2.3 - Covalent Bonding & Structure

• covalent bonds


  strong electrostatic attraction between 2 nuclei and the shared pair of electrons between them
• bonding electrons


  electrons involved in bonding
• non-bonding elections


  elections in outer shell not involved in bonding

  also called lone pairs
• importance of lone pairs


  affects shapes of molecules

  used to form dative covalent bonds

  importance in reactions of some compounds
• structures covalent bonds make


  giant covalent lattices

  simple molecular structures
• single covalent bonds- methane


  carbon only has 2 bonding electrons but it needs 4 so electron in 2s orbital is promoted to apz orbital using energy released by bonds forming so now there are 4 bonding electrons

  bonds must be all identical so must be in identical orbitals

  orbitals hybridise - atomic orbitals rearrange to form orbitals of equal energy forming sp3 orbitals

  atomic orbital of hydrogen & sp3 orbitals merge end on forming molecular orbitals (sigma bonds) which contains shared electrons
• sigma bonds


  the overlap of 2 orbitals to form sigma bonds
• why some compounds don't exist


  energy required to promote electron is too great because gap in energy level between orbitals is too great so promotion doesn't happen

  energy released by forming bonds isn't enough to compensate for energy needed to promote electron so promotion doesn't happen

  atoms will form as many bonds as possible as long as it's energetically profitable
• double covalent bonds - ethene


  electron promotion: one electron from 2s2 orbital of carbon is promoted to 2pz orbital so there are enough bending electrons and carbon is in excited state

  hybridilation: 3 orbitals hybridise to form Sp2 or bitals - 2s, 2px, 2py because they have the lowest energy levels

  sp2 orbitals hydrogen orbitals overlap end-on forming sigma bonds

  pz orbitals overlap sideways forming pi bond
• pi bonds


  A region of space where you can find the two electrons that make up the bond
• what double bonds consist of


  a pi bond and a sigma bond
• which is the weaker bond: pi or sigma band?


  pi bond because they're more exposed and on a higher energy level so less energy is required to break them

  in a reaction pi bonds break first and are easier to react with
• triple covalent bonds - ethyne


  promotion of electron: electron in 2s2 orbital of carbon is promoted to 2pz orbitalso there are 4 bonding electrons

  hybridisation: only 2s and 2px or bitals hybridise because carbon only bonds to two atoms so forms sp orbitals

  hydrogen and sp orbitals overlap end-on to form sigma bonds

  2py and 2px orbitals overlap sideways to form 2 pi bonds between carbons
• dative covalent bond


  a bond in which two atoms share a pair of electrons and both electrons are donated by one atom
• aluminium chloride


  exists as a dimer because it's electron deficient and a dimer is more stable than separate molecules because energy is released when dative covalent bonds form

  when heated it sublimes
• dimer


  2 molecules joined together
• hydrated ions


  ions with water molecules attached
• oxoniam ions


  aqueous hydrogen ions
• what happens when aluminium chloride dissolves in water


  hexaaluminium ion forms with a dative covalent bond because Al3+ ion has 3 level orbitals empty so hybridises to form 6 new orbitals that accept Ione pairs of mater molecules
• why does an atom try to form as many bonds as possible?


  to release the most energy so becomes most energetically stable
• bond length


  intranuclear distance between two covalently bonded atoms
• what does band length depend on?


  number of electrons: the higher the electron density the stronger the attraction between atoms so the stronger and shorter the bond

  size ofatoms: the smaller the nuclei the shorter and stronger the bond
• bond energy


  energy required to break one mole of the bonds in a substance in the gaseous state
• how to measure bond length in covalent solids


  use x-ray diffraction studies
• how to measure hand lengths in covalent vapours


  microwave spectroscopy
• covalent bonds in simple molecular substances


  strong so molecules don't break up into atoms easily
• intermolecular forces in simple molecular substances


  weak so substances are liquids or gases at room temperature and solids are easy to melt and evaporate
• giant covalent structures


  hard and have high melting temperatures and are hard due to strong covalent bonds
• properties of simple molecular structures


  gases, liquids or soft metals at room temperatures

  low melting and baling points

  don't conduct electricity because they don't contain ions or free electrons

  more soluble in non-polar solvents