Module 2.2.2- Bonding and structure

• Ionic bonding


  The electrostatic attraction between oppositely charged ions
• MgCl2 dot and cross diagram

  
• Giant ionic lattice


  A 3D structure of oppositely charged ions, bonded together by strong ionic bonds

  
• Properties of giant ionic structures: high melting and boiling point


  High temperatures are needed to provide the large quantity of energy needed to overcome the strong electrostatic attraction between the ionsmelting point increases with greater ionic charges
• Properties of giant ionic structures: solubility


  Most ionic compounds dissolve in polar solvents. Polar water molecules break down the lattice and surround each ion in solution.Solubility requires 2 main processes:- ionic lattice must break down- water molecules must attract and surround the ionssolubility decreases as ionic charge increases
• Properties of giant ionic structures: electrical conductivity


  Not conduct when solid since ions are in a fixed position with no mobile charge carriersConduct when liquid since ions are free to move as mobile charge carriers
• Covalent bonding


  The strong electrostatic attraction between a shared pair of electrons and the nuclei of the bonded atoms
• Difference in attraction between ionic and covalent


  Ionic- attracts in all dimensions

  Covalent- localised
• Single covalent bonds dot and cross diagram

  
• Multiple covalent bonds dot and cross diagram

  
• Dative covalent bond


  A shared pair of electrons which has been provided by one of the bonding atoms only

  
• Average bond enthalpy


  The average enthalpy change that takes place when breaking by homolytic fission 1 mole of a given type of bond in the molecules of a gaseous species.Measurement of covalent bond strength
• Electron pair repulsion


  electron pairs repel one another so they are arranged as far as possible
• Lone pair repulsion in comparison to bonding pair


  Lone pairs repel more than bonding pairs
• explaining the shape of a molecule


  There are X bonded pairs and X lone pairs

  Electron pairs repel one another so they are arranged as far apart as possible

  Lone pairs repel more than bonding pairs(decreasing bond angle)
• Trigonal pyramidal


  3 bonded pairs

  1 lone pair

  107

  e.g. NH3
• Tetrahedral shape


  4 bonded pairs

  symmetrical

  0 lone pairs

  109.5

  e.g. CH4
• Non-linear


  2 bonded pairs

  2 lone pairs

  104.5

  e.g. H2O
• Linear


  2 bonding regions

  0 lone regions

  180

  4 bonded pairs count as 2 bonding regions due to the double bond e.g. CO2

  
• Trigonal Planar


  3 bonds

  0 lone pairs

  120
• Octahedral


  6 bonds

  0 lone pairs

  90
• Shape of ions NH4+

  Tetrahedral structure

  109.5

  
• Shape of ions CO3^2-

  Trigonal Planar 120

  
• Shape of ions NO3^2-

  Trigonal Planar 120

  
• Shape of SO4^2-

  Tetrahedral 109.5

  
• Electronegativity


  electronegativity as the ability of an atom to attract the bonding electrons in a covalent bond
• Non-polar bond


  The bonded electron pair is shared equally between the bonded atomsBonded atoms are the same or have similar electronegativity e.g. C-H bonds
• Pure covalent bond


  A covalent bond with equal sharing of electrons between atoms.
• Polar covalent bond


  A covalent bond between atoms that differ in electronegativity. The shared electrons are pulled closer to the more electronegative atom, making it slightly negative and the other atom slightly positive.
• Permanent dipole


  A small charge difference across a bond resulting from a difference in electronegativities of the bonded atoms.
• Intermolecular forces


  Weak interactions between dipoles of different molecules
• 3 types of intermolecular forces


  London forces

  Permanent dipole-dipole

  Hydrogen bonding
• Strength of London forces

  -The more electrons in each molecule: the larger the instantaneous and induced dipoles, the greater the induced dipole-dipole interactions, the stronger the attractive forces between molecules.-Larger numbers of electrons mean larger induced dipoles. More energy is then needed to overcome the intermolecular forces, increasing the boiling point.
• Simple molecular substance/lattice

  substance made up of simple molecules held together by covalent bonds and has weak intermolecular forces between these molecules
• Properties of simple molecular lattice- low melting and boiling point


  due to the weak intermolecular forces between the molecules, which break under low temperaturescovalent bonds do not break!
• Properties of simple molecular lattice- solubility of non polar substances


  non-polar simple molecular substances tend to be soluble in non-polar solvents

  simple molecular substances tend to be insoluble in polar solvents
• Properties of simple molecular lattice- solubility of polar substances


  polar covalent substances may dissolve in polar solventssolubility depends on the strength of the dipole
• Properties of simple molecular lattice- electrical conductivity


  no mobile charged carriers in simple molecular structurestherefore they are non-conductors of electricity
• Hydrogen bonds with...


  Nitrogen

  Oxygen

  Fluorine

  
• Why is ice less dense than liquid water?


  hydrogen bonds holds water molecules apart in an open lattice structure (fixed)the water molecules in ice are further apart than in liquid water