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 2 April 2009