Bonding & Structure

• Metallic bonding


  Each atom loses one or more electrons & are delocalised between lattice of cations

  The electrostatic attraction between lattice of positive metal ions and delocalised electrons
• Ionic bonding

  Electrons are transferred between atoms, becoming ions

  The electrostatic attraction between oppositely charged ions in a giant ionic lattice
• Covalent bonding

  Sharing a pair of electrons (generally metal + non-metal atoms)
• Why do the positive charges in metallic bonding not repel?

  • Sea of delocalised electrons move between positive ions
  • Electrostatic attraction between these negatively charged electrons and ions keep the structure together
• Metal Melting Point


  • High melting point
  • Lot of energy need to overcome strong electrostatic attraction between metal cations and delocalised electrons
• Metal Electrical Conductivity 


  • High electrical conductivity
  • Delocalised electrons can carry a current, acting as mobile charge carriers
• Metal Solubility 


  • Low solubility
  • Metallic bonding is stronger than attraction of polar water molecules for metallic ions
  • There are some interactions between polar solvent and metallic lattice but these lead to reactions rather than dissolving

  Some metals will react with water
• How does the structure of metals allow them to be ductile and malleable?


  • Because electrons can move within the structure it has a certain amount of 'give'
  • Allowing the atoms or layers to slide past each other
  • Bonds will not break as they are non-directional
• Why does the strength of metallic bonding increase from Na to Al

  • From sodium to aluminium, the charge on cations increases
  • Also number of delocalised electrons increases
  • ° attraction between cations and delocalised electrons increases
  • More heat energy required to break the bonds
• Ionic bonding involves the transfer of electrons from a metallic element to a non-metallic element
• The ionic bond is the electrostatic attraction formed between the oppositely charged ions, which occurs in all directions (non-directional bonding)
• Why is the melting point of MgO higher than NaCl

  • Magnesium ions have a greater charge that sodium ions
  • Oxide ions have a greater charge than chloride ions
  • Electrostatic attraction between oppositely charged ions is much greater in MgO than NaCl ° ionic bonding stronger
• Bonding can be ionic, covalent, or metallic
• Structure can be giant (strong bonds holding the structure together) or simple (weak intermolecular forces between molecules)
• Most ionic, covalent, and metallic solids are crystalline lattices. The ions, atoms, or molecules are arranged in a regular and repeating pattern
• m.p. & b.p. of Ionic Compounds

  • Most solid at RTP
  • Large amount of energy required to overcome the strong electrostatic forces of attraction between the positively charged ions making up the lattice
  • m.p. & b.p. increase with greater ionic charge
• Ionic Compound Solubility


  Many dissolve in polar solvents. Solubility depends on:

  • Breaking down the ionic lattice
  • The polar molecules attracting & surrounding the ions

  e.g. water can break down/disrupt ionic lattice, surrounding each ion

  Solubility of an ionic compound depends on the relative strength of electrostatic forces of attraction between the ions (greater ionic charge: less soluble)
• Ionic Compound Electrical Conductivity


  • Do not conduct elec when solid- ions in fixed positions in the lattice so no mobile charge carrier
  • Can conduct elec when molten or aqeuous: ions no longer in fixed positions as lattice has broken down- ions are free to act as mobile charge carriers
• Paired electrons not shared in a covalent bond are called lone pairs
• Covalent bond


  The overlap of atomic orbitals, each containing 1 electron, to give a shared pair of e⁻. Shared pair is attracted to nuclei of both boning atoms .

  Attraction is localised, acting only between shared pair of e⁻ and nuclei of the 2 bonded atoms. Result is molecule consisting of 2 or more atoms
• Bond energy


  The energy required to break one mole of a particular covalent bond in the gaseous state. In kJmol⁻¹
• Bond Length


  Internuclear distance of 2 covalently bonded atoms. The greater the forces of attraction between electrons and nuclei, the more atoms are pulled closer to each other, decreasing bond length
• BF₃


  Boron has elec config 1s² 2s² 2p¹ so only 3 electrons can be paired
• For Phosphorus, sulphur, chlorine (+ other elements below them in their groups) outer shell electrons that can take part in bonding results in more electrons than noble gas config, breaking octet rule
• A dative covalent bond can be formed when a molecule with a lone pair of electrons donates their lone pair to an electron-deficient atom (atom with unfilled outer orbital)
• Carbon allotropes


  • Graphite
  • Graphene
  • Diamond
  • Silicon (IV) Oxide
• Diamond


  Giant covalent lattice (macromolecule) of carbon atoms, each bonded to 4 other Carbons. Tetrahedral bond angle 109.5°. Giant lattice structure with bonds in all directions. Hard
• Graphite


  Each C bonded to 3 others in a layered structure; made of hexagons bond angle 120°. Spare electrons are delocalised and occupy space between layers. Atoms in the same layer are held together by strong covalent bonds but layers are held together by weak intermolecular forces- layers can slide over each other
• Graphene


  Single layer of carbon atoms bonded together by weak intermolecular forces- layers can slide
• Silicon (IV) Oxide


  Giant covalent lattice/macromolecular structure made of tetrahedral units bonded by strong covalent bonds. Each silicon shared by 4 oxygens, each oxygen is shared by 2 silicons. SiO₂
• Giant Covalent Substance m.p.


  High- a lot of energy is needed to break the many strong covalent bonds between atoms
• Giant Covalent Substance Electrical Conductivity


  Poor - no mobile charge carriers

  (Not the case for graphite)
• Giant Covalent Substance Solubility


  Insoluble - bonds are very strong
• Lone pair electrons repel more than bonded pairs
• Order of Repulsion


  lone pair-lone pair > lone pair-bond pair > bond pair-bond pair
• Shape & bond angle

  
• Shape & Bond angle

  
• • Lone pairs repel bonded pairs slightly closer
  • Bond angle reduced about 2.5° per lone pair
• Why electronegativity occurs


  • Different nuclear charges
  • Atoms may be different sizes (-> shared pair of electrons closer to one nucleus than another)

  If atoms are the same, this doesn't apply
• Non-polar bond


  Bonding pair of electrons is shared equally