 lone pairs 


  shape: octahedral

  bond angle: 90 degrees

  
• electron pairs, whether bonding or lone, repel each other

  lone pairs cause more repulsion than bonding pairs because they are closer to the nucleus

  electron pairs arrange themselves as far apart as possible to minimise repulsion
• lone pair-lone pair > lone pair-bonding pair > bonding pair-bonding pair

  lone pairs occupy more space, affecting the molecule's shape by compressing bond angles
• shape and angle of CH4

  • 4 bonding pairs
  • 0 lone pairs
  • tetrahedral
  • 109.5 degrees
• shape and angle of NH3

  • 3 bonding pairs
  • 1 lone pairs
  • trigonal pyramidal
  • 107 degrees
• shape and angle of H2O

  • 2 bonding pairs
  • 2 lone pairs
  • non-linear / bent
  • 104.5 degrees
• electronegativity is a measure of an atom's ability to attract the shared electron pair in a covalent bond towards itself
• in a covalent bond between atoms with different electronegativities, the bonding electrons are more strongly attracted to the more electronegative atom. this unequal sharing of electrons makes the bond polar
• polar bonds have a permanent electric dipole. a dipole is a separation of positive and negative charges within a polar covalent bond or polar molecule, resulting from an uneven distribution of electrons
• the overall polarity of a molecule depends on how its polar bonds are arranged in 3D

  molecules like CCl4 are non-polar because their symmetric arrangement causes their dipoles to cancel out, eventhough they have polar bonds. in CCl4, the 4 polar C-Cl bonds are arranged tetrahedrally, resulting in the dipoles cancelling each other out and no net dipole for the molecule
• the overall polarity of a molecule depends on how its polar bonds are arranged in 3D

  molecules like CHCl3 are polar because their asymmetric arrangement doesn't allow their dipoles to cancel out, leading to a net dipole. in CHCl3, the three polar C-Cl bonds and one C-H bond are arranged tetrahedrally, but the difference in electronegatively between H and Cl causes the dipoles to not fully cancel each other, resulting in a net dipole for the molecule
• polar molecules have permanent dipoles arising from unequal sharing of electrons in covalent bonds. The partial positive (δ+) and partial negative (δ-) charges on polar molecules enable them to experience permanent dipole-dipole forces.
• permanent dipole-dipole forces are electrostatic attractions between the partial positive end of one polar molecule and the partial negative end of another

  
• induced dipole-dipole forces also known as London dispersion forces or Van der Waal's forces are present between all atoms and molecules, even non polar ones. they arise due to temporary fluctuations in the electron distribution around atoms
• how induced dipole-dipole forces arise

  electrons in atoms are constantly moving. at any instant, there may be more electrons on one side of the atom than the other, creating a temporary dipole

  this temporary dipole can induce an opposite dipole in a neighbouring atom, causing a weak electrostatic attraction between the atoms

  this induced dipole can then induce further dipoles in other nearby particles

  although these dipoles are constantly forming and disappearing as the electrons move, the overall effect is a net attraction between the atoms or molecules