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   
ENC= 2.5,
ENO= 3.5
Carbon dioxide  linear  
ENC= 2.5,
ENO= 3.5
   None: two dipoles are of equal magnitude, but opposite in direction and cancel.  
 Water  bent


ENH= 2.1,
ENO= 3.5
    ball and stick model of water with dipolee
 Ammonia  trigonal pyramidal  


ENH= 2.1,
ENN= 3.0
    ball and stick model of ammonia with dipole
Ammonium ion tetrahedral   


ENH= 2.1,
ENN= 3.0
 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
















Energy of Formation for Ionic Compounds

Ionization Energy  Na right arrow Na+ + e-  DeltaH = +495 kJ/mol
Electron Affinity Energy  Cl + e- right arrow Cl-  DeltaH = -348 kJ/mol
Total   DeltaH = + 147 kJ/mol
  However, this value is for the free ions. If we allow them to come together by coulombic attraction into a crystal lattice a large additional amount of energy is released:   
Lattice Energy Na+(g) + Cl-(g) right arrow NaCl(s)  DeltaH = - 449 kJ/mol
  Overall   DeltaH = - 302 kJ/mol


© R A Paselk

Last modified 4 April 2011