the covalent model

• what is the a covalent bond?


   A chemical bond formed when electrons are shared between two atoms. Usually each atom contributes one electron to form a pair of electrons that are shared by both atoms.
• Covalent bonds involve shared electron pairs between atoms. Each atom contributes one electron to each shared pair, and effectively gains an additional electron from the shared pair. Atoms share the same number of pairs needed to fill their valence shell, usually with eight.

  
• Bond formation in chlorine molecule. Illustration of the sharing of electrons (dots and crosses) between two chlorine atoms (Cl) to form a molecule of chlorine (Cl2). This is an example of covalent bonding, with the single bond formed by a shared electron pair consisting of an electron from each atom.

  
• what is a Charge cloud?


  Shared electron pairs are not in a fixed position, they are in a state of constant motion and are best regarded as charge clouds. Regions of negative cloud charge are known as domains and can have 1, 2, or 3 pairs of electrons
• The strength of a covalent bond depends on how many electrons are being shared. The more electrons being shared, the stronger the bond will be
• what is the Octet rule?


  The tendency of atoms to gain a valence shell with a total of 8 electrons - achieve an electron configuration similar to a noble gas (makes each atom more stable)
• is there a Expanding the octet rule?


  Sometimes the central atom of a covalently bonded molecule can accommodate more than 8 electrons in the outer shell
• what is the lewis structure ?


  A Lewis Structure is a very simplified representation of the valence shell electrons in a molecule. It is used to show how the electrons are arranged around individual atoms in a molecule. Electrons are shown as "dots" or for bonding electrons as a line between the two atoms
• what are some Elements with incomplete octets?


  H, Li, Be, B, Al
• what is Bond energy?


  Energy required to break one mole of a particular covalent bond in the gaseous state (kJ mol-1)The larger the bond energy, the stronger the covalent bond is.
• what are the Bond length and strength?

  Bond length decreases and bond strength increases as the number of shared pairs of electrons increases. This is because the greater the number of shared electrons the greater the attraction between two covalently bonded atom - pulled closer to each other.

  Triple bond - shortest and strongest (large electron density between the two nuclei)

  Single bond - longest and weakest
• what is Coordinate bond?

  Some molecules have a lone pair of electrons that can be donated to form a Bond with an electron deficient atom (unfilled outer orbital)

  So both of the shared electrons in the shared pair originate from the same atom

  Also called dative bond - e.g: Carbon Monoxide (CO) or Ammonium ion (NH4+)
• what are electron domains ?

  the electron domain refers to the number of lone pairs or bond locations around a particular atom in a molecule. Electron domains may also be called electron groups. Bond location is independent of whether the bond is a single, double, or triple bond.
• define Two electron domains


  Bond angle is 180° - LINEAR
• define Three electron domains


  Bond angle is 120° - TRIGONAL PLANAR
• define Three electron domain (one is a lone pair)

  Bond angle 118° - BENT LINEAR

  Bond angle will be slightly less than 120° due to stronger repulsion of the lone pairs, forcing the bonding pairs closer together.

  Expands the octet - 10 electrons around sulfur atoms
• define Four electron domains


  109.5° - TETRAHEDRAL
• define Four electron domains (one is a lone pair)

  Bond angle 107° - TRIGONAL PYRAMIDAL

  Extra lone pair repulsion which pushes bonds closer together
• define Four electron domains (two are lone pairs)

  Bond angle 105° - V-SHAPED/BENT LINEAR

  Lone pairs are pulled more closely to the central atoms so they exert a greater repulsive force than bonding pairs
• define Five electron domains

  Bond angle 90°, 120° and 180° - TRIGONAL BIPYRAMIDAL

  E.g: PCl5
• what are multiple covalent bonds?

  Lewis structures can be drawn for molecules that share multiple pairs of electrons. A double covalent bond is a covalent bond formed by atoms that share two pairs of electrons. A triple covalent bond is a covalent bond formed by atoms that share three pairs of electrons.
• Valence shell electron pair repulsion theory (VESPR)


  The electrons in the different bonds and the non - bonding electrons in the outer shell all behave as negatively charges clouds and repel each other - to avoid repulsion outer shell electrons spread out as far in space as possible. Double and triple bonds count as one electron domain