Trigonal planar with angles of 120°


image of trigonal planar structure

 

 

 

 

Tetrahedral with angles of 109.5°


drawing of tetrahedron

 

 

 

 

CO2 linear molecular geometry

ball and stick model of carbon dioxide

 

 

 

 

 

 

Trigonal planar molecular geometry (formaldehyde, CH2O)

ball and stick model of formaldehyde

ball and stick model of formaldehyde

 

 

 

 

 

Tetrahedral molecular geometry (methane, CH4)

ball and stick model of methane viewd in plane of HCH

ball and stick model of methane - offangle view

 

 

 

 

 

 

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

ball and stick model of ammonia - side view

rotated to view molecule from below

ball and stick model of ammonia - view from below

 

 

 

 

 

Bent molecular geometry (water, H2O)

ball and stick model of water

 

 

 

 

 

 

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 e- = 6y + 2 where y = # atoms other than H, then octet rule is followed with single bonds only.
        • If e- < 6y + 2 then probably have multiple bonding with the number of multiple bonds = /2 (remember a triple bond is 2 multiple bonds!). However, note the exceptions below with small atoms (H, Li, Be, and B).
        • If e- > 6y + 2 then have an expanded valence shell. Note that if = 2, then pentavalent (10 electrons in the valence shell) , and if = 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.

 

 

© R A Paselk

Last modified 28 March 2008