GEOMETRY AND MEASURING STRUCTURAL PROPERTIES

Introduction

The simple theories of bonding that we learn in General Chemistry are powerful and useful. We have learned that there is no direct relationship between the formula of a compound and the shape of its molecules. We have to rely on different bonding theories, such as Lewis structures, valence-shell electron-pair repulsion theory (VSEPR), and hybridization or valence bond theory (VBT), molecular orbital theory (MOT) to be able to predict chemical properties.

With all its deficiencies in the prediction of the shape of isoelectronic species, VSEPR method remains efficient and powerful in determination of the molecular geometry. VAEPR method relies heavily on the Lewis structures. The table below demonstrates how the geometry of the molecule can be predicted from the Lewis structure.

Table 1. Determine the molecular geometry using VSEPR

It looks like, other than Lewis structure, you need to know the correlation between the geometry of electron domain and the geometry of molecular. Table 2 exhibits these correlations.

MoleculeLewis structure | Number of e-Domain | e-Domain Geometry | Molecular Geometry |

SO2 | 3(1 e lone pair2 double bonds,Each double bondCounts as 1 eDomain) | Trigonal planner | ~ 120o

~ 120o

Bending |

Table 2. The correlation among the geometry of electron domain, number of electron lone pairs, and the geometry of molecules

# of e domain | Geometry of e-domain | # of electron lone pair | Geometry of molecule |

2 | Linear | 0 | linear |

3 | Trigonal planner | 0 | Trigonal planner |

3 | Trigonal planner | 1 | Bending (120o) |

4 | Tetrahedral | 0 | Tetrahedral |

4 | Tetrahedral | 1 | Trigonal pyramidal |

4 | Tetrahedral | 2 | Bending (~109o) |

5 | Trigonal bi-pyramidal | 0 | Trigonal bi-pyramidal |

5 | Trigonal bi-pyramidal | 1 | Sea Saw (~120o and 90o) |

5 | Trigonal bi-pyramidal | 2 | T shape (90o) |

5 | Trigonal...