Bonding Supplement

 

 

 

Exceptions to the "Octet Rule."

To help determine if the octet rule is followed recall Clark's Method (abbreviated) for determining bonding in covalent Lewis Structures:

      • Add up all of the valence electrons in the structure (remember to add one electron for each negative charge, or subtract one for each positive charge)
        • If Sigma e- = 6y + 2 where y = # atoms other than H, then octet rule is followed with single bonds only.
        • If Sigma e- < 6y + 2 then probably have multiple bonding with the number of multiple bonds = Delta/2 (remember a triple bond is 2 multiple bonds!). However, note the exceptions below with small atoms (H, Li, Be, and B).
        • If Sigma e- > 6y + 2 then have an expanded valence shell. Note that if Delta= 2, then pentavalent (10 electrons in the valence shell) , and if Delta= 4, then hexavalent (12 electrons in the valence shell).
      • If you can draw more than one structure, then chose the most symmetrical.
        • If two or more structures are equally symmetrical, then you probably have resonance and should show all structures connected by double arrows.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Trigonal bipyramidal with angles of 90° & 120° (PCl5)

ball and stick model of phosphorus pentachloride

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Seesaw with angles of 90° & 120° (SF4)

ball and stick model of sulfur tetrafluoride

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

T-shaped with angles of 90° (ClF3)

ball and stick model of Chlorine trifluoride

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Linear with angles of 180° (I3-)

ball and stick model of triodide ion

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Octahedral with angles of 90° (AsF6-)

ball and stick model of Arsenic hexafluoride ion

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Tetragonal pyramidal with angles of 90° (ICl5)

ball and stick model of Iodine pentachloride

 

 

 

 

 

 

 

 

 

 

 

 

Square planar with angles of 90° (XeF4)

ball and stick model of Xenon tetrafluoride

 

 

 

 

 

 

 

 

 

Polarity in Covalent Molecules

Molecule Geometry Structure Electronegativities Bond Dipoles Molecular Dipole Model
Carbon monoxide  linear

  C=O
ENC= 2.5,
ENO= 3.5
 CO structure diagram with dipole arrow  CO structure diagram with dipole arrow  
Carbon dioxide  linear  O=C=O
ENC= 2.5,
ENO= 3.5
 CO2 structure diagram with dipole arrows  None: two dipoles are of equal magnitude, but opposite in direction and cancel.  
 Water  bent
  3D structural diagram of water showing lone pairs of electrons
water structure diagram

 

ENH= 2.1,
ENO= 3.5
 water structure diagram with bond dipole arrows  water structure diagram with molecular dipole arrow ball and stick model of water with dipolee
 Ammonia  trigonal pyramidal  ammonia structure diagram

 

ENH= 2.1,
ENN= 3.0
 ammonia structure diagram with bond dipole arrows  ammonia structure diagram with molecular dipole arrow ball and stick model of ammonia with dipole
Ammonium ion tetrahedral  ammonium ion structure diagram 

 

ENH= 2.1,
ENN= 3.0
ammonium ion structure diagram with bond dipole arrows  None: four dipoles are symmetrically arranged to cancel each other out and give a spherically charged but non-polar ion. ball and stick model ofammonium ion

 

 

 

 

 

 

 

 

 

 

 

 

 

 

drawing of trigonal pyramid

 

 

 

 

 

 

 



ball and stick model of Chlorine trifluoride

 

© R A Paselk

Last modified 15 April 2013