Clark's Method 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 S e^- = 6y + 2 where y = # atoms other than H, then octet rule is followed with single bonds only.
  • If S e^- < 6y + 2 then probably have multiple bonding with the number of multiple bonds = D/2 (remember a triple bond is 2 multiple bonds!).
• If you can draw more than one structure, then chose the most symmetrical.

VSEPR (Valence Shell Electron Pair Repulsion) Theory is based on three assumptions (there are more advanced versions, but unnecessary for us):

• Electron pairs will orient around a central point to minimize repulsion.
• Lone-pairs of electrons will have greater repulsion than bonded pairs of electrons (note that the atoms are ignored in terms of repulsion).
• Repulsion is strong at 90° and weaker at 120° (weakest at 180°).

VSEPR predicts geometry based on these assumptions in a few simple, sequential, steps:

1. Draw a correct Lewis Structure.
2. Determine the Steric Number = the number of bonded atoms + the number of lone pairs = "electron clouds" in valance shell of central atom.
3. Maximize the angles between electron pairs, placing the lone (unbonded) pairs at the extremes.

Trigonal planar with angles of 120°

 

 

 

 

Tetrahedral with angles of 109.5°

 

 

 

 

CO₂ linear molecular geometry

 

 

 

 

 

 

Trigonal planar molecular geometry (formaldehyde, CH₂O)

 

 

 

 

 

Tetrahedral molecular geometry (methane, CH₄)

 

 

 

 

 

 

Trigonal pyramidal molecular geometry (ammonia, NH₃) [model]

rotated to view molecule from below

 

 

 

 

 

Bent molecular geometry (water, H₂O)

 

 

© R A Paselk

Last modified 7 October 2008