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Polarity in Covalent Molecules

Polarity: So now we can predict bonding and shape in representative group molecules (and thus most biomolecules), how about electron density and thus charge distribution? Need two bits of information:

• Shape (based on VSEPR Theory)
• Electron distribution within a bond (based on electronegativity)

Examples:

Molecule	Geometry	Structure	Electronegativities	Bond Dipoles	Molecular Dipole	Model
Carbon monoxide	linear	C=O	ENC= 2.5, ENO= 3.5
Carbon dioxide	linear	O=C=O	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
Ammonia	trigonal pyramidal		ENH= 2.1, ENN= 3.0
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.

Find whether Chlorine trifluoride (ClF₃) is polar.

1. Determine Lewis structure:

• Valence electrons = 7 + 3(7) = 28
• 6y + 2 = 24 + 2 = 26
• = 28 - 26 = 2 so two extra electrons 10 electrons around central atom:
  

2. Determine steric number (SN) = 3(bonded atoms) + 2(lone pairs) = 5

3. Determine Geometry:

• Electronic geometry = Trigonal pyramidal:

• Since there are two lone pairs the trigonal pyramidal electronic geometry give a T-Shape:
  
  

4. Determine Polarity:

• Is there a difference in the Electronegativity (EN) of the bonded atoms?
  • EN_Cl = 3.0, EN_F = 4.0
  • EN = 1.0, there is a difference, and
• The Cl-F bonds are polar, with a small negative charge on each Fluorine.
• One of the bonds is not opposed (the molecule is NOT symmetrical), it is polar.

Energy of Formation for Ionic Compounds

It turns out that the transfer of an electron from a metal to a non-metal will not generally provide enough energy to favor the process. So how is it that these are in fact favorable reactions?

Let's look at the energy of the process by breaking it into steps and looking at the enthapies of formation starting with free atoms (the reality will be somewhat more complex since we would start with solid metal and molecules, each of which must first react to give free atomic state, but the results are similar). Of course we can get away with this because we are looking at state functions, which as we saw before are pathway independent!

Ionization Energy	Na Na+ + e-	H = +495 kJ/mol
Electron Affinity Energy	Cl + e- Cl-	H = -348 kJ/mol
	Total	H = + 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) NaCl(s)	H = - 449 kJ/mol
	Overall	H = - 302 kJ/mol

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© R A Paselk

Last modified 6 April 2009