These electron pairs refer to those ones involved in sigma bonds only.
Electron pairs from pi bonds of double and triple bonds do not count.
In this particular coordination geometry, each electron pair is at an angle of 120° from any of the other two electron pairs.
The illustrations of the geometry and the Lewis structures given below are for the following molecules:
| BF3 |
| BCl3 |
| SO3 |
| H2C=O |
Geometry of BF3
- shape of molecule: trigonal planar
- F-B-F bond angle: 120°
A trigonal plane is superimposed on the BF3 molecule.
Lewis structures of BF3 and BCl3
Due to its 3 valence electrons, boron does not follow the octet rule and thus forms electron-deficient compounds such as BF3 ( boron trifluoride ) and BCl3 ( boron trichloride ).
The characteristic reaction of the above compounds is to accept/share an electron pair from other atoms or compounds.
The reaction is called Lewis acid-base reaction.
A Lewis acid accepts a pair of electrons; a Lewis base donates a pair of electrons.
Geometry of BCl3
Geometry of SO3
Lewis structures of SO3 and H2C=O
Shown above is the Lewis structures of SO3 ( sulfur trioxide ) and H2C=O ( formaldehyde ) molecules.
Geometry of H2C=O
